By the action of concentrated sulphuric acid, sodium chromate gets converted into sodium dichromate in the manufacture of potassium dichromate.
Various steps involved are:
Step I Concentration of the chromite ore by Wilfley method.
Step II Conversion of the concentrated chromite ore into $Na _{2} CrO _{4}$. Concentrated chromite ore is fused with molten $Na _{2} CO _{3}$ in the presence of air when $Na _{2} CrO _{4}$ and $Fe _{2} O _{3}$ are formed.
Step III Conversion of $Na _{2} CrO _{4}$ into $Na _{2} Cr _{2} O _{7}$
Aqueous solution of $Na _{2} CrO _{4}$ is acidified to obtain $Na _{2} Cr _{2} O _{7}$
$Na _{2} CrO _{4}+ H _{2} \underset{(Dil)}{SO _{4}} \longrightarrow Na _{2} Cr _{2} O _{7}+ H _{2} O$