common laboratory operations

8/12/2019 Common Laboratory Operations

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COMMON LABORATORYTECHNIQUES AND

PRACTICES

Cleaning GlasswareA common laboratory requirement is clean glassware. If clean, the walls will

retain an unbroken film of water, not droplets. This can usually be achieved most simply

and safely by scrubbing the wet walls with some common commercial scouring powder

and an appropriate brush, followed by rinsing with tap water and finally with distilled

water. Never use scouring powder on glassware which is inaccessible to scrubbing such

as the inside of a transfer pipet. For this type of glassware use a liquid detergent in place

of the scouring powder. Occasionally, specific chemical deposits are completely

unaffected by these general cleaning methods, but if the chemical nature of the deposit is

known, a specific reagent may be selected for cleaning; ask your instructor for advice in

this circumstance.

Drying OperationsGlassware must frequently be dry as well as clean. This is best done by putting

the glassware into a drying oven, preferably one with circulating air that will quickly

remove the water vapor. Hot glassware with parallel sides, such as beakers and bottles, is

most conveniently handled with beaker tongs as shown in Fig. 2. It is unwise to dryPyrex and Kimax equipment by direct heating with a Bunsen burner; it is usually fatal (to

the glassware) to treat soft glass equipment this way. Alternatives to the drying oven are

wiping with clean toweling (but cloth and paper lint are often a problem), or rinsing with

volatile acetone followed by drawing clean air through the container; however, acetone is

rather expensive and highly flammable.

A common requirement, especially in quantitative analysis, is a very dry sample

or product. These materials are usually dried in the drying oven at a specified

temperature for a specified length of time, followed by immediate transfer to a special

container called a desiccator. The desiccator has a lid whose ground surface is greased so

as to provide a leak-proof seal. It is removed by sliding it away from you with one hand

while holding the bottom portion stationary with the other, Fig. 3(a). At the bottom ofthe desiccator is a drying agent (often the very cheap CaCl 2) that takes up any

moisture that enters


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Figure 2

(a) (b)Figure 3

when the desiccator is opened. The desiccator serves as a place for dried samples to cool

and be stored prior to weighing them. Transfer to and from the desiccator should

normally not be by fingers but by crucible tongs or by strips of lintless paper, Fig. 3(b).

Chemicals should not be dried by direct heating with a Bunsen burner because of the

inability to control the temperatures and the likelihood of decomposing the chemicals.

HeatingThe common device for heating at the laboratory bench is the Bunsen burner,

which usually burns natural gas and uses air as the source of oxygen. The amount of gas


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admitted to the burner determines the size of the flame; this is controlled by the valve on

the desk top to which the burner is connected. Depending on the type of burner that you

(a) Figure 4 (b)

have, the amount of air is regulated by a metal ring with holes that may be rotated at the

base of the burner barrel, Fig. 4(a), or by a piece of metal that may be slipped to one side

or the other to vary the extent to which the base of the burner barrel is uncovered to

admit air, Fig. 4(b). A yellow, smoky flame is not very hot and is dirty; it is a sign of

insufficient air. A flame that roars and blows itself out easily is also undesirable; it is a

sign of too much air. A properly burning flame has an almost invisible blue color with a

bright blue cone burning inside it, Fig. 5. The tip of the bright blue cone is the hottest

portion of the flame. When heating materials in a test tube with a Bunsen burner, thetest tube should be held with a wire test tube holder, Fig. 6, the mouth of the test tube

Figure 5 Figure 6


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should be directed away from yourself and neighbor, and the heating should be done

gently. When heating liquids in a beaker or a flask with a burner, the container should be

set on a wire gauze which evenly spreads the heat from the burner, Fig. 7. This will

minimize the possibility of cracking the container or splattering the liquid. When it is

important that there be no loss whatever from splashing, it is customary to place a watchglass over the container. If necessary, the watch glass can be supported on glass stirrups

Figure 7 Figure 8

to permit the escape of vapors from the container. Any condensate on the watch glass

can be rinsed back into the beaker with distilled water at the end of the heating period.

A particularly useful and convenient method of heating is with an electric hot

plate, especially one equipped with a magnetic stirrer. The beginner must be warmed

about the tendency to adjust the temperature regulator to too high a value as a result of

the often slow initial warm-up period; the resultant overheating may result in loss by

splattering or thermal decomposition.


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When evaporating a solution to dryness or near dryness, great care must be

exercised to prevent "bumping," i.e., superheating followed by explosive bursts of

splattering. This is best done by heating gently while continuously swirling the liquid,

Fig. 8. A suitable holder, such as a folded paper strip, must be used in this operation to

protect the fingers from the hot glass. When acids or other solutions which emit noxiousfumes are evaporated in this way it is imperative to carry out the operation under a

hood with strong ventilation.

Glass WorkingGlass is a poor conductor of heat and therefore it can be held in the hands rather

close to the section being heated or melted. Nevertheless, the softening point of soft

glass is about 500C and of Pyrex about 650C, temperatures which are intolerable to the

skin, paper, and desktop finishes (unless they are of rock or Transite). A square of thinTransite board is a very satisfactory surface on which to lay hot glass while it cools; it

protects the desk surface and does not cool the glass so fast as to produce strains in it. It

is assumed that something like this will be available when you are working with hot

glass. The sizes of glass tubing and rod are given in terms of the outside diameter

expressed in mm. It is important for your very survival not to hand a neighbor or the

instructor a piece of hot glass for inspection.

The techniques that are commonly used in simple glass-working operations are

described separately below.

1. Cutting tubing and rod. Make a single scratch crosswise of the rod ortubing with the corner of a file or a glass knife. The safest way to do this is while the rod

or tubing lies flat on the desk top, Fig. 9. When this is not possible, put your thumb on

one side of the tube and the file on the opposite side; the thumb can exert a bit of pressure

while the file is being drawn across the tubing with the other fingers. Great care should

be used if the glass walls are thin. Do not use a saw-like scratching action; do not use a

dull file. Moisten the scratch with a wet finger; then grip the tubing with one hand on

each side of the scratch with the thumbs opposite the scratch, Fig. 10. Break the tubing

by simultaneously giving a bending and a pulling motion away from the body. If theglass does not break easily, do not strain and run the risk of shattering the glass and

cutting your hands; scratch the glass again at the same spot, moisten it again, and repeat

the bending and pulling operation. Do not try to use this method for tubing larger than 12

mm in diameter.


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Figure 10 2. Trimming ends of tubing and rods. Jagged ends of tubing may be trimmed

by striking the jagged end with quick, easy strokes of the flat surface of a wire gauze held

at an acute angle to the tubing, Fig. 11.

3. Fire polishing ends of tubing and rods. The sharp edges left after cutting

may be smoothed for safe use by rotating the end of the tube or rod at the tip of the blue

cone in a Bunsen burner; continuous rotation is very important. It is easy to overdo this

heating, causing the ends to droop or the tubing to collapse; all that is necessary is a

surface melting of the glass in order to round the sharp edges. Usually this job is

complete by the time the flame has taken on a generally strong yellow color. In case of

doubt, remove the end from the flame for closer inspection.

Figure 11 Figure 12


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4. Bending tubing and rod A good bend requires that the tube or rod be

uniformly heated until soft over a length of about 2 to 3 inches. This cannot be done very

easily with the regular Bunsen burner, so a flame spreader called a "wing tip" is put on

the top of the burner before lighting it, Fig. 12. The flame should be blue, with a fairlyhorizontal top; it should have a bright blue "mantle" in the interior of the flame, also with

a horizontal top, the counterpart of the blue cone in the normal Bunsen flame.

Hold the tubing or rod to be bent just above the edge of the blue mantle and

rotate continuously, using both hands. Continue the rotation after the flame becomes

quite yellow and even after there is a strong tendency for the glass to bend while you are

rotating it, Fig. 13. Take the glass from the flame and bend the ends upward to the

desired angle, keeping the bend in a vertical plane to minimize sagging in some other

direction. If you have heated the glass uniformly and to a high enough temperature, you

will have a uniform bend, Fig. 14(a), and usually you will have plenty of time in which to

make minor adjustments in the angle before the glass will no longer bend. If the glass

was not hot enough, the bend will usually be pinched and under strain, Fig. 14(b). If the

wing tip was poorly adjusted so that the flame is much hotter at the ends, the bending will

occur in two places instead of one, Fig. 14(c); the wing tip orifice should probably be

widened at the center, an operation that can be accomplished with the small end of your

file while the flame is burning. Do not try to rebend a piece of tubing; it always looks

like a poor, done-over job.

Figure 13 Figure 14

5. Drawing tips on tubes. Continuously rotate a piece of tubing held just above

the tip of the blue cone in the normal Bunsen flame. Do not use a wing tip, and do not let

the flame wander along the length of the tube while rotating. After the flame has become


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bright yellow and the tubing is very flexible at its hot spot, remove the tube from the

flame and pull the two ends apart while holding the tube aligned and in a vertical

position, Fig. 15. Watch the shape of the tips during the drawing operation, drawing

faster if you want shorter tips, or more slowly for longer, more tapered tips. Hold the

tube in a vertical position until the glass has solidified. Use great care in breaking the tipsat the desired length after cooling for these glass walls are very thin. Only a slight

scratch is needed to make the thin tubing break.

6. Flaring tubing. It is often necessary to flare tubing in order to permit a snug

fit in connections to rubber. A typical example is the fitting of a rubber medicine dropper

bulb to a specially fashioned glass dropper. Continuously rotate the end of the tubing at a


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point just above the tip of the blue cone (don't use the wing tip). After the flame has

become quite yellow and the end of the tube shows a tendency to collapse, remove the

tube from the flame, insert the thin, tapered, pre-heated end of your file into the tube and

at an angle to it, and rotate the tube against it, Fig. 16. Do not press too hard against the

file or serious deformation will occur. This operation can be repeated to enlarge the flareor to give a little better shape to it.

7. Shaping the ends of rods. Sometimes, for breaking up solids or stirring in

test tubes, it is convenient to have a rod with something other than a plain firepolished

end. Continuously rotate the end of the rod at a point just above the tip of the blue cone

(don't use the wing tip). After the flame has become quite yellow and the end tends to

form a spherical blob, remove it from the flame and press it vertically against a Transite

square on your desk top; this will produce a disk on the end of the rod, Fig. 17(a). If,

instead of pressing it against your Transite square you squeeze the molten blob betweenthe pre-heated ends of your crucible tongs (perhaps quickly in two or three adjacent

positions), you will obtain a rod with a paddle on the end. If you have two pieces of

Transite available to you, a more even paddle will be obtained by squeezing the blob

between them, Fig 17(b). Reheating for reshaping is permissible. Glass which has been

suddenly cooled by contact with metal should be gently reheated in order to remove the

strains in the glass. Glass from which strains have not been removed is liable to break

suddenly and without warning, often with serious consequences.

FilteringThe most convenient and rapid method for filtration in quantitative analysis is by

means of a Selas crucible, a glazed crucible with an unglazed porcelain or sintered glass

bottom, through which the liquid may be sucked into a filter flask; the precipitate is

retained on the bottom of the crucible. The usual arrangement for achieving this is shown

in Fig. 18; the crucible is held in place by a rubber adapter that also possesses a glass tube

which prevents the solution from being partially sucked directly into the vacuum line.

When the filtration operation is complete the vacuum line should be closed and air should

flow through the crucible until atmospheric pressure is reached in the flask beforeremoving the crucible from the holder.


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Figure 18

Figure 19

A less convenient method of filtration involves the use of a funnel into which is

placed a piece of filter paper that has been folded into quarters. Before placement in the

funnel the outer folded comer is torn as shown in Fig. 19. Before use, the filter paper is

wet with distilled water and pressed against the glass to squeeze out the air bubbles; the

torn corner minimizes the possibility of an air leak at the fold where the paper goes from

three thicknesses to one thickness. For filtration, the funnel is held in a special wooden

rack, Fig. 20, or in an iron ring. In certain experiments (but not in this manual) the filter

paper containing the precipitate is carefully folded into a crucible and then heated at red

heat in order to burn away the paper and to obtain the ignited weight of the precipitate.

When this is done, special ashless filter papers must be used. Use of the Selas crucible

avoids the necessity of the paper burning operation and the concern for the many

chemical reagents that would render filter paper useless for filtration.


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Figure 21

Pouring of liquidsThe quantitative transfer of liquids from one container to another requires skill in

order to avoid spilling or the running of liquid down the side of the container after

pouring from it. Only normal care is usually required when pouring from a narrow

mouth bottle; the lip of the bottle should be touched to the container rim before returning

the bottle to a completely upright position in order to avoid dribbling down the side of the

bottle.The quantitative transfer of liquids from a wide-mouth vessel, such as a beaker,

can be aided by a simple technique. The running of liquid down the outside of the beaker

from the lip when returning the beaker to an upright position can be eliminated by putting

an invisible film of grease on the outside of the beaker just below the lip; this can be done

very simply by rubbing your finger against the side of your nose, then drawing your

finger under the lip of the beaker.

Directing the flow of liquid from the beaker can be simplified by placing a glass

rod across the top of the beaker so that two or three inches extend out from the lip; it is

held in position by the index finger while using the other fingers to hold the beaker.

When poured from the beaker, the liquid will flow down the rod to the location where it

is directed; if rinsing is important, this can be accomplished in the same way while a

stream of water is directed from a wash bottle against the walls of the beaker as shown in

Fig. 20.


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Volume Measurement: Volumetric FlaskThe measurement of liquid volumes is accomplished in an approximate way by a

graduated cylinder of suitable size, in an accurate way by means of a pipet or buret (for

the transfer or delivery of liquids), or by means of a volumetric flask (for the preparation

of solutions of known volume). This section and the two that follow discuss these principal pieces of volumetric equipment.

A volumetric flask is commonly used in the preparation of chemical solutions.

Because it has a narrow, clearly-marked neck, it is possible to fill it in a reproducible way

and minimize the error of measurement. When the flask is filled, the bottom of the

meniscus (the curved line of the water surface) should appear to lie in the plane that

passes through the circular line etched on the neck of the flask. The eyes of the observer

must be in this same plane, Fig. 21. The error that results when the eyes are above or

below this plane is called parallax error. The close-fitting stopper prevents evaporationand permits the vigorous shaking needed for good mixing in the preparation of solutions.

As with other volumetric ware the walls must be scrupulously clean so that no

drops will cling to the neck above the mark. Care must also be exercised to insure that no

significant amount of liquid is trapped by the stopper. During the filling process the flask

should be held only by the neck above the mark; otherwise the heat of your hand will

raise the temperature of the liquid which will expand and fill the flask with less liquid

than would have been required at room temperature.

According to the minimum standards set by the National Bureau of Standards, a

first-class (a Class A) 50-ml flask should be in error by no more than 0.05 ml; a 250-mlflask by no more than 0.12 ml. For very accurate work, the volumetric flask should be

calibrated so that a suitable correction can be made. In any case, it is important to realize

that the volumetric flask will contain the volume for which it is marked, not deliver it.

Volume Measurement: PipetThe transfer and accurate measurement of relatively small volumes of liquids are

often carried out by means of pipets. Two kinds of pipets are illustrated in Fig. 22. The

ungraduated form is called a "volumetric" or "transfer" pipet, and the other is called a"graduated" pipet or Mohr pipet. The techniques of handling them may be summarized

as follows.


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Figure 22

The pipet should be clean and either be dry or rinsed three times with the liquid to

be pipetted. The liquid to be pipetted is sucked into a clean pipet by means of a rubber

bulb, not by mouth. A right-handed person should hold the upper end of the pipet in the

right hand between the thumb and fingers, leaving the index finger free, Fig. 23(a). The

rubber bulb should be held in the left hand and compressed before connecting it to the

upper end of the pipet, and the connection should be just secure enough so as to avoid air

leaks, Fig. 23(b). As soon as the liquid level rises to a bit above the desired graduation

mark, the bulb is slipped off to one side and the end of the index finger is quickly placed

over the end of the pipet, Fig. 23(c). The tip of the pipet is withdrawn from the supply of

liquid and the outside of the lower stem wiped dry with a cleaning tissue. Pressure from

the index finger is relieved slightly to permit the liquid meniscus to descend to the

desired graduation mark, then increased again so as to hold the liquid at this position.

Persons with very dry skin may have a difficult time holding the meniscus at a fixed

position

(a) (b) (c)


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Figure 23

unless they first moisten the index finger. When the meniscus is at the mark the pipet tip

is touched to a wet glass wall of something like a beaker in order to detach the drop or

partial drop of liquid held there. The filled pipet with its carefully adjusted meniscusmust not be subjected to sudden, jerky motions that may result in loss of some of the

liquid from the tip. When using the volumetric pipet the full contents of the pipet are

allowed to flow into the desired container. After waiting a few seconds for draining, the

pipet tip is touched to the wet container wall to remove the drop or partial drop held there

and in the capillary end of the pipet. Do not blow out the remaining liquid; the small

amount that stays behind is reproducible and has been allowed for in the calibration of

the pipet. A left-handed person should hold the pipet in the left hand and the bulb in the

right. Because it seriously hampers control of the pipet you should not control the flowof liquid from the pipet by means of your thumb. When the liquid drains from the walls

of the pipet there should be left a continuous invisible film of liquid, not droplets. If

droplets appear, the pipet must be cleaned before use with liquid detergent solution

followed by copious rinsing, first with tap water and then with distilled water. If the

detergent solution does not clean the pipet have the instructor clean the pipet with warm

sodium dichromate solution.

When using the graduated pipet, the handling techniques are the same as for the

volumetric pipet except that instead of draining the entire contents the meniscus is

allowed to drop only to the desired level. The partial drop hanging at the tip is alsoremoved as before.

For reproducibly accurate work, the pipet must not be drained too fast; minimums

of 20 sec for a 10-ml pipet and 30 sec for a ml 50-ml pipet are recommended. Pipets

which meet the standards set by the National Bureau of Standards will have errors that do

not exceed 0.02 ml for a 10-ml pipet or 0.03 ml for a 25-ml pipet.

Figure 24

Volume Measurement: Buret


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A buret, Fig. 24, is a common piece of equipment used to deliver known volumes

of liquids. It is a slender glass tube of uniform bore with graduation marks along its

length. A 50-ml buret usually has the milliliter marks numbered, with unnumbered 0.1-

ml subdivisions in between. A scrupulously cleaned buret is one that drains smoothly to

leave an unbroken invisible film of liquid on its walls; droplets indicate dirty walls and a buret unfit for accurate work.

Cleaning is best accomplished by the use of liquid detergent solution and

scrubbing with a buret brush with a long wire handle. This should be followed by

copious rinsing, first with tap water and then with distilled water. If the buret is kept

filled with distilled water (and stoppered with a cork) when it is not in use and stored in

your desk, there will seldom be a need for cleaning it.

Since your buret has a Teflon stopcock, you will need only to check it for proper

tightness to insure against leaking. To check your buret for leakage, clamp your buret ona buret stand and fill it to above the "O" mark with distilled water. Use a funnel (on this

and similar occasions) for filling and use good judgment to avoid overflow. After

eliminating any air bubbles from the tip, lower the meniscus to approximately "O" or a

bit below. Let the buret stand for 5 minutes. If the meniscus drops, the stopcock leaks

and it must be adjusted or replaced.

The rate at which liquid is withdrawn from a buret should be relatively slow so

that the wall film will drain in a reproducible manner. To check the delivery time of your

buret, fill it to approximately "O" with distilled water, then open the stopcock wide open

and determine the time required for the buret to empty to the 50ml mark. This timeshould be reasonably close to the time printed on the buret, or correspond to

approximately 0.5 ml per sec. If the time is much longer than recommended, there is

possibly a partial plug either at the tip of the stopcock; if so, it should be removed. If the

time is much shorter than recommended, you should consult your instructor.

Before it is filled the buret should always be rinsed three or more times with

small portions of the solution to be used. Also, a small portion of the contents should be

withdrawn rapidly from the tip so as to remove all air bubbles from it. The buret tip

should be handled in the same way as the pipet tip. It should be wiped with a tissue;then, after a bit of liquid is withdrawn, it should be touched to some wet glass surface

before the initial reading is taken or before a measured volume is withdrawn. After the

desired volume is withdrawn, the tip should again be touched to the wet receiver wall

before the final buret reading is taken.

Many devices have been suggested to aid in estimating the position of the bottom

of the meniscus. Two of these are shown in Fig. 26; one is a white card on which has


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been stuck a strip of black plastic electricians' tape, the other is a strip of black paper

clipped snugly around the buret. Use the method that gives you the most confidence in

reading your buret to the nearest 0.01 mi without parallax.

Figure 25

Burets vary in quality. A 50-ml buret that meets the standards set by the National

Bureau of Standards will have a maximum accumulated error of less than 0.10 ml, and

proportionately smaller errors for volumes of less than 50 ml. For accurate work it is

necessary to correct for the inherent errors in a buret, as is done in the calibration

experiment in this manual.

TitrationOne of the principal uses of the buret is in measuring the volume of a solution of

one reagent which is required to react completely with another reagent. The equivalence

point of such a titration is the point at which the volume of titrant delivered from the

buret is just sufficient to react completely with the second solution. The completion of

the titration is indicated by a change of some observable property of the solution being

titrated, at or near the equivalence point. The point at which this observable change

occurs is called the end point of the titration.

Standardization of a titrant is the process in which a known amount of a puresubstance or a solution of known concentration is titrated in order to determine the

concentration of the titrant solution. Pure solids that are suitable for this purpose are

known as primary standards. Such a standardized titrant can then be used to titrate

samples of unknown purity with which it reacts, in order to determine their composition.


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A great variety of properties have been used to detect the end points of titrations.

Sometimes an indicator which changes color at the end point is added to the solution

being titrated. Sometimes a property of the solution like the pH or electrical conductance

is observed instrumentally throughout the titration. In each titration experiment in this

manual the method of detecting the end point will be discussed.In the following discussion we shall assume that you have followed the

instructions in the preceding section for rinsing, filling, and preparing the buret for use.

After recording the initial buret reading to the nearest 0.01 ml, the titrant may be added

rapidly at first to the titration vessel (usually a conical flask). Near the end of the titration

the rate of addition should be decreased until the titrant is being added drop by drop.

Very near the end point partial drops of titrant can be added in several ways:

1. Let part of a drop form on the tip of the buret and wash it into the flask with

water from your wash bottle.2. Let a partial drop form on the tip and detach it by touching the tip to the inside

surface of the flask. Then rinse the sides of the flask with the wash bottle.

3. Very rapidly flip the buret stopcock through 180. This will deliver a partial

drop directly into the flask. This technique should be practiced before the end point is

reached, for the speed of the flip determines the volume of titrant added.

When the end point is first observed, the walls of the flask should be rinsed with a

stream of distilled water from the wash bottle. Additional partial drops of the titrant

should be added, if necessary.

Weighing: General ConsiderationsThe mass of an object is a measure of the quantity of matter it contains. The mass

is a fixed value which doesn't depend on the conditions of measurement. The weight of

an object is a measure of the gravitational force exerted upon the object by the earth. It

varies from place to place because of irregularities in the structure of the earth and

because the distance between the object and the center of the earth varies. Chemists

traditionally use the term weight to refer to mass. Almost all of the devices chemists use

to "weigh" objects actually determine mass since they compare the force exerted betweenthe object and the earth with that exerted between a set of known masses ("weights") and

the earth. Under these circumstances, the actual value of the gravitational force is of no

concern and it is mass that is measured. The unfortunate usage of weight for mass by

chemists is deeply ingrained and shows no signs of disappearing.

The techniques of handling materials to be weighed depend on the physical nature

of the material. Single objects of reasonable size can be weighed by simply placing them


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directly on the balance pan. Powders, collections of small crystals, or liquids which

cannot be weighed directly are "weighed by difference." The following instructions

apply particularly to the analytical balances.

1. Sample is transferred to a once-folded weighing paper (special lintless paper

with a slick surface). This is done OUTSIDE THE BALANCE CASE .2. Paper and sample are placed on the balance pan and weighed accurately.

3. Sample is carefully slid along the fold and into the beaker or flask where it

will be used.

4. The paper alone (with whatever crystals cling to its surface) is accurately

reweighed.

5. The weight of sample is found by subtracting the weight of the paper from the

weight of the paper with sample.

This method insures cleanliness of the balance case and does not require anyspecial care about removing all of the crystals from the paper. Obviously, it is important

that all of the crystals that are transferred from the paper land in the flask and not partly

on the table top, for you are measuring the loss in weight of the load on the paper and not

the increase in weight in your flask. In order that samples of approximately correct size

be obtained, it is convenient to find the approximate weight of the paper in advance. Any

addition or subtraction of material from the paper should be done outside the balance

case.

In some cases, it is appropriate to use the "tare" feature of the electronic balances.

With the tare feature, the balance can be set back to zero after the container which willhold the material to be weighed has been placed on the balance pan. On the analytical

balances taring is performed by pressing the red "T" on the front panel and on the

laboratory balances by pressing "RE-ZERO". The balance will now give the actual mass

of the material being weighed. The appropriate method for a given experiment will be

discussed in conjunction with the experiment. For some experiments the "weighing by

difference" method is preferred and for others the "taring" method is preferred.

Liquids must be weighed as the increase in weight of the flask to which the liquid

has been added. Any addition or removal of liquid should be done outside the balancecase. A watch glass laid over the flask opening will reduce loss in weight due to

evaporation.

Weighing Precautions:

Analytical balances are used for accurate, quantitative work. They are delicate,

expensive instruments and deserve your most considerate attention. In order to take


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advantage of their sensitivity and to obtain accurate and reproducible weighings, it is

necessary that certain precautions be taken. The most important of these are listed below;

they should always be considered, regardless of the type of analytical balance you use. If

your work requires that correction be made for the buoyancy of air, you may find helpful

the discussion of the principles in Appendix B.1. Static electric charges, which accumulate especially easily on glass and other

non-metallic materials, may cause serious errors because of erratic and unpredictable

attractions or repulsions to other parts of the balance. Vigorous rubbing of these

materials with a dry cloth just prior to weighing should be avoided, especially in areas

where the air is very dry.

2. Objects which have been heated to a high temperature and then cooled in a

desiccator (see Fig. 3) may be prone to pick up moisture from the air when first put on

the balance pan. All solid surfaces tend to hold a trace of moisture to a degree that isdetermined by the humidity of the air and, unless it is harmful to the experiment, a more

reliable and reproducible result will be obtained if water-vapor equilibrium is established

between the surface and the air before recording the weight. Constancy of weight with

time is a good indication that equilibrium has been established. Objects which have

recently been very wet and wiped dry may actually lose weight as water vapor leaves the

surface and water-vapor equilibrium is established. Hygroscopic materials should always

be kept in closed containers and weighed as rapidly as possible. Since moisture can also

be transferred from the hands to surfaces in contact with them, it is good practice to use a

pair of tongs, a band of lintless paper, or some other device for transferring objects to andfrom the pans.

3. Hot or cold objects should never be placed on a balance pan. The hot object,

in addition to causing possible damage to the pan, sets up convection currents in the air

around and under the pan so that a smaller weight than is correct is needed to balance it.

As the object cools, the convection current becomes less strong and the container appears

to gain weight. A convection current is set up in the reverse direction for a cold object

and, in addition, if it is very cold it may tend to condense moisture from the air onto its

surface. Both effects would result in a recorded weight that is too large. Random aircurrents from the room are excluded by closing the doors of the balance case during the

actual weighing operation.

4. Naturally CARE of the balance is important for good results. Improper

removal or addition of material from a pan can cause damage; spilling chemicals on the

pan or in the balance case leads to corrosion; exposure of the balance to lab fumes may

result in corrosion of all parts or condensation of a repulsive film on them.


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common laboratory operations

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