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
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 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, the test tube should be held with a wire test tube holder, Fig. 6, the mouth of the test tube
Figure 5 Figure 6
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.
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.
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
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 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 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.
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.
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.