® Na CrO + Fe O + CO The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na Cr O . 2H O can be crystallised. 2Na CrO + H + ® Na Cr O + Na + + H O Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.
Na Cr O + KCl ® K Cr O + NaCl Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution. The oxidation state of chromium in chromate and dichromate is the same. CrO – + 2H + ® Cr O – + H O Cr O – + OH - ® CrO – + H O The structures of chromate ion, CrO – and the dichromate ion, Cr O – are shown below.
The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr–O–Cr bond angle of °. Sodium and potassium dichromates are strong oxidising agents; the sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis. In acidic solution, its oxidising action can be represented as follows: Cr O – + 14H + + 6e – ® 2Cr + + 7H O ( E o = .33V) Thus, acidified potassium dichromate will oxidise iodides to iodine, sulphides to sulphur, tin(II) to tin(IV) and iron(II) salts to iron(III).
The half-reactions are noted below: I – ® 3I + e – ; Sn + ® 3Sn + + e H S