Potassium dichromate is usually prepared by the reaction of potassium chloride on sodium dichromate. Alternatively, it can be also obtained from potassium chromate by roasting chromite ore with potassium hydroxide. It is soluble in water and in the dissolution process it ionizes:
K2Cr2O7 → 2 K+ + Cr2O72−
Cr2O72− + H2O ⇌ 2 CrO42− + 2 H+
Potassium dichromate is an oxidising agent in organic chemistry, and is milder than potassium permanganate. It is used to oxidize alcohols. It converts primary alcohols into aldehydes and, under more forcing conditions, into carboxylic acids. In contrast, potassium permanganate tends to give carboxylic acids as the sole products. Secondary alcohols are converted into ketones. For example, menthone may be prepared by oxidation of menthol with acidified dichromate. Tertiary alcohols cannot be oxidized.
In an aqueous solution the color change exhibited can be used to test for distinguishing aldehydes from ketones. Aldehydes reduce dichromate from the +6 to the +3 oxidation state, changing color from orange to green. This color change arises because the aldehyde can be oxidized to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidized further, and so the solution will remain orange.
When heated strongly, it decomposes with the evolution of oxygen.
4K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3O2
When an alkali is added to an orange red solution containing dichromate ions, a yellow solution is obtained due to the formation of chromate ions. For example, potassium chromate is produced industrially using potash:
K2Cr2O7 + K2CO3 → 2 K2CrO4 + CO2
The reaction is reversible.
Treatment with cold sulphuric acid gives red crystals of chromic anhydride (CrO3):
K2Cr2O7 + 2H2SO4 → 2CrO3 + 2 KHSO4 + H2O
On heating with concentrated acid, oxygen is evolved:
2 K2Cr2O7 + 8H2SO4 → 2 K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3O2