AS202

FACULTY OF APPLIED SCIENCE

EXPERIMENT 1 & 2

QUALITATIVE ANALYSIS FOR IDENTIFICATION OF METAL CATIONS AND TRANSITION METAL

CATIONS.

LECTURER’S NAME : DR.LIM YING CHING

STUDENT NAME : NOOR SYAHEERAH SYAZWANI BT SURYA(2014159689)

PATNER’S NAME : ATIKAH LIYANA BT AHAMAD (20145878659

OBJECTIVE

To perform qualitative analysis to identify the metal cations.

INTRODUCTION

Qualitative analysis involves the identification of the substances in a mixture. When chemical methods are used in the identification of mixtures of metal cation, these ions are usually separated before identification can occur. After they have been separated, identification of each cation depends on the observation of a characteristic chemical reaction. Solubility equilibrium and complexion equilibrium play crucial roles in the separations and subsequent identifications.

Qualitative analysis is the process by which component of mixtures are separated and identified. Unlike qualitative analysis, when the amount of a particular materials is measured, a qualitative analysis scheme simply confirms the presence or absence of certain materials. A common analysis is the identification of aqueous ions. In such analysis, there are two distint phase. First , a scheme must be developed to separate the ions from each other. Then a different test is performed to uniquely confirm the identity of each separated ions.

In addition to analyzing the unknown for it’s component ions, the development of qualitative analysis scheme highlight some of important chemical behavior of these metal salts in aqueous solution. The principal of chemical equilibrium are emphasized, as illutstrated by precipitation reaction, acid-base reaction, complex-ion formation and oxidation-reduction reactions.

Ions are often separated in solution by their different solubilities. A metal ion in a mixture may precipitate (form a solid) in the presence of a specific anion, while the remaining metal ions remained dissolve (in aqueous form). The two ions may be separated by collecting the solid at the bottom of the test tube in a centrifudge, a device that create a centrifugal force by rotation. After the precipitation is compacted, the supernatant is decanted into a separated test tube. The soluble ions is in the liquid supernatant while the soluble ion is in the solid precipitate. Thus they are physically separated from each other when the liquid is poured off, leaving the solid behind. The solid precipitate is typically washed with water to help remove any trace of the soluble ions that remain. This prevent “false positive” test later on.

Once the ions in the mixture have been separated, their identity can be further verified by a common confirmatory test. In a confirmatory test each ions has unique response to an added chemical, such as a solution colour change or the formation of a precipitate. This unique response confirm the presence of particular ion.

PROCEDURE

Procedure A.

1. The 2 drops of 6M HCl is added to 1mL of the mixture of the metal cations.2. The suspension is centrifuged for 1 minute if the precipitate of chloride is formed.3. 1 additional drop of 6M of HCl is added to the clear supernatant to check for complete

precipitation.4. The solution is cetrifudged again for 1 minute if precipitate is formed and the supernatant

is saved for procedure B.

Procedure B.

1. The 6 drops of NH3 is added dropwise to the solution until it is basic.2. The sample is put in the boiling water bath for 2-3 minutes to remove excess NH3.3. The volume of the solution is adjusted to 2 mL with distilled water and 10 drops of 6M

HCl is added.4. The 12 drops of thioacetamide is added to the solution and stirred.5. The sample is heated in a boiling water bath for 10 minutes to allow for the hydrolysis of

thioacetamide producing H2S.6. The solution is centrifudged for 2 minutes if precipitate of sulfide formed and decanted

the supernatant to a clean test tube.7. The supernatant is saved for the procedure C.

Procedure C.

1. The 3 additional drops of 1M thioacetamide is added to the supernatant and reheated for 5 more minutes to test for complete precipitation.

2. The supernatant is cetrifudged and decanted into a clean test tube if the precipitate is formed, the precipitated is discarded.

3. The volume of the solution is adjusted to 3mL with distilled water and 10 drops of 6M HCl and 10 drops of 6M NH3 are added.

4. The 6M of NH3 is added dropwise until the solution is basic.5. The 5 additional drops of 6M NH3 and 12 drops of 1M thioacetamide are added then the

mixture is stirred thoroughly.6. The sample is heated in a boiling water bath for 10 minutes.7. The supernatant is centrifudged and decanted if precipitate of sulfide is formed8. The supernatant is saved for procedure D.

Procedure D.

1. The 6M NH3 is added dropwise to the sample until it is basic.2. The 10-15 drops of NH3 is added to buffer the solution.3. The 20-30 drops of (NH4)2CO3 is added to the sample and stirred.4. The test tube containing the precipitate of carbonate is tranfered to a warm water bath of

70-80 ˚C for 2-3 minutes.5. After the sample has been warmed, the mixture is centrifudged and the supernatant is

decanted containing either Na+ or K+.6. The precipitate is saved for procedure D(i).

Procedure D(i).

1. The precipitate is washed with 2 mL of distilled water, centrifudged and decanted the wash water.

2. The 6M acetic acid is added dropwise and stirred to dissolve the precipitate.3. A few more drops of 6M acetic acid is added until it is acidic.4. The 6 drops of 1M K2CrO4 is added to the solution and stirred.5. The solution is centrifudged if the precipitate of chromate is formed.6. The supernatant is saved for the procedure D(ii).

Procedure D(ii).

1. The 10 drops of 1M K2C2O4 to the solution and stirred about 10 minutes.2. If the precipitate of oxalate is formed the solution is centrifudged.

DATA/RESULTS

DISCUSSION

Before individual components of a mixture of cations can be successfully identified, they have to be separated from the mixture. The individual cations, once separated, may need to be identified in the ensuing confirmatory reactions. By using a flow chart and knowing the actual results of the reactions characteristic for a given cation (or a group of cations) makes it easier to identify this ion in the unknown sample, when the same results are obtained in a test. Therefore, it is useful to perform all of the characteristic reactions first on the mixture of ions whose identity is known, then proceed to the unknown sample.

In experiment 1, a known solution containing Ag+, Cu2+, Zn2+, Ca2+ , Ba2+ and Mg2+ ions. These cations belong to Analytical Groups cations of I, II, III, and IV but not all the mention ions will be observed due to any error or something else. Hydrochloric acid will has been used to precipitate Ag+ as white AgCl in mixture and the unknown I however, is not considered sufficient evidence for the presence of Ag+ in either a known or an unknown mixture. As a confirmatory test for this cation, this precipitate should dissolve in aqueous ammonia with the formation of a complexion and should reappear when the solution is treated with an acid.

Ag+(aq) + Cl-(aq) AgCl(s) white precipitate

Hydrogen sulfide is required for the next two separations. A saturated solution (0.10 M) of this substance will be generated by heating a solution of thioacetamide, CH3CSNH2. Hydrolysis (reaction with water) has the following result:

CH3CSNH2 + H2O CH3CONH2 + H2S

This reagent will be used to precipitate Cu2+ as white CuS from a H3O+ solution and to precipitate Zn2+ as white ZnS from a weakly basic solution but it can be detect in while doing this step only CuS precipitated in this solution.

3CuS + 8H+ 2NO3- 3Cu2+ + 2NO3- + 3S + 4H2O

Then Ba2+ and Ca2+ will be precipitated as white CaCO3 and BaCO3 by the addition of (NH4)2CO3. This precipitate will dissolve in an acid with the evolution of carbon dioxide. If a precipitate does not form, Ca2+ cannot be present in the unknown and mixture sonBa2+ was present as the white precipitate .

Ba2+ + CO3 2- BaCO3(S)

Next K2CrO4 is added to the supernatant to detect the present of Ca2+ or Mg2+ ions. Then yellowish solution produced as the CrO4

2- is yellow colour. Mg2+ react with it and the equation like below :

Mg2+ + CrO42 MgCrO4

(s)

The confirmatory test for this cation involves the colourless solution of MgC2O4, upon the addition of K2C2O4. The equation for this reaction is :

Mg2+ + C2O42- MgC2O4(aq)

The experiment 2, it was involving the qualitatives analysis which contain of metal cations (Ag+, Cu2+, and Zn2+). Several specific reactions will be carried out on solutions that will either separate the ions from each other to confirm the presence of particular ion. The reactions that confirm the presence of a particular ion involve the creation of visible product such as precipitates or coloured species .The reaction that involves the precipitation also be applied in the reaction. Qualitative analysis rection is simple reaction because it identify only one ion for each time.

For the first experiment , HCl was used to precipitate Ag+ as while as precipitate as AgCl. This can be shown by balance chemical equation :

Ag+(aq) + Cl-

(aq) AgCl(s)

From the observation supernatant was formed. After that add H2S and the observation is white precipitate with blue solution formed. From the observation that supernatant and white precipitate was formed. So the supernatant is separated and the precipitate into different test tube. Now the confimatory presence of Cu+ is discussed. This reaction can be showned by the balance equation below :

3CuS + 8H+ 2NO3- 3Cu2+ + 2NO3- + 3S + 4H2O

After the solution is centrifuged , the precipitate was discarded. 6 M NH3,6M HNO3 and K4Fe(CN)6 was added. Brown precipitate was formed in the unknown solution and yellowish solution and brick-red precipitated in the mixture. The confirmatory test for each cation involves the addition of potassium ferrocyanide, K4Fe(CN)6, to these solutions. A brown precipitate confirms the presence of Cu 2+ by this eequation :

2Cu2+ + Fe(CN)64- Cu[Fe(CN)6](S)

There from the equation and observation , the presence of Cu2+ was confirmed. whereas a white precipitate confirms the presence of Zn2+ is not detectable because maybe there is contamination in the solution of the apparatus and Zn2+ is not enough to be observed. The Fe(CN)6 4 is a complex ion that is considerably less toxic and dangerous than the cyanide ion, CN-.

Some error could affect the experiment are like the precipitate is washed by adding a few drops of the washing reagent (usually water), mixing thoroughly with a stirring rod, centrifuging, and removing the washings with a capillary pipette. Two or more washings are generally necessary to prevent contamination of the precipitate. Failure to wash precipitates is one of the

most common sources of error in qualitative analysis. Failure to wash precipitates is one of the most common sources of error in this experiment.

SAFETY

1. Fe(CN)64 is a complex ion that is considerably less toxic and dangerous than the cyanide ion,CN.

2. Hydrochloric acid, ammonia, nitric acid, and acetic acid can cause chemical burns in addition to ruining clothes. If any of these solutions spilled, wash the contaminated area thoroughly with tap water and report the incident to your instructor. You may require further treatment. Thioacetamide is a carcinogen. Avoid contact with skin.

3. In addition to having a foul odor, the hydrogen sulfide generated during the hydrolysis of thioacetamide is extremely toxic . Although only small amounts of H2S usually escape from the solution, work under a hood if possible. If not,use the inverted conical filter funnel and water aspirator described earlier to suck away the escaping H2S.Wash your hands thoroughly. Oxalate solutions are poisonous.

CONCLUSION

The qualitative analysis involves the identification of the substances in a mixture , so the qualitative analysis was performed. In experiment 1, the mixture and unknown containing Ag+ion, Cu2+ ion, Ba2+ ion and Mg2+ ion. From the experiment 2 , the mixture and unknown containing Cu2+ ion and Zn2+ ion.

REFERENCES.

1. Wismer, Robert K. Qualitative Analysis with Ionic Equilibrium; Macmillan Publishing Company: New York, 1991.

2. Zumdahl, S. S. Chemical Principles, 4th Ed.; Houghton-Mifflin: New York, 2002, chapter 8.

3. Robert Bruce Thompson , ALL lab ,No lecture ,Ilustrated Guide to Home Chemistry Experiments , publisher Dale Doughtery , first edition year 2008,page:331,332 and 33

4. Michael Clungston and Rosalind Flemming ,Advanced Chemistry , 2000 ,Oxford copyright ,page:379 and 380

JOTTER