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PRIMARY COLOR DEVELOPMENT 369
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This reaction opens up the entire field of azo dye chemistry for possible application to color development. It may be that the latent image does not have a sufficient potential to oxidize the hydrazo compound. In that case, perhaps the quinones formed by the action of the latent image upon hydroquinone would serve. These are relatively strong oxidizers, especially in the presence of halides, which would serve to remove the silver from solution. The hydrazo compounds can be formed by the controlled reduction of azo dyes, usually by means of alkaline alcohol and zinc dust, sodium amalgam, etc. They can also be prepared by the reduction of nitro compounds with zinc dust and alkali.
H H
alcohol
H H R— N02 Zn + KOH> R— N— N— R
In the first reaction, unsymmetrical hydrazo compounds are formed. In the second, symmetrical compounds are formed. Another possibility lies in the use of the leuco di and tri-phenyl methanes, together with hydroquinone. The quinones that are formed should oxidize the leuco derivative to the dye. If this is insufficient, then it is possible to convert the silver into a substance that is capable of oxidizing action. The disclosures of Willis and Kopp who oxidized aniline and other organic substances by dichromate or dichromate reduction products, have already been discussed. The production of dyes by such a procedure was suggested by Gusserow and Andresen (cf. above). These experimenters did not utilize a silver image, but one composed of chromic and chromous oxides formed by the action of light on dichromate sensitized emulsions. The Neue Photographische Gesellschaft in a series of disclosures (Eng. P. 18370/03, 10898/04, and 21584/06; Ger. P. 157411, 180947, and 180948) showed how a silver image could be converted into one of manganese oxide. This could then be made to react with amines or phenols to form colored residues. The silver image is first treated with one of the following:
A. Potassium ferricyanide, 0.5% solution 100 parts Manganous sulphate, 2 % solution 20 parts Potassium bromide, 10% solution 15 parts
B. Potassium ferricyanide, 0.5% solution 100 parts Manganous sulphate, 2% solution 25 parts