The anilic acid number,an analytical index for testing dibasic acid anhydrides

For testing the purity of dibasic acid anhydrides and for analysing mixtures of these anhydrides with the corresponding free acids the determination of a new index number is recommended, indicated as the “anilic acid number”. The process is based on the fact that these acid anhydrides, when dissolved in acetone, under suitable experimental conditions can be added readily and quantitatively to aniline with resulting formation of the corresponding anilic acids.Immediately after addition, titration is effected with alcoholic potassium hydroxide on phenolphthalein. Pure dibasic acid anhydrides in this case yield the calculated anilic acid number. In the presence of free acids the values found are too high. The free acid content can be easily determined from the difference between the theoretical and the found anilic acid number.