safer chmeicals and safer reactions

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Safer chemicals,safer reactions

L195

July 2006

Ju

ly2006

L195

Electrolysis of molten

lead(II) chlorideThe Thermite reaction

Saferchemicals,s

aferreactions

Microscale reduction ofcopper oxide with hydrogen

Semolina particles on castor oilillustrate the presence of an

electric field betweenthe two plates


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ContentsPage

1. Introduction 11.1 Hazard information 11.2 Reducing risks 11.3 Reducing waste and safer disposal 2

2. Safer solutions 32.1 Stock solutions 32.2 Suggested concentrations for various practical activities 3

3. Safer alternatives to chlorinated hydrocarbons 73.1 Current situation 73.2 Disposal strategy 83.3 School alternatives for the use of chlorinated hydrocarbons 8

4. Safer alternatives to asbestos 124.1 Safer mineral fibre 124.2 Heat-resistant paper 13

5. Safer combustion of liquid fuels 145.1 Safer spirit burners 145.2 D-i-y spirit burners 15

5.3 A review of procedures for measuring the energy content of fuels 156. Safer advanced-level organic chemistry 16

6.1 Phenylamine (aniline) reactions - a safer alternative 166.2 Phenol reactions - a safer alternative 186.3 Nitration of benzene - a safer alternative 206.4 A safer use of nitrated products 216.5 Reduction of nitrobenzene - a safer alternative 226.6 A safer solvent for the extraction of caffeine from tea 236.7 A safer reagent to oxidise alcohols to carboxylic acids 24

7. Hydrogen reactions - safer procedures 257.1 Hydrogen burning and exploding in air 257.2 Hydrogen/oxygen explosion 267.3 Hydrogen as a reducing agent 27

8. Diffusion of gases - a safer alternative 288.1 Diffusion into air 288.2 Diffusion into a vacuum 29

9. Exothermic reactions - safer procedures 309.1 Ammonium dichromate(VI) decomposition 309.2 Iron/sulfur reaction 319.3 Sodium burning in chlorine 329.4 Thermite reaction 339.5 Fat-pan fire 349.6 Cigarette smoking machine 359.7 Sulfuric acid dilution 36

10. Electrolysis of molten salts 3711. Crude-oil distillation 39

12. Cooling curves 4012.1 Substances used for cooling curves 4012.2 Manual logging for the cooling curve of naphthalene 4212.3 Using a datalogger to monitor cooling 42

13. A safer procedure for the thiosulfate/acid reaction 43Index 45

This guide replaces L195, Substitute Chemicals and Alternative Procedures,published in 1994.

Strictly Confidential - Circulation to Members and Associates only.As with all CLEAPSS materials, members and associates are free to copy

all or part of this guide for use within their own establishments.

CLEAPSSBrunel University

Uxbridge UB8 3PHTel: 01895 251496 Fax: 01895 814372

E-mail: [email protected]

CLEAPSS 2003(amended 2006; section 5) Web site: www.cleapss.org


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L195 Safer chemicals, safer reactions

1. IntroductionMany people in science education have blamed health & safety and environmental legislationfor removing some of the well-known demonstrations and activities from school chemistry.

This, they believe, has made chemistry less exciting. There is no doubt that hazard informationon chemicals and legislation has altered the approach to practical work (see Guide L196,

Managing Risk Assessment in Science). However, CLEAPSS has always maintained that almost allstandard demonstrations, such as the Thermite reaction, can still be carried out and have aplace in todays curriculum.

1.1 Hazard informationThe Health and Safety Executive, through the CHIP Regulations1, publishes information con-cerning the level of hazards together with risk phrases2for many substances used in industry.This guidance is a result of carefully-controlled tests. Consequently, for example, the hazard forcopper(II) sulfate is harmful with the risk phrase R22: Harmful if swallowed. All suppliers ad-

here to this information.If a chemical is not on the Approved Supply List, suppliers must provide hazard informationfrom data supplied to them by the manufacturer of the chemical. This can lead to confusing andconflicting information. Thus, copper(II) chloride, which is not on the Approved Supply List, istoxic in some catalogues but harmful in others.

A Materials Safety Data Sheet (MSDS) must be provided when the chemical is first supplied orif the hazard data are changed. These sheets should be kept for reference. However, they arenot always helpful; some carry very little information (especially when the formulation is acompany secret!), while others have pages of information about animal tests, covering everyeventuality. There is also a difference between European and American-based suppliers, with

the latter often adopting an overcautious approach. It is useful to note that MSDSs apply toindustrial situations rather than to education where only very small amounts or dilute solutionsof these chemicals are used. MSDSs are also not risk assessments since they do not take intoaccount the circumstances in which the chemicals are used.

1.2 Reducing risksThis guide describes how control measures, required by the COSHH3andManagement of Healthand Safety at Work Regulations, allow procedures with hazardous chemicals to continue. Controlmeasures used to make procedures safer include substituting chemicals with a lower hazardrating (see Table 1) and/or lower concentrations, reducing the scale of an operation andensuring that vapours are contained. In this document, the hazard of a solution is printed in

small capitals; if there is no reference to a hazard, it may be assumed that the solution has a lowhazard.

Table 1 The hazards of chemicals

Physico-chemical effect Health effects

FLAMMABLE HARMFUL IRRITANT

HIGHLY FLAMMABLE TOXIC CORROSIVE: causes burns

Decreasingrisk

EXTREMELY FLAMMABLE VERY TOXIC CORROSIVE: causes severe burns

1 The Chemicals (Hazard Information and Packaging for Supply) Regulations2002.Approved Supply Lists: Information for

the Classification and Labelling of Substances and Preparations Dangerous for Supply, HSE. This publication is updatedfrequently and the ISBN number changes. Changes to the hazards of chemicals used in education are highlightedin the CLEAPSS Bulletin, published three times a year.

2 A list of risk phrases can be found in the CLEAPSS Laboratory Handbook, section 7.8.3 Control of Substances Hazardous to Health, 4th edition,Approved Code of Practice and Guidance, HSE Books, 2002, ISBN

0717625346.


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CLEAPSS can only advise on the science involved in a proposed activity. Those withresponsibility for chemistry, when directing practical work, should assess the suitability ofrooms (eg, size and ventilation), experience of staff and behaviour of students when adaptingtheir employers risk assessments for the particular circumstances of the school or college. Forfurther advice, again see Guide L196.

Ideally, an activity should be modified so that there is no, or a reduced, need for personal prot-ective equipment (PPE) as a control measure. Chemicals, or concentrations of solutions, shouldbe chosen so that, if possible, only safety spectacles need be worn as eye protection and thatthere is no need to wear gloves. All PPE should be as comfortable as possible.

Another source of guidance4 on risk assessment in educational activities is Safety in ScienceEducation. This emphasises the dynamic nature of assessing risk rather than it being a static,form-filling exercise. The guidance also states that the control measures arising from the assess-ment of risks should be included in the instructions for demonstrations and practical work.

1.3 Reducing waste and safer disposal

Any activity involving chemical changes will generate chemicals for disposal. Issues about dis-

posal need to be considered when the activity is planned. Many schools have rows of shelvesfull of chemicals no longer required by current courses or over-ordered in the past. These surp-lus chemicals may present hazards and certainly clutter up scarce space. They should be disp-osed of in a safe and responsible manner. Prep room clear-outs like this will normally involvepaying an authorised waste contractor - obtain at least three quotations5.

When planning an activity, in order to minimise waste disposal, consider:

arranging for the product of one reaction to be the starting material for the next (see sections 6.3,6.4, 6.5);

reducing the scale of the procedure or even adopting microscale techniques6;

recycling the products, if possible.

Situations where recycling may be practicable include the following. Recover, eg, silver, copper or nickel by electrolysis or displacement of the metal with iron.

Collect lead iodide from precipitation reactions and use later for electrolysis.

Collect copper sulfate crystals, crush, and use next year.

Wire-form copper oxide which has been reduced to copper can be reoxidised and reused or keptfor use as copper metal.

Recover solvents by redistillation, but not peroxidisable ones (eg, ethoxyethane, cyclohexene),unless first tested for peroxides.

Waste products which are hazardous can often be treated to make them low hazard or at leastless hazardous; see the examples below.

Table 2 Treatment of wasteWaste chemical Possible treatment

Acids or alkalis Neutralise one with another. Aim to discharge to waste between pH 5 and 9. If there isno surplus of one, soda ash and ethanoic acid are cheap materials.

Oxidising agents andreducing agents

React one with another. Iron(II) sulfate, acidified with a little dilute sulfuric acid, is acheap general-purpose reducing agent.

Metal salts It may be possible to precipitate, usually as the carbonate or hydroxide. Barium saltscould be precipitated as the (low hazard) sulfate. Chromium(VI) compounds, and othersalts in high oxidation states, may need reduction before precipitation.

For further information on the disposal of small amounts of waste products from chemicalreactions, see Hazcards.

4 Safety in Science Education,1996, DfEE, The Stationery Office, ISBN 011270915X, sections 4.7.1 and 4.7.2.5 See CLEAPSS leaflet PS5, Waste Disposal Contractors.6 See CLEAPSS guide L216,Microscale Organic Chemistry.


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2. Safer solutionsMyths: All bench solutions must be removed from laboratories. You must only use 0.4 M7

solutions at KS3. Corrosive and toxic solutions must not be used at KS3.

All of the above statements arefalse. Many years ago, reagent bottles containing 2 M acids, alk-alis and salt solutions, were often found on open shelves and benches within easy reach ofstudents for them to carry out qualitative analysis. Such situations can still be found but theready accessibility of hazardous solutions presents some risk. There may be valid reasons tokeep chemical solutions in a laboratory (eg, it is some distance from the prep room) but hazard-ous chemicals (ie, any with a hazard classification) should normally be locked in cupboards.Any chemicals on open shelves should be low hazard unless the assessment of risk has shownthat students are sufficiently reliable to make an exception.

In its scheme of work for science at KS3, QCA suggested the use of 0.4 M acids and alkalis. Theadvantage of using such concentrations is that none of the common acids or alkalis would beCORROSIVE. Using guidance from the CLEAPSS Laboratory Handbooksection 3, safety spectaclescould be used rather than goggles, which are the required form of eye protection when students

handle corrosive or toxic chemicals. Using the COSHH Regulationsas guidance, a risk assess-ment for a procedure to be used by pupils should identify the minimumsafe concentration of asolution at which the desired effects of the reaction can be observed. For most purposes, 0.4 Mor even 0.1 M is adequate but, in certain cases, to obtain the desired reaction, more concentratedsolutions are required (eg, the preparation of hydrogen using the reaction between zinc andhydrochloric acid). Guidance is given in Table 3 which relates an activity to a reasonable conc-entration of reactants. It is unlikely that, at KS3, schools will need to use concentrations strongerthan 0.4 M for sodium hydroxide and 2 M for hydrochloric acid. 1 M sulfuric8 acid mayoccasionally be required. All of these are classified as IRRITANT, rather than CORROSIVE. There isno justification in continuing to use 2 M solutions just because they always have been used inthe past.

Whether students are allowed to use corrosive and/or toxic solutions depends upon the riskassessment, not only for the activity but also a consideration of the behaviour and number ofstudents in the class, the experience of the teacher, the size of room and its ventilation etc. Someresponsible students could use 1 M sodium hydroxide (CORROSIVE) at Y7 and others could evenmake soap with 5 M sodium hydroxide at Y9, if the risk assessment was positive, but these arelikely to be exceptional cases.

2.1 Stock solutionsIt is recommended that large volumes (eg, 10 litres) of the following stock solutions areprepared for further dilution as and when required.

1 M sodium hydroxide (corrosive),

2 M hydrochloric acid and/or 1 M sulfuric acid (irritant).

Smaller volumes of more concentrated solutions may be required but these should be preparedas needed. Diluted solutions of ammonia should be prepared just before use because the contin-ual loss of gas from the solution reduces its concentration.

2.2 Suggested concentrations for various practical activitiesTable 3 provides concentrations and guidance for various activities.

7 M is used as a convenient method of writing mol dm-3.8 Sulfur is the British Standard spelling and has been used throughout this guide.


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Table 3 Suggested concentrations for various practical activities (continued)

Activity Suggestedconcentration

Comments(Hazards of these concentrations are shown. If no ha

Salt preparation usingdilute acids with metalhydroxide, metal oxidesor metal carbonate

2 M hydrochloric acid (IRRITANT) or1 M sulfuric acid (IRRITANT) with 2 Malkalis (CORROSIVE) includingammonia solution

10 ml of 2 M hydrochloric acid with 1 ml of 2 M sodium hsodium chloride salt.

10 ml of 1 M sulfuric acid (IRRITANT) with copper(II) oxid

(HARMFUL) produces 2.5 g of copper sulfate crystals (HAcompared to the use of 0.4 M acid because filtration, bo

Note that 1 M sodium carbonate solution has a lower hasolution and may be preferable. However, the chemistrymore complex.

Soap preparation 5 M sodium hydroxide solution(CORROSIVE)

A risk assessment should be carried out to ascertain whforming the procedure safely. Goggles or even face pro

Sodium hydroxide solu-tion with aluminium foil

1 M sodium hydroxide solution(CORROSIVE)

Hydrogen (EXTREMELY FLAMMABLE) is produced.

Stoichiometry ofprecipitation reactions

1 M solutions of metal salts These concentrations are required to obtain a measurarating of the salts to be used (see introduction to this se

Sulfur dioxide test 1% potassium dichromate(VI)solution

This concentration is still classified as TOXIC.

Testing for carbon-carbon double bonds

0.01 - 0.005 M bromine solution inwater [0.16 - 0.08% (w/v) solution]0.001 M acidified potassium mang-anate(VII) solution

The weaker solution should be used for the cracking odepth of reagent in the test tube so that not much of the

Titrations (volumetric,coulometric)

0.1 M alkali (IRRITANT) and acidsolution

Alkaline solutions more dilute than 0.01 M are notsuitaolved carbon dioxide interferes with the results. Solutionused but evaporation of the solvent at the tip of the bure


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3. Safer alternatives to chlorinated hydrocarbonsMyth: You cannot use chlorinated hydrocarbons9in schools.

3.1 Current situation

Table 4 illustrates the hazards of chlorinated solvents that have been used in school chemistry.

Table 4 Chlorinated hydrocarbons used in school chemistrySubstance Hazard rating and OEL

(ppm)10

Risk11

Schooluse?

Comments

1,2-Dichloroethane TOXIC, HIGHLY FLAMMABLE5 (MEL) (LTEL)

R45, 11, 22,36/37/38

No (B pt is 84 C.) Formally used tomake Thiokol rubber. It is notrecommended for school use.

Dichloromethane(Hazcard28)

HARMFUL; Limited evidence of acarcinogenic effect100 (MEL) (LTEL)300 (STEL)

R40 Yes (B pt 40 C.) Used in paint strip-pers and brush cleaners. It canbe used with care by post-16pupils.Although classified as acategory 3 carcinogen, the evi-

dence has been challenged.Tetrachloroethene(Hazcard99)

HARMFUL; Limited evidence of acarcinogenic effect,DANGEROUS FOR THE ENVIRONMENT

50 (LTEL), 100 (STEL)

R40, 51/53 Yes (B pt is 121 C.) This is not acommon school chemical but itcan be used with care by post-16 pupils.

1,1,2,2-Tetrachloro-ethane

VERY TOXIC, DANGEROUS FOR THEENVIRONMENT

No OEL assigned

R26/27,51/53

No (B pt is 146 C.) It is not recom-mended for school use.

Tetrachloromethane(Carbon tetrachloride)(Hazcard100)

TOXIC; Limited evidence of acarcinogenic effect,DANGEROUS FOR THE ENVIRONMENT

2 (Sk12) (LTEL)

R23/24/25,40, 48/23,52/53, 59

No (B pt is 77 C.) Ozone depleter(not now available).This used tobe a common solvent in schoolsespecially for bromine. Schools

should dispose of existing stocks.1,1,1-Trichloroethane(Hazcard103)

HARMFUL,DANGEROUS FOR THE ENVIRONMENT

200 (LTEL), 400 (STEL)

R20, 59 No (B pt is 74 C.) A comparativelysafe substance but no longeravailableas it is an ozonedepleter. Schools should disposeof existing stocks.

1,1,2-Trichloroethane HARMFULNo OEL assigned

R20/21/22 No (B pt is 112 C.) A very expensivechemical.

Trichloromethane(Chloroform)(Hazcard104)

HARMFUL; limited evidence of acarcinogenic effect2 (Sk) (LTEL)

R22, 38, 40,48/20/22

Yes (B pt is 61 C.) It can be used ina fume cupboard with care bypost-16 pupils.

Trichloroethene(Hazcard99)TOXIC: category 2 carcinogen100 (LTEL).150 (STEL) R45, 36/38,52/53,67 Only bytrained

staff

(B pt is 87 C.) It is used to testfilter fume cupboards by trainedemployees. It should not be usedby pupils.

9 Chlorinated hydrocarbons are organic compounds in which one or more halogen atoms are substituted for a hyd-

rogen atom.10 See CLEAPSS Laboratory Handbooksection 7.9.1 for information about Occupational Exposure Levels.11 See CLEAPSS Laboratory Handbooksection 7.8 for information about risk phrases.12 Sk means that a chemical can be absorbed through the skin.


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The chronic effects of most chlorinated hydrocarbons are still not fully understood but wherethere may be a cause for concern, a R4013 risk phrase is given. The R45 risk phrase is givenwhere there is a more serious problem14. More details on carcinogenic hazards can be found inTopics in Safety15, section 12.

This country has signed up to the Montreal protocol which forbids the sale of chemicals with a

R59 risk phrase (dangerous to the ozone layer) for general use. Schools cannot now legally buytetrachloromethane and 1,1,1-trichloroethane (see Hazcard103). Trichloromethane (chloroform),however, can still be bought from suppliers.

The COSHH Regulationsstate that the three substances discussed above cannot be used in diff-usive applications such as surface cleaning and the cleaning of fabrics except for the purposesof research and development or for the purpose of analysis. It is not clear how the word diff-usive applies to a few millilitres in a test tube stoppered with a bung, as the rate of evapor-ation would be negligible. It would need, however, a court of law to rule on this!

Some chlorinated hydrocarbons, eg, dichloromethane, can be used, although of course there arestill hazards (see Table 4). Further legislation is anticipated and any alteration to this inform-ation will be provided in future editions of the CLEAPSSBulletin.

3.2 Disposal strategyAfter use as solvents, chlorinated hydrocarbons should be collected by the students or teacherfor disposal by a hazardous waste contractor. Unopened or nearly-full bottles of tetrachloro-methane and 1,1,1-trichloroethane should also be included. Stocks of trichloromethane (seeTable 5) might also be disposed of because the inclusion of investigations of Raoults Law in 6th

form schemes of work is now highly unlikely.

Treatment for the removal of materials such as bromine and iodine from chlorinated solventscan be achieved by washing the solvent with 1 M sodium hydroxide solution (CORROSIVE).When the liquid is colourless, the aqueous layer should be removed and the solvent placed in

organohalogen waste bottles, suitable labelled, with their contents and the date. They should beremoved by a hazardous waste contractor within a year. This could be an expensive operation.Test-tube washings can go down the foul-water drain with plenty of soapy water.

Recycling is also an option for larger volumes of solvents that can still be used legally. Theliquid would need to be stripped of contaminants, dried and redistilled for further use. Thisprocedure, however, takes time and should be carried out only by those with expertise inorganic chemistry.

Destruction of chlorinated hydrocarbon solvents, as opposed to non-chlorinated hydrocarbons,is more involved16and expensive. Any procedures that specify the use of organohalogen solv-ents should be reviewed to see if safer alternatives are available. If information in Table 7 does

not identify a suitable alternative, contact CLEAPSS.

3.3 School alternatives for the use of chlorinated hydrocarbons

The COSHH Regulationsdemand that if a safer alternative reagent is available for a certain act-ivity, it should be used in preference to the more hazardous substance. Table 5 considers activ-ities for which there appear to be no suitable alternatives.

13 Limited evidence of a carcinogenic effect:this phrase is associated with category 3 carcinogens.14 See CLEAPSS Laboratory Handbooksection 7.8.15 Topics In Safety, 3rdedition, 2001, ASE, ISBN 0863573169.16 Toxic dioxins may be formed during combustion and these need to be removed from the waste fumes.


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Table 5 Activities for which there are nosuitable alternatives

Activity Comment

Investigating the hydrolysis of mono-sub-stituted halogenoalkanes. (An activitycarried out on a test-tube scale.)

Mono-substituted butanes, which are usually used for thisactivity, are not included in the Montreal protocol. Thewashings may be disposed of down the foul-water drainwith plenty of water.

Investigating the negative deviation ofRaoults Law: mixtures of trichloro-methane (TOXIC) with ethyl ethanoate(HIGHLY FLAMMABLE).

There seems to be no satisfactory alternative to usingtrichloromethane. See Hazcard104. The mixtures need tobe kept in a labelled bottle for disposal by a licensedcontractor. However, it is most unlikely that this activitywill now be carried out in schools as the topic has largelybeen removed from post-16 curricula.

Preparation of Thiokol rubber:1,2-dichloroethane is used.

It is no longer recommended that this substance shouldbe made in schools

17.

Chlorinated hydrocarbons were usually used as degreasing agents in industry. Examples ofalternatives available to industry are shown in Table 6.

Table 6 Industrial solvents available to schools

Name Recommendedsource

18Chemical Hazard Comments

EvolveCH15

Breckland ScientificSupplies or Timstar6.95 for 500 ml.

A blend of hydro-carbons.

B pt about 170 C.

R10: FLAMMABLE.R65: HARMFUL; Maycause lung damage ifswallowed.

Not suitable for dissolvingbromine. A 1% bromine sol-ution decolourises withinminutes.

Lotoxane Griffin and George12.75 for 1 litre.

C11-13 branchedhydrocarbons.

B pt about 180 C.

Minimal. Not suitable for dissolvingbromine.

Volasil

244

VWR (Merck)19

13.86 for 500 ml

Octamethylcyclo-

tetrasiloxane.(CH3)8Si4O4M pt 17.5 C

B pt 175.5 C.

Supplier quotes R53,

62: May cause long-term adverse effectsin the aquatic envir-onment; possible riskof impaired fertility.

Despite these hazard warn-

ings, the substance is usedextensively in face creamsand shampoos. It dissolvesbromine but the solutiondoes not keep; see Table 7.

Despite comments to the contrary in chemical catalogues, there is no single substitute for thechlorinated hydrocarbons used in schools; different activities may need different alternat-ives.

It is necessary to review the activities for which chlorinated hydrocarbons are used in schoolchemistry and identify suitable alternatives. Table 7 provides guidance. Of course, the suggest-ed alternative may not work as well as the original chlorinated hydrocarbon!

17 Safety in Science Education, DfEE, 1996, The Stationery Office, ISBN 011270915X.18 Addresses may be found in the index to the CLEAPSS Laboratory Handbook. Obtaining chemicals from other

suppliers often increases the price.19 Merck Eurolab (formally BDH Ltd) is now trading as VWR International. Its telephone number is 0800 223344 but

schools may be referred to a local agent for VWR products.


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Table 7 Activities for which there aresuitable alternatives

Activity or substance Substitute

Bromine solution in a non-aqueoussolvent; tetrachloromethane was thebest solvent because the solutioncould be kept for a long time.

Cyclohexanecan be used if it can be stored out of the light (eg,in a dark bottle or a clear bottle in a labelled cardboard box).However, plastic screw tops or bungs will be affected if storedfor over a month. In a clear bottle, noticeable deterioration will

occur within 2 days.

Volasil 244 can be used in clear bottles in the laboratory. A3% (w/v) solution is stable overnight. A 1% (w/v) solution isstable for several days but not for a week. Solutions that are3% (w/v) or stronger should be used as quickly as possible. Inchemical terms, bromine opens the ring structure of Volasil toinitiate polymerisation. The resulting jelly is not easy to removefrom bottles.

Chromatographic separation ofaspirin, paracetamol and caffeine.

Ethyl ethanoate(HIGHLY FLAMMABLE) (10 ml) with a drop ofconcentrated ethanoic acid (CORROSIVE).

Chromatographic separation of the

nitration products of phenol.

A 1:1 solution of ethoxyethane(EXTREMELY FLAMMABLE) andpetroleum ether(100-120 C) (HIGHLY FLAMMABLE). Rf value forphenol is 0.60 with products at 0.72 and 0.41 plus other spots.

Degreasing agent (eg, lenses andmetals).

Lotoxane, Evolve CH15& Volasil 244can be used. However,the rate of evaporation is slower than with 1,1,1-trichoroethane.

Displacement reactions of the halo-gens (ie, the addition of chlorine solu-tion to potassium bromide solution.

Cyclohexaneor Volasil 244are suitable but, being less densethan water, they form an upper layer. Cyclohexane is much lessexpensive than Volasil 244. The collected waste can be treatedwith 1 M sodium carbonate solution before recycling or disp-osal. See Hazcards45 and 106 for disposal.

Distinguishing between soda andborosilicate glass.

Propan-1,2,3-triol. Borosilicate glass disappears from view inpropan-1,2,3-triol (glycerol) because it has a similar refractiveindex.

Distribution of ammonia betweentrichloroethane and water.

Ethyl ethanoate(HIGHLY FLAMMABLE) may be used but theconcentration of ammonia solution must be 1 M or less.

Distribution of iodine betweentetrachloromethane and water orpotassium iodide solution.

Volasil 244.This is a complex system. Details can be obtainedfrom CLEAPSS.

Electric field patterns of semolinagrains trapped between layers of1,1,1-trichloroethane and castor oil.

Castor oilcan be used on its own without using a chlorinatedhydrocarbon at all. Small bottles of castor oil can be obtainedfrom a local pharmacist.

Enthalpy of formation of inter-molec-ular forces (hydrogen bonding)between trichloromethane and ethylethanoate molecules.

It is preferable to measure the enthalpy change when the inter-molecular forces (hydrogen bonds) between ethanolmoleculesare broken by the addition of cyclohexane.

Extraction of caffeine from tea orlimonene from orange peel.

Dichloromethane(HARMFUL). Another method of extractingcaffeine into an organic layer can be achieved by addingenough sodium chloride to saturate the solution and then add-ing propan-1-ol(HIGHLY FLAMMABLE). The caffeine extracted willcontain sodium chloride and will need to be separated (seesection 6.6).

Hydrolysis of group IV halides:Tetrachloromethane does not reactwith water but silicon tetrachloridedoes.

A (1:1:1) mixture of tetrachloroethene(HARMFUL), ethanol(HIGHLY FLAMMABLE) and water, which are present in the samephase, showed no changes in conductivity when left for a day.This indicated that no hydrolysis had taken place. It isquestionable whether this isa valid substitution!

Iodine value determination of unsat-urated fats. Dichloromethane(HARMFUL) can be used. Tetrachloro-methane was used to amplify the iodine during the titration of

Wijs solution against sodium thiosulfate solution.


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Table 7 Activities for which there aresuitable alternatives (continued)

Activity or substance Substitute

Lead analysis using dithizone. Methylbenzene(HARMFULand HIGHLY FLAMMABLE) is analternative solvent for dithizone.

Molecules attracted to electrostatic

fields: an electrostatic field generatedby rubbing a plastic rod is broughtclose to a liquid running out of aburette. The direction of the flow isdeflected by the polarised moleculesattracted to the field. Tetrachloro-methane shows no deflection.

Tetrachloroethenecan be substituted for tetrachloromethane.

Dichloromethane and trichloromethane can still be used. Theburettes can be set up in a fume cupboard. However, a highface velocity may well affect the observations so the fumecupboard should be switched off while the demonstration iscarried out. If separate burettes are used, the liquids can be putback into their bottles for further use.

Nylon preparation: solutions of 1,6-diaminohexane in water and an acidchloride in an immiscible solvent (itused to be tetrachloromethane) arebrought together. The nylon polymerforms at the interface of the two

liquids.

Cyclohexane, petroleum ethers, Lotoxane, Evolve CH15and Volasil 244can all be used as suitable alternative solventsto dissolve the acid chloride. However, these solutions have tobe applied after the aqueous layer has been placed in thebeaker. Organic solvents are never completely free from tinyamounts of water

20and a white solid in these solutions indicates

that the acid chloride has hydrolysed to the dicarboxylic acidand will not form nylon. See Hazcard100 for further exper-imental points. For disposal, stir the liquid, place the nylon inwaste paper and put it in the waste. See the relevant Hazcardfor the removal of the solvent waste.

Perspex cement use. Dichloromethane(HARMFUL).

Tin(IV) iodide preparation. Tin andiodine are heated under reflux in1,1,1-trichloroethane.

Dichloromethane (HARMFUL)can be used but the boiling pointis lower. Tetrachloroethene(HARMFUL) may also be used.

20 Tetrachloromethane was the ideal solvent because water absorption is extremely small and solutions of acid

chlorides kept for years.


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4. Safer alternatives to asbestos4.1 Safer mineral fibre

Machine-made mineral fibre21, used as a replacement for asbestos wool in school science, is stillan emotive issue because of fears that it might induce cancer as a result of inhaling airbornefibres. Most mineral fibre, including Triton KaowoolCeramic Fibre22 (a white fibre material

made by Thermal Ceramics UK Ltd23), is given an R40 rating (limited evidence of a carcino-genic effect) in the Approved Supply List (see footnote 1). This risk phrase indicates that it is acategory 3 carcinogen (substances which cause concern but as yet there is no direct evidence tolink them to human cancers). The following statement24should be noted.

Man-made mineral fibres are coarser than asbestos fibres and do not split longitudinally into fib-

rils. Most currently available products do not readily release airborne fibres. Some fibres are fine

enough to be breathed in, but few will reach the deep lung and those that do will not persist as

they are generally much more soluble than asbestos. While there is limited evidence of an

increased lung cancer risk in workers producing mineral wool insulation in the early days of the

industry, there is no evidence of an increase in risk of mesothelioma.

Thermal Ceramics UK has also produced Superwool 607, a fibre which, when tested, did notgive results which warranted a R40 risk phrase and is safer to use. It is now available fromPhilip Harris (cat. no. C5H71636, 500 g, 10.38), Scientific & Chemical Supplies (cat. no. MI020,500g, 4.66) and Griffin Education (cat. no. SPW-607-010L, 5.5 m x 610 mm x 6 mm blanket,14.18).

Ceramic fibre is useful because it is an inert material, which keeps reactants apart or retainsheavy vapours, dusts and powders and stops them entering the atmosphere. Some examples ofits use follow.

Keeping reactants apart

Diagram A shows a common arrangement ofequipment used for passing steam over magnes-ium and cracking hydrocarbons or alcohols. Itis also known as the Arculus method.When heat-ing nitratesand testing for oxygen, it has beenknown for pupils to put glowing splints into moltennitrate, accidentally at first and then on purpose, towitness the very violent reaction. Placing mineralfibre at the end of the ignition tube (Diagram B)allows oxygen to diffuse through the fibre butprevents the glowing splint from reaching themolten nitrate.

The fibre absorbshe liquid

Other reactants orcatalysts can be placed here

Diagram A

Retaining heavy vapours and dusts

The fibre allows gases or vapours to passthrough but retains dusts and powders. However,the denser the gas the slower the rate of diffu-sion so hazardous vapours take a comparativelylong time to diffuse through the fibre.

The arrangement in diagram B can be usedwhen heatingammonium chloride, ammoniumdichromate(VI)(see page 30), coal, iodine,iron/sulfur mixtures (see page 31), metalnitrates, potassium chlorate(V)andpotassium manganate(VII).

Substance tobe heated isplaced here

Mineral fibre isplaced in theopening orfurther down

Diagram B

21 This used to be called Man-Made Mineral Fibre (MMMF)!22

Available from Griffin & George: T/3740/48 for 100 g: (look under T in the chemicals section of the catalogue.From Beecroft: CC3500-52 for 100 g; CC3500-56 for 500 g; (look under Ceramic fibre wool).23 Thermal Ceramics UK Ltd, Tebay Road, Bromborough, Wirral CH62 3PH. Tel0151 334 4030, Fax0151 334 1684,

Web site:www.thermalceramics.com.24 Asbestos and Man-Made Mineral Fibres in Buildings: Practical Guidance, ISBN 0727728350, Thomas Telford Publish-

ing, 2000, or from www.environmental-center.com/articles/article725/article725.htm.


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13

4.2 Heat-resistant paperMachine-made mineral fibres can be reconstituted into heat-resistant paper using an organicacrylic binder. This binder should be burnt off in a fume cupboard before the paper is used.After the burning process, the paper is more brittle and should be handled carefully, otherwiseit breaks up. If required, the paper should be shaped beforethe binder is burnt off.

Heat-resistant paper (2 mm thick) made from Superwool 607 (minimal hazard) is suitable foruse in schools and is now available from Scientific & Chemical Supplies (250 x 250 mm sheet,cat. no. APS020010, 3.73; 50 mm x 10 m roll, cat. no. APS020020, 10.71) and Griffin & George[but unfortunately only in bulk quantities (cat. no. SPW-607-P; pack of 5 rolls 20 m x 500 mm)].

The copper(II) oxide/zinc reaction

Procedure Wear eye protection. Cut a piece of 2 mm heat-resistant paper 80 x 40 mm. Fold it lengthways

in a V shape. Alternatively, use two sheets of 1 mm paper together.

In a fume cupboard, hold the paper in tongs to burn off the binder. Cont-inue burning until black carbon left on the paper is burnt away, leaving awhite surface. The paper is quite brittle now so take care when handling it.

In two separate weighing boats (or other dry receptacles) measure 0.25 g

of copper(II) oxide and zinc powder respectively. Pour the contents of onereceptacle into the other and then pour back again. Do this several timesto ensure good mixing.

Place the mixture along the centre of the V in the paper.

Light a Bunsen burner and apply the flame directly to the solid. Removethe flame immediately the reaction starts.

Have ready a 250 ml beaker containing 1 M sulfuric acid. Add the paperand its contents to the beaker and heat to boiling.

The acid dissolves zinc oxide, unreacted zinc and unreacted copper(II)oxide. Note the colour of the solid remaining.

Disposal: pour all contents of the beakers through a sieve to remove thesolids. The liquid can be diluted and poured down the foul-water drain.

The solids can be placed in the waste bin.

Burning calciumThis demonstration should be carried out by an adult with the students at least 3 m away, allwearing eye protection. They should cover their eyes with open fingers to reduce some of thelight. A Bunsen burner flame using natural gas is not hot enough to light calcium but a kitchenblowtorch25works well.

Procedure Make a 4 mm layer using four 40 x 40 mm pieces of 1 mm heat-resistantpaper

26. There is no need to remove the binder.

Place them on a gauze supported by a tripod.

Wear a face shield. Wear thermal protective gloves.

Using forceps, place one or two granules of calcium on the paper. Light the kitchen blowtorch and, holding it at arms length, apply it to the

calcium. Place the other hand in front of the eyes with open fingers. Askthe class to do the same.

The calcium will ignite with a bright red flash.

A kitchen blowtorch

25 A kitchen blowtorch is available from a cooking gadget shop. It uses LPG from lighter fuel which gives a very hot,

narrow flame. (It can also be used to make holes in borosilicate test tubes, which converts them into combustiontubes, and repair chipped borosilicate glassware.)

26 Two pieces of 2 mm thick Superwool 607 heat-resistant paper could be used.


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14

5. Safer combustion of liquid fuels5.1 Safer spirit burners

Myths: Arent spirit burners banned? We have heard that explosions have occurred. We are

not allowed to use methanol as a fuel as it is toxic.

A few explosions have been reported but, so far, only with methanol, the most volatile of the

alcohols. For an explosion to occur, a mixture of air and alcohol vapour is required. The explos-ive range for methanol is between 7 and 37%, which is the widest range of all the alcohols; forethanol it is between 3.3 and 19%. We have been able to reproduce a small explosion by using aburner with a wick much narrower than the hole through which it fits, leaving a gap betweenthe wick and the holder. If the spirit burner is only partially filled with methanol, there is anair/methanol mixture in the vessel. As methanol rises up the wick and is ignited (with a lightedsplint), the flame backfires through the gap into the vessel, creating a small explosion. Spiritburners, currently available for purchase, appear to be made of glass although you may haveolder versions made of brass or zinc27.

Methanol is TOXIC by inhalation, skin absorption and if swallowed; it is also HIGHLY FLAM-

MABLE. The level of methanol in the atmosphere of the laboratory will be minimal during use ofthe burners. If the experimental procedure is carried out correctly, there is no reason whypupils should be exposed to vapour levels hazardous to health.

Table 8 describes the control measures that should be observed when buying and preparingspirit burners for pupil use.

Table 8 Control measures for buying and preparing spirit burners

Control measure Comment

Purchase spirit burners witha capacity of 50 ml or less.

Many designs have a capacity of over 100 ml, eg, the five burners onthe left in the picture below. The burner on the far right was the small-est with a capacity of 50 ml.

Make sure that the design is

suitable for the intendedpurpose and the burner iswell constructed.

Many spirit burners on the market are sold for aromotherapy and dinner

parties (eg, the four on the left in the picture). For some, their glassconstruction is rather thin. The tops on some burners also do not fitwell. Of the spirit burners in the main suppliers catalogues, the designon the right below is probably the best

28. Quality control may well be a

problem with all designs, so examine new purchases carefully.

Use a small funnel to fill aspirit burner.

The openings on the burners are narrow and it is easy to spill highlyflammable liquid.

Fill the spirit burner morethan half full.

This will ensure that there will not be an explosive mixture of air andalcohol vapour in the burner. If you have to use a large-capacity spiritburner, eg, > 50 ml, partially fill it with a resin or with cotton wool.

Replace a wick with one ofthe same diameter.

Explosions occur when the flame travels down a narrow wick andignites the alcohol/air mixture in the burner.

KS3 and 4 pupils should notfill spirit burners.

It is good practice to have several burners filled, labelled accordinglyand ready for use. They should be refilled in the prep room before thestart of any practical. If methanol is to be used as a fuel, burners shouldbe filled in a fume cupboard with no sources of ignition near by. Thetechnician or teacher should wear eye protection.

Lighted spirit burners must

not be moved.

Neverhold in the hand a spirit burner about to be used. It should be

placed on the bench and lit only with a lighted spill or match. Once

alight, do notmove a burner around the laboratory.

27 Old metal burners have exploded as well as those made of glass!28 Available from Griffin & George, Beecroft and Timstar.


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Table 8 Control measures for buying and preparing spirit burners (cont.)

Control measure Comment

The holder in which the wickfits should not be cracked orseverely chipped.

Many wick holders are made of porcelain which becomes chippedbecause of poor handling by the maker or supplier, misuse in labora-tories or thermal stress. The tops must be loose so the burner can berefilled but many are soloose that they fall out when the burner is

tipped up, either by accident or on purpose. If very loose, it is possiblethat there is a sufficient gap to ignite the alcohol in the container.

Ensure good pupilbehaviour.

Misbehaviour in this activity could lead to serious consequences. Theteacher must be vigilant at all times during the activity. Perhaps thetechnician, if given time, could help the pupils when they are using thebalance. Teachers with unruly pupils should consider performing ademonstration or making a video of the procedure so that recordings ofthe mass and temperature can be made by watching the recording.

5.2 D-i-y spirit burnersSpirit burners could be made in house from small jars or bottles, previously used for jam orink. Examples are shown in the diagrams below. The volume of highly flammable liquid can be

reduced still further by partially filling the bottle with cotton wool or epoxy resin.

35 mm

40 mm

Highly flammableliquid

Small jam jar

1.5 mm expansion holeSuper glue to fixthe glass to the lid

Cotton string wick 6 mm medium-wall borosilicatetubing fed through a 6.2 mm holein the lid

Ink bottle

Resin

5.3 A review of procedures for measuring the energy content of fuelsConfusion arises when comparing the energy content of fuels, such as the alcohols, and in foods(eg, burning peanuts or alternative materials29). Usually, the small amount of oil in a nut orsnackfood is used to heat water in a large test tube containing about 10-20 ml of water; a burn-ing peanut can boil this volume of water30. This activity is a flawed technique, giving rise toinaccurate estimates of energy content. Students should be asked how they could improve it,eg, by surrounding the test tube with an empty tin to reduce draughts.

The information in Table 9 uses enthalpy of combustion data to estimate the temperature risewhen heating 250 g of water by burning 1 g of an alcohol.

Table 9 Temperature increase, on burning 1 g of alcohol to warm 250 g of water

Alcohol Methanol Ethanol Propan-1-ol Butan-1-ol Pentan-1-ol Hexan-1-ol Heptan-1-ol

Increase 21.6 C 28.3 C 32.1 C 34.4 C 36.0 C 37.2 C 38.1 C

Greater accuracy is obtained in this activity when using a balance to 2 decimal places, an accu-

rate thermometer marked with 1intervals, a large volume of water, a small temperature riseand protection from draughts.

29 See PS10, The Burning Peanut Investigation and Allergies to Nuts, CLEAPSS School Science Service, for further

information.30 Anti-bumping granules should be added to the test tube containing water so that boiling water does not shoot out

and scald pupils.


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6. Safer advanced-level organic chemistry6.1 Phenylamine (aniline) reactions - a safer alternative

Comment Phenylamine (aniline) (TOXIC) has a short-term exposure limit (STEL) of 5 ppm(20 mg m

-3) and a long-term exposure limit (LTEL) of 2 ppm (10 mg m

-3)31

. It isunlikely the STEL level would be reached in a laboratory where students are carry-ing out simple test-tube reactions with small amounts of material. However, thereare less hazardous aromatic amines available, which should be used in prefer-ence. Ethyl 4-aminobenzenecarboxylate (ethyl 4-aminobenzoate) is recommendedalthough it is labelled IRRITANT

32by some suppliers. It is used as a local anaesth-

etic to the skin (benzocaine) and is present in some throat sweets!

O

O

NH2

CH3

Procedure Solubility and pH Add 2 ml of distilled water to a test tube followed by a small amount (0.02 g) of

ethyl 4-aminobenzenecarboxylate. Heat to boiling, add a few drops of full-range indicator and allow the solution to cool.

Formation of salts

Add 0.02 g of ethyl 4-aminobenzenecarboxylate to 4 ml of 1 M hydrochloricacid in a test tube. Warm the solution, if necessary, to dissolve the solid. Nowadd 5 ml of 1 M sodium hydroxide solution (CORROSIVE).

Addition of copper(II) ions

Add 0.02 g of ethyl 4-aminobenzenecarboxylate to 2 ml of 0.1 M copper(II)sulfate solution. Warm the mixture and then allow it to cool.

The addition of bromine water

Dissolve 0.02 g of ethyl 4-aminobenzenecarboxylate in 4 ml of 1 M hydro-chloric acid in a test tube. Add 2 ml of bromine water ( IRRITANT).

Ethanoylation

Place 0.5 g of ethyl 4-aminobenzenecarboxylate into a boiling tube and add0.5 ml of ethanoic anhydride (CORROSIVE) in a fume cupboard. Note what hap-pens during the reaction. In the fume cupboard, add 2 ml of water and heat toboiling. Cool the solution in an ice bath with stirring and filter off the crudesolid. Add the solid to 4 ml of 50% ethanol/water mixture (HIGHLY FLAMMABLE),recrystallise, filter and find the melting point of the solid when it is dry.

Diazotization and coupling reactions Dissolve 0.25 g of ethyl 4-aminobenzenecarboxylate in 1 ml of 1 M hydro-

chloric acid in a boiling tube. Cool this solution in an ice bath. Dissolve 0.2 g ofsodium nitrate(III) (OXIDISINGand TOXIC) in 1 ml of water in another boiling tubeand cool this solution. Dissolve 0.5 g of naphthalen-2-ol (HARMFUL) in 5 ml of1 M sodium hydroxide (CORROSIVE). Add the ethyl 4-aminobenzenecarboxylatesolution to the sodium nitrate(III) solution and then add this mixture to thenaphthalen-2-ol solution.

Disposal These procedures are carried out with minimum quantities and the materials canbe flushed down the foul-water drain.

31 See CLEAPSSLaboratory Handbook, section 7.9.1 for information on Occupational Exposure Limits.32 R43: May cause sensitisation by skin contact (VWR International Ltd [formerly Merck Ltd]); R38: Irritating to the

skin (Aldrich): Minimal risk (Fisher).

Ethyl 4-aminobenzenecarboxylate


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Notes

Br

Br

O

O

NH2

CH3

CH3 O

O

CH3

O

NH

O

O

NH2

CH3

O

O

N

CH3

N

OH

IEthyl 4-amino-3,5-

dibromobenzenecarboxylate

IIEthyl 4-(ethanoylamino)

benzenecarboxylate

III

Ethyl 4-[2-(2-hydroxy-1-naphthyl)hydrazino]benzenecarboxylate

Ethyl 4-aminobenzenecarboxylate

Ethyl 4-aminobenzenecarboxylate is soluble in hot water. It cools to form long,needle-shaped crystals. The solution has a neutral pH. As the ester group is elec-tron withdrawing, the substance is a slightly weaker base than phenylamine. Ethyl4-aminobenzenecarboxylate dissolves in 1 M hydrochloric acid and is precipitatedon addition of the alkali. The solution turns lime green on warming with copper(II)ions indicating the formation of a complex ion with ethyl 4-aminobenzenecarboxyl-

ate acting as a ligand. The blue colour returns on cooling. With bromine wateradded to ethyl 4-aminobenzenecarboxylate, a white precipitate (structure I)isformed.

Ethyl 4-aminobenzenecarboxylate dissolves in ethanoic anhydride. As the reactionproceeds, heat is evolved and another solid appears. It may be necessary to addmore water if the solid does not recrystallise on cooling. The melting point of theproduct (structure II) should be 110 C. Diazotization of ethyl 4-aminobenzene-

carboxylate and couplingwith naphthalen-2-ol produces a red dye (structure III).Azo dyes should not be isolated. The diazonium salt of ethyl 4-aminobenzene-carboxylate appears to be stable at room temperature but cooling is included sothat, in the procedure, students are less likely to be confused about reactionconditions for phenylamine. Heating the diazonium salt results in only a smallamount of coupling, with a yellow dye being formed.


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6.2 Phenol reactions - a safer alternative

Comment Phenol vapour has a maximum exposure limit (MEL) of 2 ppm for long-term exp-osure although it is doubtful that this level would be reached in a laboratory atroom temperature where only small amounts are being used

33. Phenol is TOXIC in

contact with the skin and if swallowed and CORROSIVE, causing burns. It solidifiesinto a lump when kept for long periods. It then becomes very difficult to removefrom the bottle.

However, there are less hazardous aromatic phenols available. Methyl 4-hydroxy-benzenecarboxylate (methyl 4-hydroxybenzoate) has an IRRITANT warning fromAldrich but other suppliers issue no hazard warnings. Unlike phenol, it does notabsorb water from the atmosphere.

CH3

O

O

OH

Methyl 4-hydroxybenzenecarboxylate

Procedure Solubility and pH Add 2 ml of distilled water to a test tube followed by 0.02 g of methyl 4-

hydroxybenzenecarboxylate. Heat to boiling, add a few drops of full-rangeindicator and cool slowly.

Formation of salts

Add 0.02 g of methyl 4-hydroxybenzenecarboxylate to 2 ml of 2 M sodiumhydroxide solution (CORROSIVE). Warm the solution if necessary. Now add 3 mlof 2 M hydrochloric acid (IRRITANT).

Addition of iron (III) ions

Add 0.02 g of methyl 4-hydroxybenzenecarboxylate to 2 ml of 0.01 M iron(III)chloride solution.

Ethanoylation

Dissolve 0.5 g of methyl 4-hydroxybenzenecarboxylate in 4 ml of 2 M sodiumhydroxide solution (CORROSIVE) in a boiling tube. In a fume cupboard, add 1 mlof ethanoic anhydride (CORROSIVE) and swirl the liquid round for a few minutes.Note what happens during the reaction. Let the liquids settle, then place theboiling tube in an ice bath. Decant off the liquid, add 8 ml of 50% aqueousethanol solution (HIGHLY FLAMMABLE) to the solid and remove from the fumecupboard. Warm until the solid dissolves, cool in an ice bath (if the solid doesnot form, scratch the sides of the container with a glass rod or spatula) andfilter off the solid. When the solid is dry, obtain its melting point.


Add 0.02 g of methyl 4-hydroxybenzenecarboxylate to bromine water

(IRRITANT).Nitration

Add 0.04 g of methyl 4-hydroxybenzenecarboxylate to 4 ml of 1 M nitric acid(CORROSIVE). Heat the solution to boiling, allow it to cool slowly for 2 minutesand then quickly in an ice bath. Filter off the solid. Dissolve the solid in 1 ml ofethoxyethane (EXTREMELY FLAMMABLE) and use chromatography to analyse thesolution using silica plates (80 x 40 mm) alongside a reference solution ofmethyl 4-hydroxybenzenecarboxylate. The chromatography solvent is a 1:1hexane/ethoxyethane mixture (EXTREMELY FLAMMABLE) and the productsshould be viewed under U-V radiation (do not look directly at the lamp).

Disposal These procedures are carried out with minimum quantities and the materials canbe flushed down the foul-water drain.

33 See CLEAPSSLaboratory Handbook, section 7.9.1 for information on Occupational Exposure Limits. The value for

phenol was altered in EH/2002, Occupational Exposure Limits 2002, HSE Books, ISBN 0717620832.


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19

Notes H3

O

O

OH

Br

BrCH3

O

O

OH

CH3

O

CH3

O

O

O

Methyl 4-(ethanoyl)benzenecarboxylate Methyl 2,6-dibromo-4-hydroxybenzenecarboxylate

I II

Solubility and pHMethyl 4-hydroxybenzenecarboxylate is soluble in hot water. It cools to form long,needle-shaped crystals. The solution is acidic, giving pH values between 4 and 5.

As the ester group is electron withdrawing, the substance is a slightly stronger acidthan phenol.

Formation of saltsMethyl 4-hydroxybenzenecarboxylate dissolves in alkali and is precipitated onaddition of acid.

Addition of iron(III) ionsThe solution turns violet as a complex is formed.

EthanoylationAn emulsion forms and the solution becomes warm. The ester then separates fromthe aqueous solution as the bottom layer. It solidifies on cooling. The melting point

of the ester (structure I) is 85 C.


A white precipitate is formed. Structure IIis a possible structural formula for theproduct.

NitrationA yellow solid is obtained. The chromatogram shows the nitro product (Rf= 0.62)and the original product (Rf= 0.55). Other coloured products may be seen on thechromatogram.


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6.3 Nitration of benzene - a safer alternative

Comment The use of benzene has been prohibited34in places of work as it is a class 1 carc-inogen. Methyl benzenecarboxylate (benzoate) (I), a suitable alternative, is HARM-FULif swallowed but there is no evidence of carcinogenic, mutagenic nor terato-genic effects. There is little hazard information about the product, methyl 3-nitro-

benzenecarboxylate (3-nitrobenzoate) (II), but no adverse effects have been

reported. The important risks remain with the use of concentrated acids.

CH3

O O

O

CH3

O O

N+

O-

conc H2SO4+HNO3 H2O+

I II

Procedure Wear goggles or a face shield to guard against chemical splashes. Do not sit

down when handling concentrated acids. In a clean, 100 ml DRY conical flask,add 10 ml of concentrated sulfuric acid (CORROSIVE) and cool it to below 10 Cin an ice bath.

Place about 4 ml of methyl benzenecarboxylate in a pre-weighed 10 ml meas-uring cylinder and obtain its mass. Add the liquid to the cold acid. Calculate themass of methyl benzenecarboxylate added to the acid.

Stir the mixture well in the conical flask and continue cooling it.

In another, DRY conical flask or boiling tube, prepare a mixture of 3 ml each ofconcentrated sulfuric and nitric acids (both CORROSIVE) and cool this mixture inan ice bath.

Using a dry dropping pipette, add the nitric/sulfuric acid mixture to the acidicmethyl benzenecarboxylate solution a drop at a time, swirling the flask,

controlling the addition so that the temperature stays in the range of 5 to 15C. After the addition, allow the flask to stand at room temperature for 10 minutes.

Pour the liquid onto about 40 g of crushed ice in a beaker and stir until theproduct solidifies.

Filter off the solid; suction filtration with a Bchner flask is quicker.

Wash the solid with three 5 ml portions of water.

Change the collection flask for a small, clean, dry flask and wash the productwith two portions of 5 ml ice-cold ethanol

35(HIGHLY FLAMMABLE). Keep the

filtrate for the extension.

To purify the main product by recrystallisation, transfer the solid to a 100 mlconical flask, add 20 ml of ethanol and place the flask in a beaker of almostboiling water from an electric kettle.

When the solid dissolves, remove the flask, filter the liquid into another conicalflask (suction filtration) and allow the solution to cool (in an ice bath if time isshort).

Filter off the solid, allow it to dry and find the mass of material formed andhence the percentage yield of the reaction.

Find the melting point of the product.

Disposal The product may be saved and recycled; see procedures 6.4 and 6.5.

Extension Carry out thin-layer chromatography on the recrystallised product [use ethanol(HIGHLY FLAMMABLE) to dissolve the solid] and the ethanolic filtrate, which will needreducing in volume. Use ethoxyethane/hexane mixture (1:9 by volume) (HIGHLYFLAMMABLEand HARMFUL) as the eluting solvent. After the chromatogram is devel-oped, the spots can be located by using either ultra-violet radiation (do not lookdirectly at the lamp) or an atmosphere of iodine vapour (HARMFUL).

34 COSHH Approved Code of Practice and Guidance, 2002, HSE Books, ISBN 0717625346.35 Ethanol should be cooled in a freezer in a plastic, screw-capped bottle.


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6.4 A safer use of nitrated products

Hydrolysis of methyl 3-nitrobenzenecarboxylate to 3-nitrobenzenecarboxylic acid

Comment Disposal of organic waste is a problem, so recycling the product from the previouspreparation saves money and helps protect the environment. This activity involvesthe saponification of the ester to form the sodium salt of the acid, followed byacidification to produce 3-nitrobenzenecarboxylic acid.

+ OHCH3

Na+

O-

O

N+

O-

O

Na+

OH-

O O

CH3

N+

O-

O

+ +

O-

O

N+

O-

O

Na+

+ H+

OHO

N+

O-

O

+ Na+

Sodium 3-nitrobenzenecarboxylate

3-nitrobenzenecarboxylic acid

The hazards of the product, 3-nitrobenzenecarboxylic acid, are not well estab-lished. It is suggested that it is treated as both a HARMFUL and IRRITANT substance.

Procedure Place 2 g of methyl 3-nitrobenzenecarboxylate in a 50 ml pear-shaped flaskand add 15 ml of 2 M sodium hydroxide solution (CORROSIVE) and an anti-bumping granule.

Attach a reflux condenser and warm the flask gently until the two layers mixand then continue for another 10 minutes.

Pour the solution into a beaker, cool it in an ice bath and add 2 M hydrochloricacid (IRRITANT) until the solution is acidic.

Filter off the product.

A small amount of crude 3-nitrobenzenecarboxylic acid can be recrystallisedfrom 3 ml of hot 0.5 M hydrochloric acid and its melting point can be found(138-141 C).

Disposal Keep the solid to illustrate reduction to a primary amine and formation of an azodye (see procedure 6.5). Otherwise, store it with other organic waste for collectionby an authorised contractor.


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6.5 Reduction of nitrobenzene - a safer alternative

Comment The reduction of a nitro-group to the amine group is not an easy procedure. Theusual method involves reducing nitrobenzene to phenylamine, a procedure requir-ing a fume cupboard. The following method uses 3-nitrobenzenecarboxylic acidprepared by the saponification of methyl 3-nitrobenzenecarboxylate (see 6.4). Thisprocedure can be carried out in the open laboratory.

O

OHO

N+

O- NH2

OHO

Sn/Conc HCl

3-Nitrobenzenecarboxylic acid 3-Aminobenzenecarboxylic acid

Unfortunately, it is not possible to isolate the pure amino acid. The solid obtainedis the chloride salt of 3-aminobenzenecarboxylic acid. The presence of the sub-stance can be illustrated by the formation of an azo dye and a copper complex.

Procedure Use a fume cupboard. Wear goggles. Place about 2 g of crude 3-nitroben-zenecarboxylic acid in a 50 ml pear-shaped flask. Add 5 g of tin and 10 ml ofconcentrated hydrochloric acid (CORROSIVE).

Set up Quickfit apparatus for reflux. The pear-shaped flask should be surr-ounded by cold water in a beaker. Add an anti-bumping granule to the water.

Heat the water to boiling with a gentle Bunsen burner flame and note thechanges inside the pear-shaped flask. Once the solid has dissolved, continueboiling for 10 minutes longer. After disconnecting the apparatus, pour thecontents into a 250 ml beaker and cool in an ice bath. In the fume cupboard,

add concentrated ammonia (CORROSIVE) until the solution is just alkaline.

Take care when handling hot glass in this step. Heat the beaker with the whitesuspension to boiling to coagulate the precipitate [tin(II) hydroxide] and filterthe suspension. Collect the solution in a 250 ml conical flask. Wash the precip-itate with a little water.

Boil off the water in the conical flask until about 5 ml of solution is left.

Cool the flask and the filtrate in an ice bath. When cold, swirl the flask and filteroff the solid product which is the chloride salt.

Azo dye formation On dissolving the chloride salt in dilute hydrochloric acidand adding cold sodium nitrite solution, a diazonium salt is formed whichreacts with an alkaline solution of naphthalen-2-ol to give a red dye.

Copper(II) complex formation If the chloride salt is added to 2 ml of 0.1 M

copper(II) sulfate solution, a green complex is formed.

Disposal The materials can be added to water and flushed down the foul-water drain.


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6.6 A safer solvent for the extraction of caffeine from tea

Comment Chlorinated hydrocarbons have been used as the extracting agent for this activity.In view of the hazards both to health and the environment of these substances,use can be made of propan-1-ol which, when saturated with sodium chloride, isimmiscible with water

36.

About 50 mg of caffeine can be extracted from two bags of tea. Slow evaporation

of a propanone solution of caffeine produces attractive, needle-shaped crystals.

Procedure Wear eye protection. Boil pure water in a kettle. Pour about 50 ml of boiling water onto two tea bags in a 250 ml beaker. Let

this stand for 5 minutes.

Stir and squeeze the tea bags with a glass rod to extract the tea solution.

(Take care: hot glassware.) Decant the solution into a 250 ml conical flaskleaving the bags behind. Add about 20 ml of boiling water to the bags, stir andsqueeze with the rod and again decant the solution into the conical flask.

Add approximately 16 g of sodium chloride and heat the solution to boiling ona tripod and gauze.

While hot, add approximately 1 g of calcium hydroxide to the tea solution.

Prepare an ice bath and cool the flask and its contents. Try not to disturb the

precipitate, which is very fine.

Place about 0.5 g of Superwool37

(or other mineral fibre) in the neck of afilter funnel and slowly add the cold, cloudy tea liquid. A clear solution shouldcome through. This can be further filtered in the normal way if required.

Pour the solution into a separating funnel and add 15 ml of propan-1-ol(HIGHLY FLAMMABLE) and replace the stopper. (Note which layer is which!)Agitate the funnel to dissolve the caffeine into the propan-1-ol layer.

Remove the stopper and drain off the lower, aqueous layer into a conical flask.Now drain the propan-1-ol layer into another flask.

Return the aqueous layer to the separating funnel and add a further 10 ml ofpropan-1-ol. Repeat the separation and draining processes, combining thepropan-1-ol which contains caffeine, tannins and some sodium chloride into

one flask. (Wear goggles.) Return the combined propan-1-ol solution to the separating

funnel and this time add 25 ml of 2.5 M sodium hydroxide solution ( CORRO-SIVE). This will remove the sodium chloride and remaining tannins from thepropan-1-ol. Repeat the agitation action, drain off the unwanted aqueous layerand drain the propan-1-ol layer into another conical flask.

Add anhydrous sodium sulfate and swirl. All water will be removed when theliquid is clear and the sodium sulfate is free flowing.

Decant the solution into a clean, pear-shaped flask and set up for distillation.Distil until there is about 5 ml of propan-1-ol left. Remove the remainingalcohol and caffeine with a Pasteur pipette into a weighed, glass Petri dish andallow the alcohol to evaporate overnight. It could be warmed gently on a hotplate.

Scrape the solid into a test tube. Add about 2 ml of propanone (HIGHLY FLAM-MABLE), swirl and decant the liquid into another Petri dish. Allow the propanoneto evaporate in a working fume cupboard.

Disposal Tea bags can be placed in the waste. The waste sodium hydroxide solution shouldbe washed away with plenty of water down the foul-water drain.

Controls andhints

The version from which this method is adapted stated that the washing withsodium hydroxide solution removed sodium chloride as well as the tannins fromthe propan-1-ol solution but we were not totally convinced by this; hence thewashing with propanone.

36 The method is adapted from the following article: Hampp, Andreas, The Extraction of Caffeine from Tea: A Modific-

ation of the Procedure of Murray and Hansen,J. Chem. Educ, 1996,731172.37 See section 4.1 of this guide.


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6.7 A safer reagent to oxidise alcohols to carboxylic acids

Myth: As chromium(VI) compounds are carcinogenic, this procedure is now not allowed.

Comment Preparative oxidation reactions are usually carried out with sodium dichromate(VI)dissolved in sulfuric acid of various concentrations. Chromium(VI) compounds areall deemed TOXICas they are carcinogens. The risk to health is extremely low asthe route is via inhalation and there is not much chance of breathing in aerosols of

chromium(VI) compounds in the school laboratory. Poor mixing of concentratedsulfuric acid with water causes a more common accident. This standard procedureis not banned but a less hazardous reagent can be used. The reaction can berepresented by the following equation.

4MnO-4(aq)+ 3C6H5CH2OH(l)3C6H5COOH(aq)+ 4MnO2(s)+ 4OH

-(aq) + H2O(l)

The product, benzenecarboxylic acid, is neutralised by sodium carbonate to formthe benzenecarboxylate ion, which is soluble in water. After filtering off the mang-anese(IV) oxide and removing the unreacted manganate(VII) ion, the addition ofhydrochloric acid produces benzenecarboxylic acid. Yields of 55% of crude benz-enecarboxylic acid were obtained but rose to 73% on reducing the volume offiltrate.

Procedure Wear eye protection. On a weighing boat, place 1.5 g of potassium mangan-

ate(VII) and 0.5 g of sodium carbonate. Carefully pour this into a 50 ml pear-shaped flask and add 20 ml of water.

Swirl the mixture so that the solids dissolve.

Stand the pear-shaped flask in a beaker and tare it on a balance. Add about0.5 ml of phenylmethanol and note the reading on the balance. This is themass of starting material. Add an anti-bumping granule to the flask.

Set up the pear-shaped flask for reflux. Lower the flask into a 400 ml beakeron a tripod and gauze.

Add water to the beaker to about the 300 ml level. Add a couple of anti-bumping granules to the beaker. (Your teacher should examine the equipmentbefore you proceed.)

Light the Bunsen burner, place it below the gauze and let the water boil for at

least 15 minutes. Keep the water level topped up. After 15 minutes, when the Bunsen burner is turned off, turn off the cooling

water to the condenser and raise the pear-shaped flask. Using a thick cloth,remove the very hot beaker containing water.

Using the cloth, remove the clamp from the boss and pour the reaction mixturethrough filter paper to remove the manganese(IV) oxide. Note the time whenthe reaction has stopped. Collect the filtrate in a conical flask. The solution ispurple because of unreacted potassium manganate(VII).

Add a magnetic stirrer bar and place the flask on a magnetic stirrer. As it stirs,add a little sodium metabisulphite. Add this steadily until the solution clears.

Wear safety goggles. IN A FUME CUPBOARD, place a pH meter in the mix-ture, add drops of concentrated hydrochloric acid until the pH is less than 2.

(Take care: sulfur dioxide, aTOXIC

gas, is produced.) Place the solution IN THE FUME CUPBOARD, and heat to boiling. This

removes sulfur dioxide and dissolves precipitated benzenecarboxylic acid.Remove the flask from the fume cupboard and allow it to cool to room temp-erature; ice can be used to accelerate cooling.

Filter off the benzenecarboxylic acid crystals.

More benzenecarboxylic acid can be obtained by boiling the filtrate until halfthe original volume remains. Again, cool and filter off the crystals.

Place the benzenecarboxylic acid in 5 ml of water in a test tube, heat toboiling, filter and collect the filtrate in a 100 ml beaker. On cooling, the crystalsof benzenecarboxylic acid can be filtered off, dried in an oven and the meltingpoint can be measured.

Disposal The pear-shaped flask must be rinsed in water immediately. Any stains can beremoved with sodium metabisulfite solution in the fume cupboard.

Controls andhints

A stirring rod can be used in place of a magnetic stirrer but it is not as efficient.There is scope for more research with this reagent at school level.


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7. Hydrogen reactions - safer procedures7.1 Hydrogen burning and exploding in air

Myth: Hydrogen is explosive and cannot be used in schools.

Comment When hydrogen has been prepared or used on a large scale, demonstrators havesometimes failed to flush out all the air from a gas generator, so that impure hydro-

gen is collected and ignited, resulting in several accidents. Many chemicals, eg,methane and ethanol, explode when ignited in the presence of air but the range ofratio of air to chemical is often quite narrow. However, mixtures of hydrogen andoxygen explode over a wide range (4 to 77%). The ignition temperature of hydro-gen is reduced by the presence of metals and their compounds, [eg, copper(II)oxide].

With the procedure and apparatus below, hydrogen can be safely prepared andburnt with the inevitable final explosion.

ProcedureHoffman clipRubber tubing

Hydrogen in

Clamp

Glass tube5 mm bore

1 litre plasticsoft drinksbottle cut offat the bottom

Filling the chamber Burning the hydrogen

Fill the explosion chamber (a 1 litre, plastic, soft drinks bottle) with water byopening the Hoffman clip and sucking up the water until it is just below theglass tube in the bung. Tighten the clip sufficiently that the level does not drop.

Unless a hydrogen cylinder is available, generate hydrogen with at least

100 ml of 3 M hydrochloric acid and granulated zinc, adding about 5 ml of 1 Mcopper(II) sulfate solution to increase the speed of reaction. The volume ofdead space in the generator must be less than 1 litre to ensure that all the airin the generator has been expelled.

Pass hydrogen into the explosion chamber until it is full of gas. If the hydrogenis prepared chemically, a considerable volume of air will also come over. Asthere must be noair in the chamber, keep the delivery tube under water and,after opening the Hoffman clip, lift out and invert the chamber so the air/hydrogen mixture is lost. Refill the chamber with water, reset as above andcontinue to collect pure hydrogen in the chamber. (If a hydrogen cylinder isused, this step is not required.)

When bubbles escape from the chamber, it will be full of pure hydrogen.Remove the generator away from the equipment.

Light a Bunsen burner about 1 metre from the apparatus.

Slowly lift the clamp up the retort stand raising the bottle out of the trough, re-tighten the boss on the clamp stand and remove the trough. Hydrogen isretained because it is less dense than air.

Light a spill and hold this in one hand while the rubber tubing is removed fromthe glass tube with the fingers of the other hand. Ignite the hydrogen at the topof the tube with the spill. Move away to a safe distance.

Controls andhints

If a chemical generator is used, see Hazcard48.

Make sure the glass tube does not protrude through the bung. If it does, it collectswater which blocks the passage of gas when the clip and rubber tube are rem-oved. If the glass tube is too narrow, it may not support the flame as it backfires

into the chamber.Practice the demonstration before performing it. It does require some co-ordin-ation! Do not become over-ambitious, using larger explosion chambers. Theexplosion is quite loud enough!


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7.2 Hydrogen/oxygen explosion

Myth: The very loud bang can cause damage to the room and distress to pupils.

Comment If carried out on a large scale in glass apparatus, exploding large volumes ofhydrogen and oxygen in their stoichiometric ratios can shatter windows and glassapparatus and the noise can cause deafness.

The function of risk assessment is to adjust the procedure to avoid these events.Soap bubbles make a useful alternative means of holding the gases. Using themethod below ensures that there is no flying glass and the explosion is loudenough for a laboratory.

Two holes are drilled with a very fine drill bit through the rubber bung that fits thewide-necked, 100 ml bottle. A wider hole is bored though the bung which is thenfitted with glass tubing

38. Two copper wires are fed through the bung and the

nickel-foil electrodes are soldered onto the wire.

The glass tubing is bent into shape using a non-luminous Bunsen burner flame tomelt the glass and being careful not to heat the bung.

Procedure

Nickel electrode solderedonto copper wire

Detergent solutionina crucible

on wood blocks or

a laboratory jack

0.2M sodium sulfate solution

Pour the 0.2 M sodium sulfate solution into the 100 ml bottle so it reaches thevery top.

Have a beaker handy to collect the overspill as the bung containing theelectrodes is inserted into the neck of the bottle. The overflow rises up thedelivery tube and empties into the beaker.

Connect the copper wires to the low-voltage supply (6-8 volts is a suitablesetting) and pass current until no more solution is pushed out of the bottle intothe beaker. All this can be prepared earlier and the current turned off.

Place the crucible filled with 50% liquid detergent solution on the laboratoryjack or wooden blocks so that gases from the bottle will bubble through. Switchon the current to collect bubbles of gas.

Switch off the current, lift the bottle so that the tube is clear of the crucible andmove the crucible closer to an ignited Bunsen burner.

Light a splint with the Bunsen burner flame and apply to the bubbles on the topof the crucible.

Controls andhints

Wear eye protection.

The Bunsen burner required for this demonstration should be at least 1 m from thebottle.

The first gas sample may not explode with a loud crack as air might be present, sorepeat the procedure.

The glass tubing should be flush with the bottom of the bung.

The electrolyte is 0.2 M sodium sulfate solution rather than sulfuric acid. Moreadvanced students will appreciate that water molecules are being oxidised andreduced at the electrodes to form oxygen and hydrogen.

This method uses nickel electrodes in place of platinum which are expensive anddifficult to solder. However, the nickel anode does oxidise during the process if theelectrolysis cell is left on for long periods. The solution becomes green andnickel(II) hydroxide precipitates out.

Disposal All solutions can be poured down the foul-water drain.

38 Use 6 mm medium-wall thickness borosilicate glass tubing.


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7.3 Hydrogen as a reducing agent

Myth: A teacher was prosecuted for carrying out this reaction so it must be banned.

Comment Using large-scale apparatus to reduce copper(II) oxide (HARMFUL) with hydrogen(EXTREMELY FLAMMABLE), an experienced teacher ignited the hydrogen before allair had been flushed through. The ensuing explosion sprayed glass and acid, con-tained in the hydrogen generator and drying bottle, over the students and himself,

none of whom were wearing eye protection. No safety screens were used and thestudents were too close to the demonstration. The teacher was successfully pros-ecuted by the HSE because of his gross negligence in not using basic safety proc-edures. However, the procedure he used is not banned but it is seldom performedthese days.

Since this accident, safer alternatives to hydrogen, including methane and/or pass-ing methane through ethanol, have been used as the reducing agent. However,this complication can detract from the chemistry to be taught and confuse thepupils. In this microscale version, the dead space has been reduced to a mini-mum and a borosilicate glass tube is used as the combustion tube. Mineral fibreplugs prevent spray reaching the copper(II) oxide and the copper(II) oxide fallingout of the tube. The black copper(II) oxide becomes red as copper is formed.Water, the other product of the reaction, condenses further up the tube.

Procedure

Hydrogengenerator

Mineral fibre Copper(II) oxide

Apply heat with a mini-Bunsenburner or spirit burner

Mineral fibre

To make the hydrogen generator, bore a rubber bung that fits a specimenbottle (eg, 40 x 25 (d) mm) and insert a 6 mm medium-wall borosilicate glass

tube, so that it is flush with the bottom of the bung. Bend the glass tube to 90using a Bunsen burner flame and allow it to cool.

To make the combustion tube, use a micro-spatula to place copper(II) oxideinside a straight 6 mm medium-wall borosilicate glass tube and add a littlemineral fibre

39to each end, poking it down the tube.

Use rubber tubing to connect the combustion tube to the bent glass tube.

Place 8 ml of 1 M sulfuric acid (IRRITANT), 1 ml of 0.1 M copper(II) sulfate andzinc granules in the specimen tube. Place the specimen tube in a clamp. Insertthe bung, with the combustion tube attached.

Wait for 30 s so that hydrogen fills the combustion tube and expels the air.Light the mini-Bunsen burner (adjusting the flame to just non-luminous) orspirit burner with ethanol as the fuel.

Heat the copper(II) oxide in the combustion tube. As the colour of the solidchanges, move the flame further along the combustion tube until all thecopper(II) oxide has reacted.

Continue passing hydrogen over the copper until the combustion tube hascooled and tip out the copper metal onto white paper.

Controls andhints

Wear eye protection. Copper(II) sulfate is added to catalyse the zinc/acid reaction.

The microscale approach will require extra dexterity by students, teachers andtechnicians! Using a flex-cam, this procedure could be demonstrated and shownon a television screen.

Disposal Pour the solution in the specimen tube down the sink with lots of water. Keep thezinc granules for future use in hydrogen generators.

39 See section 4.1.


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8. Diffusion of gases - a safer alternativeMyth: Bromine is banned. It is too dangerous to handle.

8.1 Diffusion into air

Introduction Bromine(VERY TOXIC and CORROSIVE) (see Hazcard15) has caused a number ofaccidents to both teachers and technicians, with serious burns to the skin or breath-

ing difficulties. Bromine is not banned but if it is to be used, a knowledgeable coll-eague should be in the vicinity to provide assistance in case of an accident. Anyperson handling bromine for the first time, or who does not handle it regularly,should receive training from an experienced colleague.

Nitrogen dioxide (VERY TOXICand CORROSIVE) is a heavy, brown gas. Despite thesimilar hazard warnings, there is a lower risk of serious injury with nitrogen dioxidethan bromine and the gas provides a safer alternative. A known volume of concen-trated nitric acid (CORROSIVE) is added to an excess of copper turnings to produceenough nitrogen dioxide so that a gas jar of known volume is nearly filled. Anothergas jar of air is placed over a gas jar of nitrogen dioxide. Over the next 20 minutes,the brown gas diffuses into the upper jar.

Procedure

Copper turnings

Fig i Fig ii Fig iii

Clamp

Using water and a 250 ml measuring cylinder, establish the volume of the gas

jar. Do notuse this wet gas jar for the following demonstration. Using a retort stand, boss and clamp, adjust the fitting of a dry inverted gas jar

over another dry gas jar of the same size and set it to one side.

Place at least 1 g, but no more than 2 g, of copper turnings in the gas jar (fig i).Knowing that 8 ml of concentrated nitric acid produces 1000 cm

3of nitrogen

dioxide at room temperature and pressure, estimate the volume of acid neededto just fill the gas jar with gas. Wearing eye protection and suitable gloves,place 1 ml less than the estimated volume of nitric acid (CORROSIVEand OXID-ISING AGENT) in a 10 ml measuring cylinder. Empty the contents of the measur-ing cylinder into the gas jar with copper and watch the brown gas rise (fig ii).

Once the reaction stops, invert the second jar over the jar containing the gas.Clamp this jar into position with care (fig iii). Diffusion takes place in 20 minutes.

Controls andhints

If the above procedure is followed, a fume cupboard is not required because nitro-gen dioxide, being heavier than air, remains in the gas jar. Gloves are not requiredwhen an automatic pipettor is used.

Disposal If possible, move the gas jars to a fume cupboard. Add water to each gas jar andpour the contents down a foul-water drain, adding more water. Unreacted copperturnings can be dried and reused. If there is no fume cupboard in the room, care-fully insert gas-jar lids to cover both jars. Seal with sellotape and remove to a fumecupboard.

Extension The demonstration can be performed along with a similar set up using bromine toshow that gases diffuse at different rates. To fill a 1 litre gas jar, use no more than2 ml of liquid bromine. Adjust the volume of bromine liquid to the capacity of the gasjar that is available. It takes time for bromine to vaporise. Use a fume cupboard,

wear goggles or a face shield and nitrile or latex chemical-resistant gloves

40

. Abucket of 1 M sodium thiosulfate solution should be available in case brominesplashes onto the skin or is spilled.

40 See PS50, Gloves as Personal Protective Equipment (PPE), CLEAPSS School Science Service.


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8.2 Diffusion into a vacuum

Introduction In school laboratories, bromine gas diffusion kits41are often not available, unusedor broken. Vacuum pumps are very heavy, difficult to use by untrained staff andvery noisy. For this demonstration, a hand-operated pump is all that is required toreduce the pressure in one of the containers. Schools with redundant Quickfitapparatus can use round-bottom flasks in place of thick-walled borosilicate glassbottles. Nitrogen dioxide (VERYTOXICand CORROSIVE) is a safer alternative tobromine.

Procedure Prepare tubing, stopcocks and suitable size bungs to fit thick-walled bottles, asshown in the diagram below.

The bottles, with their screw tops removed, should be clamped about the neckto hold them steady. Do not place the stopper into Bottle A.

B A

Stopcock X Stopcock YTo a hand-operated

vacuum pump

Nitrogen dioxide2 x 500 ml

thick-walledbottles

(Wear eye protection and chemically-resistant gloves42

.) In bottle A, place 1 gof copper turnings and add 3.5 ml of concentrated nitric acid (CORROSIVE andOXIDISING AGENT).

When the reaction is over, place the bung into Bottle A and arrange the equip-ment as shown in the diagram. Ensure that stopcock Y is closed and stopcockX is open.

Connect the hand pump and withdraw air from Bottle B. Close stopcock X anddisconnect the pump.

Open stopcock Y and the brown flask diffuses quickly into Bottle B.

Controls andhints

Students should wear eye protection. There is a very remote risk of an implosionoccurring so a safety screen should be placed between the demonstration and thepupils. The demonstrator should wear a face shield. If the instructions above arefollowed, there is no reason to use a fume cupboard to prepare the gas as it isheavier than air and stays in the bottle.

Hand-operated vacuum pumps43

, such as that illustrated below, can be obtainedfrom most educational science equipment suppliers for 60-65.

Disposal The screw tops for these bottles can be replaced and the bottles removed to afume cupboard where water is added to each, into which the gas dissolves. Theliquid can be flushed down the foul-water drain.

Extension This procedure may be repeated with bromine but great care is required (seeHazcard15).

41 Bromine diffusion kit, Philip Harris, cat. no. COA

safer chmeicals and safer reactions

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