A SIMPLIFIED LABORATORY STIRRER and MELTING-POINT BATH L. S. KEYSER University of Illinois. Urbana. Illinois
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0 MOST students taking the laboratory work in qualitative organic analysis the desirability of having a simple and compact set-up for accurate determination of melting points has real significance. That the use of any device depending on convection currents t o accomplish stirring gives rise to errors of order of magnitude up to five degrees is well known, and while the use of a small beaker with hand stirring is less open to criticism, it is inconvenient, to say the least. Various types of stirrers have been proposed, but are for the most part complicated and difficult to make. The stirrer herewith described is capable of operation on either laboratory suction or compressed air supply and possesses in addition the following advantages. It is very simple in construction and has considerable power, so that it-may be used for other purposes than melting-point baths. There is no draft of air to blow out the flame or cool off the thermometer. All the parts are either standard laboratory apparatus or can be easily fabricated by anyone. The drawings are almost self-explanatory, but the following description is given so that any points which might cause confusion may he clear. In F i p e 1, A is an ordinary thistle tube cut off and carefully firepolished about two inches from where the small tube joins to the bulb. B is a glass rod having any desired length and of such diameter as to just fit the bore of the A thistle tube. The fit should be just loose enough so that free turnine is ~ossible. (A bit of vaseline makes an .. ideal lubric>nt.j C and C' are rubber stoppers with F ~ c m 1 holes cut as illustrated. D and Dl are small metal washers which serve as bearings and lessen the inrush form of a saew by grasping the end and twisting while .of air a t D. The jet E is drawn in such a way that the glass is still pliable. This type of blade has the maximum turning effect may be obtained. Although advantage of giving adequate stirring without splashing only one such jet is shown, otheis may be put in in a or frothing and a t the same time lessening the danger of similar fashion. F is a small fan cut fmm sheet metal breaking a thermometer. The fan should be adjusted to fit the opening of the thistle tube. It may be so that i t rotates in the plane of the maximum diameter fastened to the glass rod in a variety of ways, but per- of the bulb. The end of the glass rod may or may not haps the simplest is a friction joint. If, when making be used as a bearing by adjusting the height of the the hole in the center of the circle from which the fan rubber stopper. Actual adjustment of the stirrer will is to be made, the hole is not bored but punched with a make this point clearer. me or other blunt instrument, the small flares of metal The stirrer as described, when properly adjusted, left will furnish ample footing for a friction joint with will work smoothly and efficiently. As such it has the aid of a small rubber tube or cork, G, which serves many laboratory uses and may be preserved as a unit to hold the fan on the glass rod. H is a screw-type itself or incorporated with a two-necked flask fashioned blade which is easily made by heating the end of the from a long-necked boiling flask by sealing in a side tube rod in a flame until soft and then flattening by means of having a bore of about 15 millimeters, as shown in pincers or other suitable tool. After the end is flattened Figure 2, to form a convenient and useful melting-point $or about a half inch or so, it can easily be shaped in the bath having all the advantages outlined above. One 590
If a two-necked flask such as is shown is not available disadvantage likely to arise in the use of the stirrer with such a flask, unless precautions are taken against and facilities for making i t are not a t hand, a small it, is that fumes of the liquid of the bath will be drawn three-necked flask lends itself admirably to use as a up through the thistle tube with resultant complica- melting-point bath when used in conjunction with the tions. This difficulty can be entirely obviated by stirrer, or simpler still, a small beaker may be used. careful fitting of the bearing a t D, Figure 1, and by the The only advantage afforded by the use of a beaker use of more than one inlet jet. A similar compensation is its ease of changing medium and cleansing. Howis obtained by increasing the size of the opening in the ever, if sulfuric acid saturated with potassium bisulinlet jet. The stopper C' should also have a good- fate is used as the medium, the tendency to discolor is sized slit or a second hole in i t to allow the air to be not great and changes need not be made very often. Many modifications of the stirrer here desaibed are, drawn up rather than vapors from the bath medium. If the stirrer is not used in a closed system, these pre- of course, possible. The design of the fan may take almost any form, but in the experience of the writer a cautions are of course unnecessary. The use of the two-necked flask has the advantages simple one having eight blades twisted to an angle of 45 that i t gives an accurate means of getting stem correc- degrees works as well as any. If the reader has trouble tion, furnishes a compact unit that may be put away in a getting his stirrer adjusted, his difficulty may lie in the laboratory desk, magnifies the melting-point tube, and inlet tip. The best angle can usually be found easily by has sufficient volume of bath medium to allow delicate the trial-and-error method. It is not difficult to get one control of the bath temperature. The whole assembly of these stirrers to work, and the little time and effort may be fastened to a ring stand by means of a single spent in making one of them will be well repaid in satisfaction gained from service. clamp.
