GB1351464A - Bleaching of silver developed from silver halide emulsions - Google Patents

Abstract

1351464 Bleaching photographically developed silver MINNESOTA MINING & MFG CO 13 April 1972 [14 April 1971] 9420/71 Heading G2C A process for bleaching photographically developed silver comprises contacting the silver with an aqueous solution containing azodicarbonamide and/or at least one substituted azodicarbonamide preferably having the following general formula in which R<SP>1</SP> to R<SP>4</SP> represent H, alkyl, cycloalkyl, substituted alkyl (e.g. alkoxyalkyl, hydroxyalkyl and alkyl substituted by a heterocyclic radicle), an unsaturated aliphatic group which may be substituted (e.g. allyl), aryl, substituted aryl (e.g. haloaryl, alkaryl and alkoxyaryl), heterocyclic or pairs of groups R<SP>1</SP> -R<SP>2</SP> and/or R<SP>3</SP>-R<SP>4</SP> complete a heterocyclic ring with the associated N atom, except that R<SP>1</SP> - R<SP>4</SP> do not all represent H. Preferably at least one of the amide nitrogen atoms is covalently bonded to a group having a centre of positive charge e.g. a quaternary ammonium grouping, the substituted azodicarbonamide having at least one negatively charged moiety e.g. I# or toluene - p - sulphonate, associated with it. The compounds given are the di-toluene - p - sulphonate and the di-iodide of N, N' - di (ethyl- 2- trimethylammonium) azodicarbonamide. The solution may also contain free halide ions, preferably Br<SP>-</SP>, and is buffered to a p H of 4 to 7 by, for example, sodium acetate/acetic acid. The solution may also contain a silver halide solvent compatible with the optionally substituted azodicarbonamide e. g. sodium thiocyanate.