Analysis of Mixtures of Carboxylic Acids

In the course of investigations of the reactions of diazo compounds with various types of acids, the authors have had occaaion to develop methods for analysis of mixtures of carboxylic acids by measurement of their reaction rates with diphenyldiazomethane. The carboxylic acid-diphenyldiazomethane reaction has obvious advantages for this purpose, as it is very convenient to follow spectrophotometrically (the diazo compound has a deep permanganatelike color), occurs in a wide range of solvents, and may be carried out with small amounts of materials. Furthermore, different carboxylic acids often show substantial differences in rate-for example, p-nitrobenzoic acid reacts approximately 25 times faster than p-aminobenzoic acid in ethyl alcohol at 30°. The principal disadvantages of the reaction for analytical purposes are the lack of simple stoichiometry in solvents like ethyl alcohol, where the reaction rate is strictly second-order, and nonintegral kinetic orders in carboxylic acid in aprotic solvents, where the reaction is quantitative. It is the purpose of the present paper to show how these disadvantages may be overcome in the analysis of binary acid mixtures