4.1 Properties of potassium dichromate

  d and f Block Elements

• On reaction with $KOH$, it forms yellow coloured potassium chromate. $${K_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ 2KOH }} \to {\text{ }}{K_2}C{r_2}{O_7}{\text{ + 2}}{{\text{H}}_2}{\text{O}}$$

• In acidic medium chromate ions are changed in dichromate ion and reverse in alkaline medium. $$C{r_2}{O_7}^{ - 2}{\text{ + }}{{\text{H}}_2}{\text{O }} \rightleftharpoons {\text{ Cr}}{{\text{O}}_4}^{ - 2}{\text{ + 2}}{{\text{H}}^ + }$$

• It acts as a strong oxidizing agent in acidic medium.

• During this process potassium dichromate ($C{r^{ + 6}}$) is converted to chromium sulphate ($C{r^{ + 3}}$). $$C{r_2}{O_7}^{ - 2}{\text{ + }}{{\text{H}}_2}{\text{O }} \rightleftharpoons {\text{ Cr}}{{\text{O}}_4}^{ - 2}{\text{ + 2}}{{\text{H}}^ + }$$

• Dichromate ion gain six electrons and acts as oxidizing agent.

• Action of $FeS{O_4}$: It oxidise ferrous sulphate to to ferric sulphate and the colour of reaction mixture changes from orange red to green. $${K_2}C{r_2}{O_7}{\text{ + 6}}FeS{O_4}{\text{ + 7}}{H_2}S{O_4}{\text{ }} \to {\text{ }}{{\text{K}}_2}S{O_4}{\text{ + C}}{{\text{r}}_2}{\left( {S{O_4}} \right)_3}{\text{ + 3}}F{e_2}{(S{O_4})_3}{\text{ + 7}}{{\text{H}}_2}{\text{O}}$$

• Action of Potassium Iodide: It oxidize iodides in iodine and from brown colored solution with the reduction ${K_2}C{r_2}{O_7}$ in chromic sulphate. $${K_2}C{r_2}{O_7}{\text{ + 6KI + 7}}{H_2}S{O_4}{\text{ }} \to {\text{ 4}}{{\text{K}}_2}S{O_4}{\text{ + C}}{{\text{r}}_2}{\left( {S{O_4}} \right)_3}{\text{ + 7}}{{\text{H}}_2}{\text{O + 3}}{{\text{I}}_2}$$

• Action of ${H_2}S$: This is oxidized by ${K_2}C{r_2}{O_7}$ in pale yellow precipitate of sulphur. $${K_2}C{r_2}{O_7}{\text{ + 4}}{H_2}S{O_4}{\text{ + 3}}{{\text{H}}_2}{\text{S}} \to {\text{ }}{{\text{K}}_2}S{O_4}{\text{ + C}}{{\text{r}}_2}{\left( {S{O_4}} \right)_3}{\text{ + 7}}{{\text{H}}_2}{\text{O + 3S}}$$

• Action of $S{O_2}$: It oxidize $S{O_2}$ in ${H_2}S{O_4}$ and color of the solution changes in green due to reduction of ${K_2}C{r_2}{O_7}$ in chromic sulphate. $${K_2}C{r_2}{O_7}{\text{ + }}{H_2}S{O_4}{\text{ + 3S}}{{\text{O}}_2} \to {\text{ }}{{\text{K}}_2}S{O_4}{\text{ + C}}{{\text{r}}_2}{\left( {S{O_4}} \right)_3}{\text{ + }}{{\text{H}}_2}{\text{O}}$$

• Oxidation of Alcohols: It oxidizes primry alcohols to aldehydes and then in carboxylic acids while Secondary alcohols are oxidized to form ketones. \[{\text{C}}{{\text{H}}_3}{\text{ - C}}{{\text{H}}_2} - OH{\text{ }}\xrightarrow[{dil.{H_2}S{O_4}}]{{{K_2}C{r_2}{O_7}}}{\text{ C}}{{\text{H}}_3}CHO{\text{ }}\xrightarrow[{dil.{H_2}S{O_4}}]{{{K_2}C{r_2}{O_7}}}{\text{ C}}{{\text{H}}_3} - COOH\]

• Mixture of potassium Dichromate, $NaCl$, $KCl$ and ${H_2}S{O_4}$ on heating form vapours of chromyl chloride which when cooled gives oily red liquid. $${K_2}C{r_2}{O_7}{\text{ + 6}}{H_2}S{O_4}{\text{ + 4NaCl}} \to {\text{ 2KH}}S{O_4}{\text{ + 4NaHS}}{{\text{O}}_4}{\text{ + 2Cr}}{{\text{O}}_2}{\text{C}}{{\text{l}}_2}{\text{ + 3}}{{\text{H}}_2}{\text{O}}$$

• It is used as oxidizing agent in tanning of leather, in dying and to decrease the glass wares.
  

• Used in manufacture of chromium compounds like lead chromate, chrome alum, inks, etc.

• Used in volumetric analysis for the determination of amount of FAS (Mohr's Salt) and iodine.

• Used in detection of chloride ion by chromyl chloride test.