Potassium dichromate

Structure

X-ray crystallography shows that it is a salt. Two polymorphs are known, but the structure of the dichromate anion is the same in each. It ionizes when dissolved in water, releasing Cr₂O2−7. Thus in aqueous solution, it is the dichromate ion that matters in terms of chemical reactions and environmental impact. This anion is a corner-shared bitetrahedron, resembling pyrophosphate.

Reactions

Inorganic

When heated strongly, it decomposes with the evolution of oxygen.

4 K₂Cr₂O₇ → 4 K₂CrO₄ + 2 Cr₂O₃ + 3 O₂

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 (CrO2−4). For example, potassium chromate is produced industrially using potassium carbonate:

K₂Cr₂O₇ + K₂CO₃ → 2 K₂CrO₄ + CO₂

Treatment with cold sulfuric acid gives red of chromic anhydride (chromium trioxide, CrO₃):

K₂Cr₂O₇ + 2 H₂SO₄ → 2 CrO₃ + 2 KHSO₄ + H₂O

On heating with concentrated acid, oxygen is evolved:

2 K₂Cr₂O₇ + 8 H₂SO₄ → 2 K₂SO₄ + 2 Cr₂(SO₄)₃ + 8 H₂O + 3 O₂

Potassium dichromate is readily reduced by sulfur dioxide:

K₂Cr₂O₇ + H₂SO₄ + 3 SO₂ → K₂SO₄ + Cr₂(SO₄)₃ + H₂O

In addition to providing a route to chromium(III) sulfate, this reaction was once the basis of a test for sulfur dioxide.

Potassium trichromate (K₂Cr₃O₁₀) is prepared by evaporating a solution of potassium dichromate in nitric acid (d = 1.19) over concentrated sulfuric acid at low temperature. The tetrachromate (K₂Cr₄O₁₃) is prepared by evaporating a solution of the trichromate in nitric acid (d = 1.4-1.5) slowly on a sand bath.

Potassium dichromate reacts with hydrogen chloride to give chromyl chloride:

K₂Cr₂O₇ + 6 HCl → 2 CrO₂Cl₂ + 2 KCl + 3 H₂O

Analytical reagent

Because it is non-hygroscopic, potassium dichromate was a common reagent in classical "wet tests" in analytical chemistry.

Organic chemistry

Potassium dichromate is an oxidising agent in organic chemistry. It is a member of a large collection of chromium-based oxidants. Oxidations are often conducted in the presence of sulfuric acid, which favors the formation of chromium trioxide, which is the active oxidation reagent.

It is milder and more selective than potassium permanganate. It is often used interchangeably with sodium dichromate, although the use of chromates has declined owing to environmental concerns (see Hexavalent chromium). Especially in the presence of acids such as sulfuric acid, it is well known 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. Tertiary alcohols are not oxidized.

Oxidations by dichromate are accompanied by a color change from yellow-orange for Cr(VI) to green for Cr(III). The color change, a colorimetric test, exhibited can be used as a test to distinguish aldehydes from ketones. Aldehydes are oxidized further, whereas ketones resist oxidation by dichromate salts.

In some cases, potassium dichromate can cleave C=C bonds.

Potassium dichromate is a classical reagent for the preparation of naphthoquinones.

Leather

Potassium dichromate has few major applications, as the sodium salt is dominant industrially. The main use is as a precursor to potassium chrome alum, used in leather tanning.

Wood treatment

Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produces deep, rich browns that cannot be achieved with modern color dyes. It is a particularly effective treatment on mahogany.

Natural occurrence

Potassium dichromate occurs naturally as the rare mineral lópezite. It has only been reported as vug fillings in the nitrate deposits of the Atacama Desert of Chile and in the Bushveld igneous complex of South Africa.

Safety

Potassium dichromate is a prevalent allergen in patch tests (4.8%). Its presence in cement can cause contact dermatitis in construction workers after extended exposure. In general, it is one of the most common causes of chromium dermatitis. Aquatic organisms are vulnerable to poisoning by dichromate salts, but far less so than organic pollutants.

As with other Cr(VI) compounds, potassium dichromate is carcinogenic. The compound is also corrosive and exposure may damage eyes. Human exposure further causes impaired fertility.