Mechanism of the “Hydrazine Effect” Caused by N'-acyl-N-phenylhydrazine

Abstract

N'-acyl-N-phenylhydrazines are known to cause an increase in speed and contrast of photographic development of silver halide grains. This “hydrazine effect” is attributed to the formation of phenyldiazene, being a strong nucleating agent. The overall mechanism is explained by the three successive reaction processes, namely, (1) oxidation of N'-acyl-N- phenylhydrazine, (2) removal of the N'-acyl group from N'-acyl-N-phenyldiazene by base catalyzed hydrolysis, and (3) nucleation of phenyldiazene to unexposed or underexposed silver halide grains. The oxidation of N'-acyl-N-phenylhydrazine is effected by the oxidation products of the developing agent such as p-bcnzoquinone and p-benzoquinone-2-monosulphonate. Of the three reactions, the process (2) is rate limiting, being influenced by the nature of the N'-acyl group and the alkalinity of the developer.