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HINTS ON LABORATORY TECHNIQUE MORRIS M. GRAFF Naval Stores Research Division, United States Department of Agriculture, New Orleans, Louisiana
SINCE few schools offer courses in laboratory technique, the average graduate student enten the field of research with only the meager experience he has gained in performing the experiments required in his undergraduate courses. A knowledge of the proper techniques greatly facilitates the solving of many manual problems encountered in laboratory research. The experienced research scientist uses many such short cuts involving various simple devices and laboratory apparatus. In this article i t is the author's purpose to describe some of these laboratory aids which he has had the opportunity to observe and use. Originality is claimed in only a very few instances. It would be di5cult to give credit for the original ideas because the material referred to either has never been published or has been described but has long since been forgotten. By the publication of such aids the author hopes to encourage the inclusion of similar useful techniques in textbooks and laboratory manuals.
This stirrer consists of a rod, a, sleeve, b, joints, c, blades, d, and setscrew, e. The blades, fabricated from thin strips of any flexible metal, are attached to the sleeve and rod by ring joints, allowing a certain amount of freedom a t the junctions. The blades may be constructed with blunt, sharp, or notched edges, depending on the purpose for which the stirrer is to be used, and their number may be varied. For operation, the stirrer is lowered into the mixing vessel in position I; the rod is held firmly; the sleeve is lowered to produce the desired expansion of the blades, and fastened by means of the set-screw. The blades may be adjusted so that they lie in a plane parallel to the rod, as in position 11, or oblique to it, as in position 111, by holding the rod 6rm and rotating the sleeve to the desired position before tightening the setscrew. This type of stirrer permits high-speed stirring, the degree of agitation being varied easily by altering the expansion of the blades. Variable cutting or shearing action can be obtained by altering the type and setting An Efficient Adjustable Laboratory Stirrer of the blades. In round-bottomed containers, with A DEVICE for stirring various kinds of mixtures that blades of the proper length (&inch blades, for example, om easily be inserted through the small neck of a flask are ideal in a 1-liter round-bottomed flask), the exis sketched in Figure 1. panded stirrer conforms closely to the interior, and by keeping the reaction mass or slurry from adhering to the inner surfaces, insures good agitation. 0
Three Types of Medicine Droppers
ONE OF the most valuable and inexpensive pieces of laboratory equipment is the medicine dropper. After
Irig-
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m
I. stirrerin vuiou. ~ ~ d t i ~ ~ .
a little practice a t the blast lamp the student is able to draw droppers of varying reservoir length, different 182
APRIL. 1947
capillary length, bore, and shape. A suitable rack for the different droppers while in use can be readily made by notching filter paper boxes. Three useful types of laboratory medicine droppers are shown in Figure 2. The regular shape ( A ) is useful for adding small amounts of solvents, as in recrystallization or for testing solubility. Shape (B), having a long h e capillary but so constructed that the glass a t the tip is thick enough to withstand slight pressure, among many other uses, serves particularly well when solvents must be filtered without loss by evaporation, as in the removal of filter paper fibers, particles, etc., from samples prepared for optical rotations. A small piece of filter paper igdropped into the flask containing the solution and the tip of the capillary is pressed against the paper on the bottom of the flask. When pressure on the bulb is released, the solution rising in the capillary is filtered free of solid particles without a significant change in the concentration. A flat tip may be obtained by scratching the capillary with a piece of porcelain before breaking it. This type of capillary is also useful when many recrystallizations must be performed on a small amount of material. Loss of sample will result if the crystals are filtered off and transferred from the funnel to another flask, etc. But if the recrystallizations are carried out in conical centrifuee tubes, the crvstals may be centrifuged to the tip and the supernatant liquor removed by carefully inserting the capillary through-the crystal mass so that it reaches the bottom of the tube. With release of pressure on the bulb, the clear liquor is drawn up into the capillary, leaving the crystals ready for the next recrystallization. The amount of solvent removed and the separation performed will depend on the cross section of the capillary opening. Removal of liquid from containers without disturbing the crystal formation may also be carried out with this type. To withdraw the liquid from odd-shaped flasks, etc., it may be more convenient to use a bent capillary such as form (C). Micro Filter Stick
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A HANDY in repeated recrystallizations of small amounts of material is a micro filter stick similar to that described in textbooks and in micro analysis. A broken fritted-glass funnel may be ground down
F i w m 4. Filterins Adapt-.
to form this type of filter stick. The tube for rec&stallization is chosen of sufficient size to allow the filter stick to enter with a small amount of clearance, and its bottom is flattenqd. The material, dissolved in the tube in the usual manner, is allowed to crystallize. During filtration, the tube may be held in an ice bath if low melting solids are to be handled. The filter stick presses the crystals to the bottom while slight suction is applied. (Figure 3.) 0
rigure 3. rilt..in.
6 t h the &id of MI-O riltu stick
A Beaker a s a Hot,Air Bath
DURINGA recrystallization operation where i t is necessary to filter the hot material through a fluted Paper, varying amounts of crystalline material are often lost due to crystallization in the funnel. A large part of this may be readily reicovered by setting the flask and funnel on a hot plate under a large beaker, or bell jar, which prevents cooling of the funnel by air drafts. Gentle refluxing causes the material to be dissolved and percolated into the flask in a very short time. 0
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Filtering Adapter
AMONG THE many uses of a filtering adapter (Figure 4) are its use in ( A ) filtering from one flask or tube into another without the aid of a suction flask, avoiding transferring and dilution, and (B) evaporation of solutions by allowing dry or filtered air or inert gases to be drawn over them. The latter is done by substituting a drying tube or cotton filter plug for the Biichner funnel.
JOURNAL O F CHEMICAL EDUCATION
This device, when used where rubber stoppers are to be avoided, can be constructed of interchangeable ground-glass joiqts, as in A . The size of Erlenmeyer flask used is limited by the amount of suction required, but such a size limitation does not apply to round-bottomed flasks. Ruhber Dam
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THEUSE of a rubber dam in drying material collected on a Biichner funnel is an old trick that needs reviving. If the funnel is covered with the rubber dam (Figure 5), and held in place with rubber bands while suction is applied slowly, the rubber dam is drawn tightlyagainst t h e material, literally squeezing the solvent out without exposing the material to an unnecessary amount of air, thus avoiding oxidation. Another method for pressing out solvent is to use a flat-topped glass stopper. For maintaining suction in a "slow filtration" which occurs a t "quitting time" when water aspirators must be turned off, the rubber tubing can be clamped near the outlet. A surgical clamp is ideal for this purpose. Effective removal of a large cake from a Biichner funnel is obtained by placing the Biichner with bpen side down on a large evaporating dish or watch glass and blowing sharply into the tip opening of the funnel. The cake will usually drop out in one piece. Steam Generator
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A
boiling results, the rate being adjusted and controlled by varying the voltage. The pinch clamp on the water inlet is closed and the steam is allowed to escape through the outlet. The control of steam generation is very precise, and where inflammable materials are being distilled, this generator is far safer than a gas-flame heated boiler. 0
Micro Steam Bath
A MICRO steam bath of the. t&e illustrated in Figure 7 can be readily coustructed from a test tube of proper size. This device is useful in crystallizing from centrifuge or test tubes. Rubber stoppers with holes of different sizes serve well as "covers" for this steam bath.
C
steam generator which lends itself to excellent control is sketched in Figure 6. The h e a t ing unit consists of 10 to 12 feet of Nichrome wire, No. 24, having a resistance of 1.6 ohms per foot. The coil is formed by winding the wire around any tube of suitable diameter. Two '/,-inch welding rods, sharpened to a needle point, are forced through the rubber stopper, bent into form before attaching to the wire coil, and their ends, extending out of the rubber stopper, used as terminals to be attached to a variable voltage transformer of sufficient wattage. The rubber stopper is fitted with a mercury-filled safety manometer (which may be constructed, if desired, from a pipet so as to provide a reservoir for the mercury), a water inlet closed with a piece of rubber tubing and pinch clamp, and a steam outlet. The generator is assembled with all connections made for use. and the water inlet valve o~ened. The current is allowed to pass into the water reservoir until vigorous LABORATORY
Figurn 8 . A Precisely Controllad Typa of Labor.. tow Steam Generator
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Hot or Cold MicroBYchner Funnel
A SMALL sized steam-heated or ice water-cooled Biichner funnel may be constructed with an Alundum discsealed into a piece of glass tubing enclosed within a jacket having an inlet and an outlet (Figure 8). Hot or cold filtration forthe re- : 2 moval of im~.th purities is made a simple operation by circulatina either steam or ice waterthrough the outer jacket. The material to be filtered may be added to the funnelwith a medicine dronY per and suction may be apFiwr.8. Micro Biichno~Funnel tor notor cold F ~ I + . . ~ ~ O ~ plied if necessary.
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Evaporation Under Bell Jars
THEUSE of a bell jar for evaporating solutions under streams of inert gases or dry air is another handy laboratory aid. The bottom of the bell jar is ground to form a vacuum tight fit on a metal or glass plate (Figure 9). For evaporation of small amounts of material a micro bell jar with a thick glass plate is now on the market. This size has many other uses. 0
An IcaWater Circulating Device
A GOOD substitute for a circulating pump is shown in Figure 10. I n operation the condenser and connections are filled with water and a slow stream of compressed air is turned on. This pushes the water up into the ice bath while ice water siphons out of the ice bath through the condenser jacket.
MIXINGPHOSPHOROUS pentoxide with broken glass, glass beads, or porcelain prevents waste of this material when it is being used in drying tubes, desiccators, etc.,. and also provides a greater active surface. It is convenient if the drying agents used in desiccators are placed in petri dishes instead of directly in the desiccators. WHENA capillary melting-point tube must be filled with low melting solids, such as fats, oils, or waxes, a. convenient method is to start with a capillary open at. both ends. The sample is melted and after a small amount has been drawn up into the capillary, the opposite end of the capillary is sealed. The tube is then dropped into a previously warmed centrifuge cup. The sample is centrifuged into the sealed end and allowed to solidify by placing the capillary tube in ice water. INREMOVING a tight screw cap from a bottle, wrapping several mbber bands about the cap before twisting is advantageous. AN INFRARED lamp has many laboratory uses as a source of heat. Among these are heating material in a vacuum desiccator, refluxing low-boiling inflammable solvents, heating constant-temperature baths, and softening grease in frozen joints, as in desiccator tops and other lubricated mound-elass - .ioints where freezing is due to hardening.
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Figure 9. Evapor.tion with U ~ of F 6-11 Juond Hot P1-tq
AN EXCELLENT method for keepinfa water bath clear is to suspend a piece of cheesecloth or cotton in it. When obse~ationsare to be made in glass baths that tend to "dew up" or become foggy because of the low temperature, "painting" the surface with glycerin is advantageous. ONE OF the oldest methods for cleaning mercury involves the use of a chamois skin. A small amount of mercury is poured into a cup made by pressing the skin into the opening formed between the thumb and forefinger. The chamois is then twisted while being held over a large beaker. Thus a fine mist of clean mercury falls into the beaker.
Figure
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C i r c u l s t i n ~Ice Water by M.sm of Cornpros~d-AirHoist
