THE methods which have been used so far in the assay of p-aminosalicylic acid are based on the estimation of amino, phenolic or carboxyl groups. Those involving bromination1v2 and diazotisati~n~*~v~ usually yield erroneous results owing to the presence, presumably, of m-aminophenol as an impurity, which is itself estimated by a modification of the latter method6. The spectrophotometric method' is believed to be accurate when highly sensitive instruments are used. However such instruments are not always available and with less sensitive ones considerable variations in results are seen.p-Aminosalicylic acid has also been assayed by the estimation of the carboxylic group either by the formation of the insoluble silver saltss (Volhard's method) or by dry decarbo~ylation~. A combination of alkalimetric and bromination methods has also given consistent results1. On the other hand it is difficult to bring the acid and its calcium salt into solution to form the silver salt and dry decarboxylation cannot be applied to sodium and calcium salts. Further, in the case of p-aminosalicylic acid, it is difficult to identify the residue of m-aminophenol after dry decarboxylation.In view of these facts a comparative study of the methods of assay was made and in the course of the work, a method based on the estimation of the labile carboxylic group and the amino group by combined decarboxylation and diazotisation was developed. The compounds assayed by the different methods were the acid and its sodium and calcium salts, the latter two in the form of their pharmaceutical preparations, i.e., granules and tablets. The p-aminosalicylic acid (sample "E") was purified by repeated crystallisations and the results are expressed on the assumption that this is pure as verified by its melting point and that of the derived m-aminophenol.
MethodsIn the bromination and diazotisation methods, the techniques of Dino Coppini2 and Pesez5 were followed. The spectrophotometric method as described in "New and Nonofficial Remedies, 1951," was used, the instrument employed being a Spekker photometer.
DecarboxylationApproximately 0 3 g. of the compound, accurately weighed, was placed in a reaction flask fitted with an intake for carbon dioxide-free air, a separating funnel and a reflux condenser to prevent steam passing over into a series of three absorption flasks, each containing 50 ml. of 0.2N
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