964
ANALYTICAL CHEMISTRY
together through the 3-way stopcock. If the copper block is a t room temperature, the meniscus will fall in the right leg- and rise in the leftleg. The copper block is then cooled by direct immersion in liquid air until the meniscus in the right leg is back to a Dosition near the upper tungsten contact. Theliquild air is remdved, a glass test tube slipped over the copper block, and the test tube immersed in liquid air. The position of the mercury in the manometer then turns the heater current off and on, so as to maintain the meniscus near the contact point. The rheostat in the heater circuit and the depth of the liquid air around the test tube are adjusted so that the off and on periods for the heater are approximately equal in duration. Whe: roperly adjusted the temperature remains constant to *0.3 When it is desired to increase the temperature of the distil1in.g tube to the next value, the height of the mercury in the left leg is reset to the proper position with the mercury in the right leg still a t the tungsten contact. Without further attention the tpmperature quickly rises to the desired value and then is automatically maintained constant at this point.
For use as both a method of temperature measurement and temperature control the 120" type 3-way stopcock in the man. ometer should probably be replaced with a T-type stopcock. This type of stopcock has an advantage when measuring temperatures, as it permits adding mercury to both legs of the manometer simultaneously until the meniscus in the right leg just touches thP contact point. However, when the main application is the presetting of the control point a t some particular temperature and the control of the temperature a t this point, the 120" type s t o p cock is believed to be preferable. Although the location of the stopcock above the bottom of the manometer makes somewhat inconvenient the removal of all the mercury as required when filling the thermometer with helium, it has the advantage of permitting relubrication of this stopcock without loss of the helium from the reservoir.
DISCUSSION
(1) Buffington, R. M., and Latimer, TV. M., J . Am. Chem. SOC.,48, 2305 (1926). (2) Savelli, J. J., Seyfried, W. D., and Filbert, B. M.,IND.ENQ CHEM.,A N ~ LED., . 13, 868 (1941). (3) Scott, R. B., J . Research .VatZ. BUT. Standards, 25, 459 (1940). (4) Shepherd, M., Ibid., 2, 1145 (1929); 26, 227 (1941). (5) Southard, J. C., and Miluer, R. T., J . Am. Chem. SOC.,55, 4384 (1933). (6) Ward, E. C., IND.ENG.CEIEM.,ANAL.ED.,10, 169 (1938).
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This control unit has been found to be a very satisfactory means of both adjusting and controlling the temperature required for
isothermal distillations a t low temperatures. The operation of the thermometer is very simple and it requires a minimum of attention from the operator. The apparatus should also be applicable to the control of low temperatures as required in other problems.
LITER4TURE CITED
RECEIVEDMaroh 1, 1948.
Quantitative Colorimetric Microdetermination of Methanol with Chromotropic Acid Reagent R. N. BOOS, Merck & Co., Znc., Rahway, N. J . .4 rapid, accurate, and specific method for the quantitative determination of
methanol is described in which the methanol is oxidized to formaldehyde and the latter measured colorimetrically with chromotropic acid. The method permits the determination of methanol with a relative error of
