d and f Block Elements
4.0 Potassium dichromate
4.0 Potassium dichromate
Preparation of potassium dichromate from chromate ore
Chromate ore i.e., $FeO$, $C{r_2}{O_3}$ is also chrome iron ore and it is converted in potassium dichromate as follows:
i) Concentration of ore: By washing the powdered chromate ore with water lighter impurities are removed leaving behind the heavier ore.
ii) Roasting: Conversion of chromite ore in sodium chromate:
- Concentrated ore is heated with sodium carbonate and lime stone in reverberatory furnace in air to form yellow sodium chromate.
- Lime stone is added to keep the reaction mixture porous.
$$4FeO.C{r_2}{O_3}{\text{ + 8N}}{{\text{a}}_2}{\text{C}}{{\text{O}}_3}{\text{ + }}{\text{ 7}}{{\text{O}}_2}{\text{ }} \to {\text{ 2F}}{{\text{e}}_2}{{\text{O}}_3}{\text{ + 8N}}{{\text{a}}_2}{\text{Cr}}{{\text{O}}_4}{\text{ + 8C}}{{\text{O}}_2}$$
- Sodium chromate is separated by dissolving in water.
iii) Conversion of sodium chromate into sodium dichromate:
- Sodium chromate on reaction with ${H_2}S{O_4}$(conc.) produce sodium dichromate. $$2N{a_2}Cr{O_4}{\text{ + }}{H_2}S{O_4}{\text{ }} \to {\text{ N}}{{\text{a}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ + N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}{\text{ + }}{{\text{H}}_2}{\text{O}}$$
- Sodium sulphate is separated by concentration of solution on filtration aqueous solution $N{a_2}C{r_2}{O_7}$ is obtained.
iv) Conversion of sodium dichromate into potassium dichromate:
- Sodium dichromate on reaction with $KCl$ form ${K_2}C{r_2}{O_7}$. $${\text{N}}{{\text{a}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ 2KCl }} \to {\text{ }}{K_2}C{r_2}{O_7}{\text{ + 2NaCl}}$$
- Less soluble ${K_2}C{r_2}{O_7}$ is separated on cooling and it is purified by crystallization.
