A Physical Method for Determining Residual

A method is described for determining small amounts of water and other volatile substances present as impurities in various organ ic and inorganic compounds. The impurity is separated by fusing the substance in a cell attached to a collecting system and allowin g it to freeze slowly while the vapors are collected by pumping them into a t rap of known volume, cooled by liquid air. When the trap is warmed to room t emperature, the pressure of the vapor within it , if below saturation, together with the t emperature and volume, det ermine the quantity of impurity in terms of moles and if its ident ity is known, in terms of weight. An auxiliary procedure permits t he impuri ty to be isolated as a liquid, in a capillary container, and thus to be identified . It was found that the current Standard Samples of benzoic acid (39f and 140) contain less than 0.002 percent of water and do not adsorb water on exposure to an atmosphere of high humidity. The method is further illustrated by the determination of residual solvents in acetanilide crystallized from benzene and from a mixture of alcohol and water, and of water present in potassium dichromate as entrapped mother liquor.