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The chromate and dichromate ions are fairly strong oxidizing agents. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated Cr 3+ ion is produced. Cr 2 O 2− 7 + 14 H + + 6 e − → 2 Cr 3+ + 7 H 2 O ε 0 = 1.33 V. In alkaline solution chromium(III) hydroxide is produced.
The dichromate ion, Cr 2 O 2− 7, is predominant in more concentrated solutions, except at high pH. The species H 2 CrO 4 and HCr 2 O − 7 are not shown as they are formed only at very low pH. Predominance diagrams can become very complicated when many polymeric species can be formed, [ 10 ] such as in vanadates , molybdates , and tungstates .
A dianion is an anion with a net charge of −2. While there exist many stable molecular dianions, such as BeF 4 2− and MgF 4 2− , [ 1 ] thus far no stable atomic dianion has been found: Electron shielding and other quantum mechanical effects tend to make the addition of another electron to an atomic anion unstable.
Ammonium dichromate is an inorganic compound with the formula (NH 4) 2 Cr 2 O 7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state , commonly known as hexavalent chromium .
Sodium dichromate is the inorganic compound with the formula Na 2 Cr 2 O 7. However, the salt is usually handled as its dihydrate Na 2 Cr 2 O 7 ·2 H 2 O . Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt. [ 1 ]
Silver dichromate is a chemical compound with the formula Ag 2 Cr 2 O 7. It is insoluble in water and decomposes when treated with hot water. Its anion has a charge of -2.
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The extent of dichromatism of material can be quantified by the Kreft's dichromaticity index (DI). It is defined as the difference in hue angle (Δh ab) between the colour of the sample at the dilution, where the chroma (colour saturation) is maximal and the colour of four times more diluted (or thinner) and four times more concentrated (or thicker) sample.