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V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 (2) Booth, H. S., IXD. ENG.CHEM.,ANAL.ED.4,380 (1932). (3) Dushman, S., “Scientific Foundations of Vacuum Technique,” Tt’iley, Xew York, 1949. (4) Heldman, J. D., “Techniques of Glass Manipulation in Scientific
Research,” Prentice-Hall, New York, 1946.
(5) Lawrance, R. B., Chem. Eng. Progr. 50, 155 (1954). (6) Rosenherg, P., Rec. Sci. Instr. 10, 131 (1939).
inexpensive Microtitrator Kenneth A. Allen, Oak Ridge National Laboratory, Oak Ridge, Tenn.
microtitration assemblies are both cumbersome C and expensive. In addition, they generally employ standard medical syringes as burets, vhich add substantially to the cost OKVENTIOSAL
and have the further disadvantages of rapid chemical attack by alkaline solutions and gradual wear between the ground glass plungers and barrels. Even when new, there is usually some leakage around the plungers, and while the latter are no doubt ground sufficiently accurately for medical purposes, variations of h3Y0 were found in the diameter of the plunger of a 0.2-cc. tuberculin syringe. One method of overcoming some of these difficulties is to drive a precision-ground rod through a gasket into an oversize barrel. In the apparatus described by Kelley [Kelley, If.T., Proc. Instr. SOC.Amer. 7,63 (1952)] a platinum rod was used for this purpose. Glass would be preferable from the standpoint of expense, but in either case a chemically inert gasket must make a leakproof seal between two surfaces which move relative to one another, adding to the difficulties of construction. There is the slight additional objection that the rod is external to the barrel at least part of the time and thus exposed to accidental mechanical alteration. The parts necessary for constructing the device s h o r n in the figure may be obtained for a few dollars. The possibility of chemical corrosion in the buret is reduced to a minimum, and the original model has shown no detectable leakage after several weeks of intensive use. Accuracy in the measurement of delivered volumes is controlled by the bore of the tubing used for the buret barrel. Standard precision-bore tubing is generally quoted as having a tolerance of iz0.01 mm.; the piece used here has an inside diameter of 3/8 inch (-10 mm.), which indicates possible variations in the cross-sectional area of 2 parts in a thousand. The results of actual titrations with this apparatus show average deviations which are in excellent agreement with this estimate. Volumes corresponding to a piston travel of 0.1 inch (about 0.2 ml.) may be read to within 0.1% directly on the micrometer, arid for titrations of this or larger amounts in a laboratory of reasonably constant temperature the water jacket may be eliminated. For smaller increments of the total buret volume a simple calculation based on the thermal expansion of the titrating liquid is necessary to ascertain the tolerable temperature fluctuations during any one titration. The economy actually realizable from the construction of this device over the cost of conventional assemblies is, of course, a function also of the labor expenditure required for its fabrication. A good glass blower can turn out the buret in a few hours, however, and this together with the small amount of shop nTork necessary should still allow a real saving. The latter can be greatly increased if it is possible to dispense with the water jacket. At institutions such as colleges and universities the construction of such a device would be a worth-while project in the glass-blowing and shop courses usually given graduate students. This device is readily adaptable to automatic operation; in the present installation a synchronous motor drives the micrometer a t the rate of about 1 revolution per hour. Titrations requiring 10 pl. of reagent are completed in less than 15 minutes, and the delivered volumes are read to within 2 parts in a thousand on the chart of a Brown potentiometer which records potential changes in the sample solution. Although the total piston travel in such
a run is only 0.006 inch, the results of a large number of standard oxidimetric titrations of 50-7 samples of iron a i t h 0.1X ceric sulfate indicate an average deviation of less than 5 parts per thousand. Titrations involving one tenth of these amounts are good to within 5%. For such small fractions of the buret volume, temperature control to within &0.05” C. is necessary; this requirement is a t least partially offset, however, by the advantage of being able to run more than a hundred determinations without refilling the buret. The microtitrator consists of a capillary tip, 1,0.05 to 0.1 mm. in inside diameter attached to the buret Tvith a 5/20 standardtaper joint, 3. An inlet, 2, and outlet, 8, are provided in the glass jacket for the circulation of constant temperature water. i?
The precision-bore tube, 4, of 3,’s inch inside diameter, is held rigidly in place on the micrometer by the plastic supports, 5, and the metal strap, 6. The butt of the micrometer, 10, is removed as shown. A handle, 11, allows accurate manipulation of the titrator, which may be conveniently attached to a standard laboratory support rod by the screw, 7 . The Teflon piston, 9, is turned 0.010 to 0.020 inch oversize a t point A in order to provide a liquid-tight seal in the buret.
Ethylene Carbonate as a Cryoscopic Solvent Sonia K. Gross and Conrad Schuerch, Department of Chemistry, State University of New York, College of Forestry, Syracuse 10, N. Y.
of an investigation of methods for the characterI izationcourse of isolated lignins, the use of the cyclic carbonate of K THE
ethylene glycol as a cryoscopic solvent has been studied. This
CHz-CHz I
1
0\c/o II
0 compound has a number of advantages which make it a generally useful solvent for determinations of molecular weight. Ethylene carbonate is a white crystalline solid with a melting point of 37” C. This low freezing temperature permits molecular weight determinations of compounds which decompose a t the melting point of camphor. Ethylene carbonate is nonhydroscopic; a dried sample did not change in weight on exposure to the moist laboratory air for 2 days. It is a good cryoscopic solvent for a variety of compounds, including hydrocarbons such as phenanthrene, polar compounds such as hydroquinone and vanillin, and even benzoic acid (0.02M), which is frequently associated in solution. The molar freezing point depression constant [ K = 7.031 as determined herein compares favorably with both dioxane [ K = 4.631 and benzene [ K = 5.71 (Skau, E. L., and Wakeham, H., “Physical Methods of Organic Chemistry,” A. Weissberger, ed., vol. I, p. 35 ff., Interscience,
