CBSE Class 12 Chemistry Notes: d and f Block Elements – Potassium Dichromate

Potassium Dichromate, K₂Cr₂O₇

Preparation

It is prepared from the ore called chromate or ferrochrome or chrome iron, FeO.Cr₂O₃. The various steps involved are

(a) Preparation of sodium chromate

(b) Conversion of sodium chromate into sodium dichromate.

(c) Conversion of sodium dichromate into potassium dichromate.

Properties

It forms orange red crystals. It is moderately soluble in cold water but freely soluble in hot water.

1. Action of heat

When heated, it decomposed to its chromate

2. Action of alkalis

With alkalis it is converted into chromate which on acidifying gives back dichromate.

In dichromate solution the  ions are in equilibrium with  ions at pH = 4.

3. Action of conc. H₂SO₄ solution

(a) In cold conditions

(b) In hot conditions

4. Oxidising properties
It is a powerful oxidising agent. In the presence of dil. H₂SO₄ it furnishes 3 atoms of available oxygen.

Some of the oxidizing properties of K₂Cr₂O₇ are
(a) It liberates I2 from KI

(b) It oxidises ferrous salts to ferric salts

(c) It oxidises S2 to S

(d) It oxidises nitrites to nitrates

(e) It oxidises SO2 to

(f) It oxidises ethyl alcohol to acetaldehyde and acetic acid.

5. Chromyl chloride test

When heated with conc. HCl or with a chloride in the presence of sulphuric acid, reddish brown vapours of chromyl chloride are obtained.

Thus reaction is used in the detection of chloride ions in qualitative analysis.

Uses
1. In volumetric analysis for the estimation of Fe²⁺ and I^–.
2. In chrome tanning in leather industry.
3. In photography and in hardening gelatin film.

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