COMMON LABORATORY
TECHNIQUES AND
PRACTICES
Cleaning Glassware
A 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 Operations
Glassware 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 dry
Pyrex 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 of the desiccator is
a drying agent (often the very cheap CaCl2) that takes up any moisture that enters
9
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.
Heating
The 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.
(a)
Depending on the type of burner that you
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, the
test tube should be held with a wire test tube holder, Fig. 6, the mouth of the test tube
Figure 5
Figure 6
11
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 watch
glass 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.
12
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 noxious
fumes are evaporated in this way it is imperative to carry out the operation under a
hood with strong ventilation.
Glass Working
Glass 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 500ºC and of Pyrex about 650ºC, temperatures which are intolerable to the skin,
paper, and desktop finishes (unless they are of rock or Transite). A square of thin
Transite 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 or tubing
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 the
glass 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.
13
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
14
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 fairly
horizontal 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
bright yellow and the tubing is very flexible at its hot spot, remove the tube from the
15
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 tips
at 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
point just above the tip of the blue cone (don't use the wing tip). After the flame has
16
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 flare
or 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 between
the 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.
Filtering
The 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 before
removing the crucible from the holder.
17
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.
18
Figure 21
Pouring of liquids
The 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.
19
Volume Measurement: Volumetric Flask
The 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 evaporation
and 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-ml
flask 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: Pipet
The 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.
20
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)
Figure 23
21
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 meniscus
must 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 flow of
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 also
removed 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
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
22
length. A 50-ml buret usually has the milliliter marks numbered, with unnumbered 0.1ml 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 on
a 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 time
should 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
been stuck a strip of black plastic electricians' tape, the other is a strip of black paper
23
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.
Titration
One 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 pure
substance 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.
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
24
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 Considerations
The 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 between
the 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
directly on the balance pan. Powders, collections of small crystals, or liquids which
25
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 any
special 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 will
hold 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 balance
case. 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
advantage of their sensitivity and to obtain accurate and reproducible weighings, it is
26
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 is
determined 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 and
from 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 air
currents 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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