Potassium dichromate is the inorganic compound with the formula K2Cr2O7. An orange solid, it is used in diverse laboratory and industrial applications. As with all hexavalent chromium compounds, it is chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.
Production Potassium dichromate is usually prepared by the reaction of sodium dichromate and potassium chloride in the presence of an acid. Potassium dichromate precipitates, because it has a lower solubility than the corresponding sodium salt. Alternatively, it can be also obtained from potassium chromate by roasting chromite ore with potassium hydroxide:
FeCr2O4 + 2 KOH + 1.5 O2 → K2Cr2O7 + Fe(OH)2
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.
References
External links
Potassium Dichromate at The Periodic Table of Videos (University of Nottingham) National Pollutant Inventory – Chromium VI and compounds fact sheet IARC Monograph "Chromium and Chromium compounds" Gold refining article listing color change when testing metals with Schwerter's Solution Archived 2018-04-03 at the Wayback Machine
