485
ANALYTICAL EDITION
June 15, 1942
conditions similar to these. At 905’ C. the loss in weight is appreciable in the course of an hour and an analytical determination would not be satisfactory if the heating were continued at this temperature for more than a few minutes. To be certain that the loss in weight was not due to the volatilization of the platinum, the empty crucibles were heated for 113 hours at 905’ C. The total loss in weight of the two crucibles was 0.2 and 0.1 mg.
Summary The dissociation of barium carbonate has been studied b y the gas saturation method of determining vapor pressures. The extrapolation of the curve of the cube root of the rate of gas flow against the logarithm of the pressure gives values for zero rate of flow or equilibrium pressures which can be used to predict the temperature at which barium carbonate will start to decompose. A prediction of the temperature at which a noticeable loss in weight mill occur when barium carbonate is ignited has been made and experiments to determine the loss in weight have borne out this prediction. Under the conditions described barium carbonate may be ignited for a short time
Glass Valve Pressure Regulator MARION J. CALDWELL
AND
H. N. BARHAM
Kansas Agricultural Experiment Station, >Ianhattan, Kans.
I
N THE course of investigations involving the evolution of highly corrosive vapors, i t became desirable to conduct experimentation at, carefully controlled pressures, both above and below atmospheric pressure. A search of the literature failed to reveal any simple device deemed capable of continued operation under the conditions to be encountered. The glass regulator here described has been found by laboratory trial to meet satisfactorily the conditions imposed. The apparatus, the construction of which is shown in the diagram, involves the sealed-chamber principle found in the vacuum regulator of McConnell ( 1 ) . It consists of two glass chambers, communicating across the ExnAu5r bottom through a mercury pool and across the top through a stopcock. Floating on the mercury in one-arm is a leadweighted float, carrying a glass rod having at its free end a ground-glass tip which serves as a needle to close the ground opening in the exhaust line. The valve parts are held in alignment by a closefitting sleeve surrounding the float stem. The vertical motion of the valve assembly is limited to about 1 mm. by the valve seat above and the glass stop beloxv. The dimensions of the regulator may be varied WEIGH within xide limits: hoxFLOAT% ever, it is essential that the H weight of the float be sufficient to ensure the opening of the valve against the
as high as 885’ C. without affecting the accuracy of the usual analytical determination, but above this temperature the loss in weight becomes appreciable.
Literature Cited Abich, Pogo. Ann., 23, 314 (1831). Brill, 2. anorg. Chem., 45,275 (1905). Dutoit, J . chim. phys., 24, 110 (1927). (4) Finkelstein, Ber., 39, 1585 (1906). (5) Hackspill and Wolf, Compt. rend., 204, 1820 (1937). (6) Hedvall, 2. anorg. Chem., 98, 47 (1916). (7) Herzfeld and Stiepel, 2. Ver. deut. Zucker-Ind.. (Z), 35, 830 (1) (2) (3)
(1898). (8) (9) (10) (11) (12)
Intern. Critical Tables, Vol. I, 393 (1926). Isambert, Compt. rend., 86, 332 (1878). Johnston, J.Am. Chem. SOC.,32,938 (1910). Nakayama, J . Chem. SOC.J a p a n 47, 197 (1926). Nichols and Schempf, IND.ENQ.CHEM.,ANAL.ED., 11, 278
(1939), (13) Nichols and White, Ibid., 13,251 (1941). (14) Pott, from Mellor, “Comprehensive Treatise on Inorganio and Theoretical Chemistry”, Vol. 111, p. 655, London, Longmans, Green and Co., 1923. (15) Ziemeris, 2. physik. Chem., B37, 231, 241 (1937).
pressure differential at the valve seat. Rough calculations show that slightly over 10 grams are required to open a 1 sq. mm. openingagainstapressure differentialof one atmosphere. As this weight should be displaced by the mercury column in as small a pressure range as possible, it is necessary that the cross section of the float be relatively large. A float made from a 25-mm. Pyrex ignition tube has been found to give satisfactory results. In this case, the lead-loaded float weighs approximately 100 grams, giving a sizable safety factor. In operation, the pressure in the system is allowed to build up to the desired level with the regulator stopcock open. The stopcock is then closed, thereby sealing in the large chamber a body of gas at the selected pressure. As the pressure in the system further increases, the mercury in the float chamber is depressed, allowing the valve to open and exhaust the accumulated gases until the mercury in rising again closes the needle valve. If, in operation, the system does not evolve sufficient gas to maintain the desired pressure, an inert gas may be supplemented, as indicated in the diagram. The pressure fluctuations necessary to actuate the valve mechanism may be largely eliminated a t the reaction flask by interposing a bottle of a few liters’ capacity to act as a cushion. This apparatus has been used in the authors’ laboratories for the regulation of both positive and reduced pressures. Positive pressures thus far have not exceeded 1500 mm. of mercury, although there appears no reason why higher pressures could not be controlled equally well, provided the weight relationships are kept in balance. As a reduced pressure regulator, the “exhaust” line is connected to the vacuum system and a controlled air leak is attached to the “inert gas” entrance. It is not expected that the valve should operate a t very low pressures, since several millimeters variation in pressure is required to actuate the mechanism. As the control is based upon the equalization of pressure between the system and the sealed chamber, temperature variations within the chamber directly affect the regulator precision. Under proper operating conditions, the apparatus has been found capable of controlling pressure either above or below atmospheric pressure, to about 1 mm. of mercury.
Literature Cited (1)
McConnell. C . W., IXD.ENG.CHEM.,ASAL. ED.,7,4 (1935).
CONTRIBETION No. 272, Department of Chemistry, Kansas State College Manhattan, Kans.
