Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Explanation

Potassium dichromate, `K_2Cr_2O_7` is prepared from iron chromite ore, `FeCr_2O_4` . Its preparation involves following steps:
i) Conversion of iron chromite ore into sodium chromate by sodium carbonate.
`4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 to 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2`
ii) Conversion of sodium chromate to sodium dichromate
`2Na_2CrO_4 + H_2SO_4 to Na_2Cr_2O_7 + Na_2SO_4 + H_2O`
iii) Conversion of sodium dichromate into potassium dichromate.
`Na_2Cr_2O_7 + 2KCl to K_2Cr_2O_7 + 2NaCl`
Here, orange crystals of potassium dichromate are separated out from the liquor. On increasing pH, potassium dichromate (orange) changes into potassium chromate (yellow).