The Microvol Apparatus for Measuring Small Volumes of Gas R. T. SANDERSONI, Western Geophysical Company, Los Angeles, Calif.
the gas, which is allowed to expand into the closed vacuum system and condense on the outer walls of the cooled bulb. When condensation is complete, as indicated by a McLeod gage connected to the system, air is admitted to the mercury reservoir, N , through K and stopcock M , to force the mercury through J to level I, where it closes off the microvol bulb from the rest of the system. The entire gas sample to be measured is thus trapped. The liquid nitrogen is simply removed by allowing the mercury to touch bulb F , causing the nitrogen to boil out in a single spurt. The bulb is then warmed to room temperature, with the hand, by contact through the mercury, or by filling with warm water. After the mercury has been drawn down to I again by exerting a rough vacuum at L, the evaporated gas is slowly compressed into the calibrated capillary system A , B, C by raising the mercury, and measured at any volume at which it exerts a readily measurable pressure. The microvol is constructed entirely of Pyrex. For measuring volumes from 0.05 cu. mm. up t o 0.5 ml., the following approximate dimensions have been found satisfactory for the measuring capillary: A , B , and C are each about 10 cm. long, and 0.4, 3, and 6 cu. mm. per mm., respectively. Capillaries A’, B’, and C’ are of the same stock as A , B , and C, since they are for use as manometers in reading the pressure of gas in the calibrated tube. Bulb F is 20 to 30 mm. in diameter, and opening G is about 10 mm. in diameter. Bulb H need be only slightly larger than F ; a diameter of 35 mm. is sufficient. The calibrated capillary is joined to the top of H through a short piece of 8-mm. tubing, D. The mercury reservoir, N , may be constructed from a 200-ml. flask. The measuring capillary must be carefully calibrated before sealing to the apparatus. In calibrating the joints, it is convenient to make a permanent mark on the uniform-bore capillary, close to the joint on each side. It is then only necessary to determine the total volume between marks, as the gas volume can be read either above or below each joint with ease. There will be no discontinuities in measurable volumes. Spare capillaries of the same stock can conveniently be calibrated and kept in stock for emergency repairs.
A
SIMPLE and useful device for measuring very small volumes of condensable gas in a vacuum apparatus is the microvol. Volumes of the order of 0.05 cu. mm. can be measured within 10 per cent, the a c c u r a c y improving rapidly with increase in sample size. A single such device can readily be constructed to measure a complete range of volumes from 0.01 cu. mm. up to several milliliters. A microvol has been found very satisfactory in the analysis of the soil atmosphere for substances present in concentrations of less than one part per million, and should be generally applicable in microanalysis of gases. As shoTvn in Figure 1, the microvol is somewhat similar in construction to a McLeod gage, with the essential added feature of an inner-sealed bulb, F , opening to the outside. In use, the microvol is connected to the vacuum system at E through a mercury valve or stopcock. The apparatus is thoroughly evacuated, the gas to be measured being contained in a storage tube or held condensed in a trap. Liquid nitrogen is poured into F through G. The microvol is then opened to the tube containing 1
Present address, T h e Texas Corn-
pany, Beacon, K. Y.
FIGURE 1
The microvol has the following definite advantages over a McLeod gage used for the same purpose: The entire gas sample, rather than a part only, is directly measured, so that the microvol is more sensitive. The pressure-volume products can be checked very simply b y readings at widely different and quickly adjusted mercury levels. A much larger range of volumes can be measured in the same apparatus.
Iodometric Semimicroprocedure for Determination of Arsenic in Sodium Cacodylate and Cacodylic Acid VICTOR LEVINE AND WALLACE M. MCNABB Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Penna.
I
N THE semimicromethod for the determination of arsenic
somewhat similar to the one used for the decomposition of in organic compounds described in a previous paper (4, minerals ( 2 ) . This consists of heating the sample with potasthe samples were decomposed with concentrated sulfuric and sium acid sulfate in the presence of small amounts of connitric acids. However, this method of decomposition is not centrated sulfuric acid. Specimens of neoarsphenamine were subjected to this treatment without successful decomposition. applicable to cacodylic acid and its sodium salt. Attempts were made to decompose the cacodylate with sulfuric and Therefore the method is recommended for sodium cacodylate nitric acids, but when the reduction with hypophosphite was and cacodylic acid only. carried out a yellow precipitate formed which volatilized upon heating. ‘C‘pon further investigation, the most satisfactory APPARATUS. The all-glass apparatus recommended for use in method of decomposition for the cacodylate was found to be the determination of mercury (3) and arsenic (4) in organic com76
