Sodium bichromate, also correctly designated by its systematic chemical name sodium dichromate and represented by the chemical formula Na2Cr2O7, is an inorganic hexavalent chromium compound appearing as orange-red crystalline granules or powder that dissolve readily in water to produce a strongly coloured orange solution. It is closely related in both chemical composition and photographic application to potassium bichromate - the two compounds sharing the same dichromate anion but differing in their cation, with sodium replacing potassium - and the two are largely interchangeable in most photographic formulations, with the choice between them typically determined by availability, cost, and the specific solubility requirements of the application.
In photographic chemistry, sodium bichromate finds application across several distinct processing treatments including chemical intensification, toning, and bleaching of developed photographic negatives and prints. As an intensifying agent, sodium bichromate based chrome intensifiers exploit the oxidising properties of the hexavalent chromium in the dichromate anion to bleach and rehalogenize the silver image, which is then redeveloped to produce a denser, more contrasty result than the original development achieved. The intensification mechanism involves the conversion of metallic silver to silver chromate or silver halide compounds through oxidation, followed by redevelopment that rebuilds and enhances the image density, with the chromium compounds contributing additional opacity to the redeveloped image.
In toning applications, sodium bichromate is used as a component in certain chromium based toning formulations that produce cool, blue-grey to neutral tones in photographic prints through a bleach and redevelopment sequence analogous to that used in intensification. The specific tone produced depends on the formulation of the toning solution, the redeveloper used, and the processing conditions applied, with different combinations producing a range of tones from warm reddish-brown through to cool blue-black. In bleaching applications, sodium bichromate serves as an oxidising bleaching agent that converts the silver image to a soluble or rehalogenizable form preparatory to subsequent toning, intensification, or complete removal of the silver image.
Sodium bichromate is also used in certain alternative and historical photographic printing processes - including gum bichromate printing, carbon printing, and carbro printing - where its property of hardening colloids such as gum arabic and gelatin in proportion to the amount of ultraviolet light they receive is exploited to produce photographic images of distinctive aesthetic character. In these processes, a mixture of the colloid, a pigment or carbon tissue, and the bichromate sensitiser is coated onto paper or another support, exposed through a negative to ultraviolet light, and developed to wash away the unhardened, unexposed areas while retaining the hardened, exposed image areas.
The health and safety implications of sodium bichromate are essentially identical to those of potassium bichromate, as both compounds are hexavalent chromium compounds sharing the same toxicological profile. Sodium bichromate is classified as a known human carcinogen, a powerful skin and respiratory sensitiser capable of causing severe allergic reactions with repeated exposure, and a strong oxidising agent that presents fire and explosion hazards in contact with organic and flammable materials. All handling must be carried out with appropriate protective equipment including chemical resistant gloves, eye protection, and respiratory protection where dust exposure is possible, in well ventilated conditions, and all waste containing hexavalent chromium must be disposed of through appropriate hazardous waste channels in strict compliance with applicable environmental regulations.