The Determination of Creatinine with Sodium 3,5-dinitrobenzoate by Wilson

The reaction of picric acid with creatinine in alkaline solution (Jaffe’s reaction), which is the basis of the Folin method (1) for the determination of creatinine, has been thought to be either a reduction of the acid or else the formation of a red tautomer of creatinine picrate. Greenwald (2) obtained evidence that the tautomerism involves picric acid as well as creatinine, and that other nitro compounds give no color under the conditions used (24). The reaction, therefore, has been thought to be peculiar to this one acid. However, similar rearrangements probably account for the color in alkaline solution of many nitro compounds, such, for instance, as trinitrobenzoic acid, which dissolves in sodium carbonate to give a pale yellow solution of the sodium salt, but gives a deep red color with excess of sodium hydroxide. Any nitro compound which behaves in this way toward alkali is obviously unsuitable for use in place of picric acid in the Folin procedure. It has been found in this laboratory that 3, Sdinitrobenzoic acid gives a brilliant garnet-red with creatinine in alkaline solution and can be used for its determination. Under similar conditions, the acid alone gives no red. We have repeatedly purified the acid by recrystallization from alcohol and obtained a product which did not change in its color-producing qualities as purification proceeded. The chromogenic property reached a maximum with the maximum purity. On the other hand, one batch of the crude acid, as purchased, contained a yellow substance which influenced the reaction in such a way that the color obtained was very pale and very quickly destroyed. Greenwald and Gross (4), who have