Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$): Preparation and Properties

Potassium dichromate, $\text{K}_2\text{Cr}_2\text{O}_7$, is manufactured industrially from chromite ore, $\text{FeCr}_2\text{O}_4$ (an iron-chromium spinel oxide, introduced earlier in this chapter as chromium's principal natural ore), in three successive stages.

Stage 1: Oxidative fusion to sodium chromate. Powdered chromite ore is fused with excess sodium carbonate in the presence of air (or another oxidizing agent) at high temperature, oxidizing the chromium(III) of the ore all the way to chromium(VI) as the yellow chromate ion:

The insoluble iron(III) oxide by-product is removed by filtration, leaving a solution of sodium chromate.

Stage 2: Acidification to sodium dichromate. The filtered sodium chromate solution is acidified (typically with sulfuric acid), which converts the yellow chromate ion into the orange dichromate ion, exploiting the chromate-dichromate equilibrium developed further in a dedicated exercise later in this chapter:

Stage 3: Conversion to the potassium salt. The sodium dichromate solution is treated with potassium chloride. Potassium dichromate is considerably less soluble than sodium dichromate, so it crystallizes out on cooling and can be separated by filtration, leaving the more soluble sodium chloride in solution:

Structure and appearance. The dichromate ion, $\text{Cr}_2\text{O}_7^{2-}$, consists of two $\text{CrO}_4$ tetrahedra sharing one corner oxygen atom, with a Cr-O-Cr bridge. Potassium dichromate itself is an orange, crystalline, water-soluble solid.

Oxidizing properties. Potassium dichromate is a powerful oxidizing agent in acidic medium, where chromium is reduced from the $+6$ state (in $\text{Cr}_2\text{O}_7^{2-}$) to the $+3$ state (as the green $\text{Cr}^{3+}$ ion), corresponding to the reduction half-reaction

$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \longrightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$ …