Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
Potassium Dichromates are generally prepared from chromates which in turn are obtained by the fusion of chromite ore $$(Fe{Cr}_{2}{O}_{4})$$ with sodium or potassium carbonate in free excess of air.
$$4Fe{Cr}_{2}{O}_{4}+8{Na}_{2}{CO}_{3}+7{O}_{2}\rightarrow 8{Na}_{2}Cr{O}_{4}+2{Fe}_{2}{O}_{3}+8{CO}_{2}$$
The yellow solution of $${Na}_{2}Cr{O}_{4}$$ is filtered and acidified with $${H}_{2}{SO}_{4}$$ to give a solution from which orange sodium dichromate $${Na}_{2}{Cr}_{2}{O}_{7}.2{H}_{2}O$$ can be crystallised.
$$2{Na}_{2}Cr{O}_{4}+2{H}^{+}\rightarrow {Na}_{2}{Cr}_{2}{O}_{7}+2{Na}^{+}+{H}_{2}O$$
Sodium dichromate is more soluble than potassium dichromate. The latter is prepared by treating the solution of $${Na}_{2}{Cr}_{2}{O}_{4}$$ with $$KCl$$.
$$Na_{2}Cr_{2}O_{7}+2KCl\rightarrow {K}_{2}{Cr}_{2}{O}_{7}+2NaCl$$ (orange crystals)
On increasing the pH of a solution of $${K}_{2}{Cr}_{2}{O}_{7}$$, it behaves like a strong oxidising agent and it oxidises iodide to iodine sulphides to sulphur, tin (II) to tin (IV) and iron (II) salts to iron (III) salts.