Answer 1

Potassium dichromate is prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr₂O₃) with sodium or potassium carbonate in free excess of air. The reaction with sodium carbonate occurs as follows:

`4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 -> 8Na_2CrO_4+2Fe_2O_3+8CO_2`

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na₂Cr,0₇.2H₂0 can be crystallised.

`2Na_2CrO_4+2H^+ -> Na_2Cr_2O_7+2Na^(+) + H_2O`

Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.

`Na_2Cr_2O_7 + 2KCL -> K_2Cr_2O_7 + 2NaCl`

Orange crystals of potassium dichromate crystallise out. The chromates and dichromates depending upon p^H of the solution. If p^H of potassium dichromate is increased it is converted to yellow potassium chromate

`2CrO_4^(2-) + 2H^+ -> Cr_2O_7^(2-) + H_2O`

`Cr_2O_7^(2-) + 2OH^(-) -> 2CrO_4^(2-) + H_2O`