An absorption apparatus for the micro-determination of certain volatile substances

Abstract

1. THE APPARATUS AND METHOD. W1LmE several methods already exist for the estimation of ammonia, that arising out of the present study has the advantage (besides applying in principle to the analysis of other substances) of combining extreme simplicity with high accuracy in the estimation of very minute quantities. It also permits the convenient carrying out at the one time of a large number of analyses. With the apparatus the error of a single estimation expressed as the coefficient of variation need not exceed 0O5 %, and this degree of accuracy is practically maintained with estimations of ammonia as low as 0-01 mg. The mean value itself does not fall below 99-5 % of the theoretical, and the delicacy of the method is such that the analysis of the blood-ammonia comes within its range. The principle of the method consists in the absorption by simple gaseous diffusion of the volatile substance from one chamber where it exerts a certain tension into another where its tension is zero on the surface of an absorbing fluid. On a first consideration this would seem a very slow process and scarcely lending itself to practical estimations. It has indeed been used by some previous workers-vide Neuberg [1911] and Durig [1907]-who with a very different technique from that here described were obliged to wait upwards of 48 hours for the conclusion of the analysis and even then obtained somewhat low results. The principle of the method may be extended to the estimation of any volatile substance the tension of which is reduced to zero on the surface of an absorbing fluid. It has been successfully extended to the estimation of chloride (as chlorine)-being very suitable for the estimation of very minute quantities-and to carbon dioxide. The apparatus. The apparatus is of a very simple kind and consists of a glass container resembling a small Petri dish with thick walls, within which is a second chamber formed by a circular wall of glass arising from the floor of the outer chamber. The wall of the inner chamber is about half the height of the outer. The apparatus is constructed of thick glass, which (for the present investigation) should give off no alkali to the acid in the middle chamber. The upper edge of the outer wall has a ground surface and is covered during the absorption with a square glass lid on which is smeared a small amount of fixative such as vaselin or a mixture of vaselin and paraffin wax, and in this 27-2