Potassium dichromate is prepared from chromite ore (`FeCr_(2)O_(4)`) in the following steps.
Step (1): Preparation of sodium chromate
`4FeCr_(2)O_(4)+16NaOH +7O_(2) to 8Na_(2)CrO_(4)+2Fe_(2)O_(3)+8H_(2)O`
Step (2): Conversion of sodium chromate into sodium dichromate
`2Na_(2)CrO_(4)+conc. H_(2)SO_(4) to Na_(2)Cr_(2)O_(7)+Na_(2)SO_(4)+8H_(2)O`
Step(3): Conversion of sodium dichromate to potassium dichromate
`Na_(2)Cr_(2)O_(7)+2KCl to K_(2)Cr_(2)O_(7)+2NaCl`
Potassium dichromate being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.
The dichromate ion `(Cr_(2)O_(7)^(2-))` exists in equilibrium with chromate `(CrO_(4)^(2-))` ion at pH 4. However, by changing the pH, they can be interconverted.
`2CrO42- to "Ac id "2HCrO_(4)- to "Acid "2Cr2O72-`
`Cr2O72- to "Base "2HCrO42- to "Base "2CrO42-`
