METAL REDUCTION REACTIONS OF The MELANINS: SILVER AND FERRIC FERRICYANIDE REDUCTION BY VARIOUS REAGENTS IN VITRO

Test tube reactions of 163 reagents with silver solutions are recorded in tabular form. The silver solutions used were silver nitrate at pH 4 and at pH 6, ammoniacal silver nitrate, all at about 25°C., and Gomori's methenamine silver at 60°C. Ferric ferricyanide reduction is also included in the tables for comparison. Of the substances tested 30 quite promptly reacted with acid silver nitrate as well as the other reagents, 28 gave delayed reactions with acid, prompt with alkaline silver, 24 gave variable acid, and delayed alkaline silver reactions, 27 gave no reaction with acid silver and delayed reactions with alkaline, 16 reacted slowly with hot methenamine silver and not or scarcely at all with cold ammoniacal silver. The remaining 38 reactants did not blacken with silver solutions. Also tabulated are the pigment formation reactions of 13 polyphenols, indoles and sulfur amino acids with chromic acid, potassium bichromate, potassium chromate, periodic acid, sodium iodate and potassium chlorate.