What does bichromate mean?
Bichromate is a chemistry term for a salt of dichromic acid — that is, a compound containing the dichromate ion, in which two chromium atoms are bonded to seven oxygen atoms. The best-known example is potassium bichromate (more commonly called potassium dichromate), a vivid orange-red crystalline solid prized as a powerful oxidising agent in laboratory work and industry. The prefix 'bi-' reflects the older naming convention indicating two chromium atoms, though modern chemists overwhelmingly prefer 'dichromate,' leaving 'bichromate' somewhat dated in tone. It survives most prominently in art and photography through the gum bichromate printing process, in which light-sensitive dichromate salts harden gum arabic on paper to create richly toned images. Because these compounds are toxic hexavalent chromium salts, the word often appears in safety and environmental contexts. For readers, it signals technical, scientific register rather than everyday conversation.
nounA salt of a hypothetical acid, H₂Cr₂O₇, in which two atoms of chromium are combined with seven atoms of oxygen; now more commonly called a dichromate.
- Chemistry: any salt containing the dichromate ion Cr₂O₇²⁻, formed from a base and dichromic acid; known for its orange-red colour and strong oxidising properties.
"The photographer prepared a gum bichromate emulsion for the printing process."
"Potassium bichromate was once widely used as an oxidising agent in analytical chemistry laboratories."
"Her portfolio featured rich, painterly prints made by the nineteenth-century gum bichromate process."
Regular plural, used when referring collectively to different bichromate compounds.
"Various bichromates were once staples of the photographic darkroom."
Before digital photography, artists printed stunning images by coating paper with gum arabic and potassium bichromate — a chemical that hardens when exposed to light.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of bichromate
The word combines the Latin-derived prefix 'bi-' ('two') with 'chromate,' itself built on Greek 'chrōma' ('colour') via the element name chromium, coined in the late eighteenth century because its compounds display vivid colours. The '-ate' ending follows standard English chemical nomenclature for salts of oxyacids. Cognates sharing this Greek root include chromatic, chrome, and chromosome. The older 'bichromate' form has largely given way to 'dichromate,' which uses the Greek-derived 'di-' instead of the Latin 'bi-.'
How bichromate is actually used
Formal and largely technical; encountered mainly in chemistry texts and historical discussions of photography. In modern chemical nomenclature, 'dichromate' is strongly preferred, so 'bichromate' can sound dated or specialised outside fixed terms like 'gum bichromate.'
Easily confused with bichromate
Dichromate is the modern preferred name for exactly the same class of salts (containing the Cr₂O₇²⁻ ion); 'bichromate' is the older term, still used in traditional names like gum bichromate printing.
A chromate contains only one chromium atom per unit (the CrO₄²⁻ ion), whereas a bichromate contains two chromium atoms (the Cr₂O₇²⁻ ion).