Dichromate - Cr2O2?7

What is Dichromate?

Dichromate is an anion with the chemical formula Cr2O2−7. It is used as a strong oxidising agent in organic chemistry as well as a primary standard solution in volumetric analysis. The chromate ion and dichromate ions are interconvertible in aqueous solution. The most common compound known is potassium dichromate which is an orange crystalline solid which readily decompose to give potassium chromate and chromic oxide.

Cr2O2−7 Dichromate
Density 2.68 g/cm³
Molecular Weight/ Molar Mass 294.185 g/mol
Boiling Point 500 °C
Melting Point 398 °C
Chemical Formula Cr2O2−7

Dichromate Structure – Cr2O2−7

Dichromate Structure

Physical Properties of Dichromate – Cr2O2−7

Odour odourless
Appearance Red-orange crystalline solid
Valency 2
pH 4
Oxidation state +6
Solubility Moderately soluble in cold water and appreciably soluble in hot water.

Chemical Properties of Dichromate – Cr2O2−7

    • Acidified solution of dichromate forms a deep blue colour with peroxide due to the formation of [CrO(O2)2].

Cr2O72- + 4H2O2 + 2H+ → 2CrO5 + 5H2O

    • It reacts with hydrogen sulphide and oxidises it to sulphur, similarly it oxidises sulphites to sulphates, chlorides to chlorine, nitrites to nitrates, thiosulphates to sulphates and sulphur to stannic salts.

Cr2O72- + 3H2S + 8H+ → 2Cr3+ + 3S + 7H2O

Uses of Dichromate – Cr2O2−7

  • Used in photography for hardening of gelatin film.
  • used in chrome tanning in leather industry. It is also used in dyeing provides Cr(OH)3 as a moderent.
  • Used in volumetric estimation of ferrous salts, iodine and sulphites.
  • Used in the preparation of other chromium compounds such as chrome alum, chrome yellow and chrome red.

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