The conversion of silver metal back to silver halide is accomplished by using a mild oxidizing agent known as a bleaching agent together with a water-soluble halide salt, such as potassium bromide, in a water solution. Together these are known as a photographic bleach. The bleaching agent is mild enough to not adversely affect the gelatin or dye in the emulsion, yet strong enough to take electrons from the silver metal in the presence of the halide, thus converting the silver back to silver halide. Typically, iron complexes such as iron EDTA (ethylenediamine tetraacetic acid), iron PDTA (propylenediamine tetraacetic acid) or less commonly ferricyanide [Fe(CN)63-] are used since they can supply the proper bleaching activity without harming the emulsion. These are all relatively nontoxic salts that are cheap, commercially available, and safe to handle. Two are commonly used in food products: EDTA is found in bread, baked goods, and pharmaceuticals , whereas ferricyanide is used to prevent table salt and foods from caking and as a blue pigment in cosmetics, inks, and paints [5,6]. Ferricyanide was formerly the photographic bleaching agent of choice because it was relatively easy to recover and reuse, but was largely removed from the marketplace because of public concern over the word "cyanide." However, ferrocyanide, or hexacyanoferrate, is an extremely stable, complex iron salt with very low toxicity compared to simple cyanide.
The reaction between the bleach and the metallic silver halide can be described in simplified fashion as:
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