ANALYTICAL EDITION
February 15, 1942
water and high enough to avoid errors due to polarization of electrodes.
Analytical Procedure In operation, a sample of the material was dried in a vacuum oven, and a solution of 0.040 per cent concentration made up in distilled water. The solution was allowed to stand 15 minutes to reach thermal equilibrium, and was then transferred t o a short specific gravity cylinder large enough to hold the dipping electrode. The solution was agitated gently wjth the electrode, which was inserted to a fixed mark, and the bndge was balanced. The temperature of the solution was read and the specific resisb ance corrected to 25" by the formula given above, and for cell constant when necessary. To ensure clean electrodes, it was found better to make another readin on a fresh sample of the solution and accept the values only ifthe two readings agreed within 1 per cent. The salt content was read directly from the calibration curve.
Any nonvolatile water-insoluble material that is present will affect the result by decreasing the apparent amount of salt present. This may be corrected for by determining the amount of water-insoluble material by a suitable method fether extraction for fatty material, filtration of aqueous solution for solids), and correcting for this in making up the solu-
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tion for conductivity reading, so that the concentration is 0.040 per cent with respect to soluble inorganic salt plus soluble active ingredient. The result is then obtained as per cent salt on a moisture-free and water-insoluble-free basis. It is undoubtedly possible to apply this procedure when more than one inorganic salt is present. I n the particular case of sodium sulfite-sodium sulfate mixtures, when the sodium sulfite is present in variable small percentages, little error is introduced by considering the sulfite present as sodium sulfate, since the specific conductivities of the pure salts are almost equal. I n other cases it would probably be necessary to determine the concentration ratios of the inorganic salts present by some other means and use a calibration curve for the mixture actually worked with.
Literature Cited Archbutt, Analyst, 37,538-43 (1912). Bishop, to General Chemical Co., U. S. Patent 933,015(1909). Callan and Horrobin, J. SOC.Chem. I n d . , 47, 329T (1928). Coster, IND. ENG.CHEM.,25, 980 (1933). Dawson, J. SOC.Dyers Colourists, 35,123 (1919). Grant, I n d . Chemist, 10,350-4,391-5 (1934). Kendall, J . Am. Chem. Soc., 38,2460 (1916). &andera,K.,Chintie & industrie, Special No., 231-4 (1933).
An Improved Sublimation Apparatus 0. A. NELSON Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Washington, D. C.
P
ROBABLY the most common type of sublimation apparatus to be found in chemical laboratories consists of two test tubes, one within the other. The substance to be sublimed is placed in the larger test tube, which is then heated in a bath or with a free flame, while the inner tube, which functions as a condenser for the sublimate, is kept cold with running water. The capacity of such an apparatus is very limited, as only a small quantity of substance will adhere to the surface of the condenser, or will drop off when the inner tube is removed. The apparatus shown in Figure 1, A , has been found to give excellent results with a number of compounds. The condenser may be a straight tube or made with a bell-shaped lower end, as shown. Heating the end of this tube with a soft brushy flame to the softening point of the glass and applying a gentle suction to the other end produce an even convex or cuplike surface. The most important feature of this apparatus is a screen to catch any crystals of the sublimate that may break away from the condenser. This screen is supported by three glass rods fused into the edge of the condenser cup and bent inward a t right angles ( B ) . If a bath is used for heating, the surface of the liquid should be a t about the level of the screen or a trifle above, to prevent sublimation on the screen and clogging of the meshes. The screen must be resistant to action by the sublimate, and of such mesh as to retain all crystals that may fall on it. A 40-mesh German-silver screen was used in this laboratory. For sublimation under vacuum, a tube may be sealed into the side of the outer jacket, as shown in Figure 1, or run through the rubber stopper supporting the condenser. Cooling water may be run in and out of the condenser through a two-hole stopper a t the top, instead of through the one-hole stopper and side tube as shown in the diagram.
B A FIGURE1. SUBLIMATION APPARATUS
