Potassium Dichromate – Preparation, Properties and Uses

Potassium Dichromate is an ionic solid and well-known chemical reagent with the formula K2Cr2O7. It is an odourless red-orange coloured solid with a crystalline nature.

It has a non-deliquescent nature which has led to its use in analytical chemistry over other salts. This compound occurs in its natural form as a unique mineral called lopezite which has only been found in South Africa and Chile.

Preparation of Potassium Dichromate

• Potassium Dichromate can be obtained by reacting potassium chloride with sodium dichromate which is gotten by the roasting of chromite ores with sodium carbonate to produce sodium chromate which is then treated with sulfuric acid.

4 FeO.Cr2O3 + 8 Na2CO3 + 7 O2 = 8 Na2CrO4 + 2 Fe2O3 + 8 CO2

2 Na2CrO4 + H2SO4 = Na2Cr2O7 + H2O + Na2SO4

Na2Cr2O7 + 2 KCl =K2Cr2O7 + 2 NaCl

• Potassium Dichromate can also be produced by the acidification of yellowish potassium chromate.

K2CrO4 + H2SO4 = K2Cr2O7 + H2O + K2SO4

Chemical Properties of Potassium Dichromate

• Acidified potassium dichromate reacts with methanol to form menthone.
• When heated, potassium dichromate breaks down to form potassium chromate, chromium oxide, and oxygen.

4 K2Cr2O7  = 4 K2CrO4 + 2 Cr2O3 + 3 O2

• Potassium dichromate reacts with potassium carbonate to release potassium chromate and carbon dioxide.

K2Cr2O7 + K2CO3 = 2 K2CrO4 + CO2

• Its treatment with cold sulfuric acid produces chromium trioxide, water, and potassium sulfate.

K2Cr2O7 + 2 H2SO4 = 2 Cr2O3 + H2O + 2 KHSO4

While its reaction with concentrated or diluted sulfuric acid produces potassium sulfate water, chromic sulfate, and three moles of oxygen.

K2Cr2O7 + 4 H2SO4 = K2SO4 + 4 H2O + 3 O + Cr2(SO4)3

• It oxidizes hydrogen sulphide to produce sulphur.

K2Cr2O7 + 4 H2SO4 + 3 H2S = 3 S +  K2SO4.+ Cr2(SO4)3 + 7 H2O

• Its reaction with ethanol and sulfuric acid produces acetic acid, potassium sulfate, chronic sulfate, and water.

2 K2Cr2O7 + 3 C2H5OH + 8 H2SO4 = 3 CH3COOH + 2 K2SO4 + 2 Cr2(SO4)3 + 11 H2O

• It reacts with alkalis like potassium hydroxide to form yellowish potassium chromate.

K2Cr2O7 + 2 KOH = H2O + 2 K2CrO4

• It oxidizes ferrous salts to produce ferric salts.

K2Cr2O7 + 7 H2SO4 + 6 FeSO4 = 3 Fe2(SO4)3 + K2SO4 + 4 H2O + Cr2(SO4)3

• Its reaction with potassium iodide and sulfuric acid helps release iodine.

K2Cr2O7 + 7 H2SO4 + 6 KI = 3 I2 + 4 K2SO4 + Cr2(SO4)3 + 7 H2O

• In the chromyl chloride test, it reacts with salt-comprising chlorides like potassium chloride and concentrated sulfuric acid to produce chromyl chloride vapour and potassium bisulfate.

K2Cr2O7 + 4 KCl + 6 H2SO4 = 2 CrO2Cl2 + 6 KHSO4 + 3 H2O

Physical Properties of Potassium Dichromate

Potassium Dichromate is an odourless red-orange crystalline ionic solid with a molar mass of 294.185 g/mol, a density of 2.676 g/cm3, and a refractive index of 1.738.

It boils at 500°C (932°F) and melts at 398°C (748°F). It is soluble in water and insoluble in both acetone and alcohol.

Uses Of Potassium Dichromate

• It is most often used as an oxidizing agent.
• It is used as an analytical reagent.
• It is used as a component in the production of potassium chloride alum which is used in the tanning of leather.
• It is used in the production of cement to improve its texture and density.
• It is used to analyze the purity of silver.
• It is used in the treatment of certain kinds of wood.
• It was previously used as a constituent in the production of chromic acid which was used to clean glassware until the practice was stopped.
• In the printing industry, It is used in the printing of photography screens and as an oxidizing agent. It can also be used in the production of negatives.
• Potassium dichromate can also be used as a hardening agent.
• It is used in the production of waxes and glues.

Health Concerns

Potassium Dichromate is lethal and highly toxic to human health, so it should be handled with utmost care.

• If ingested, it could lead to massive damages to the intestines and the stomach and can also lead to death in some cases.
• Contact with the skin causes chromium dermatitis – inflammation of the skin, which is hard to treat and chronic.
• Exposure to the eye could lead to blindness or severe damage to the eye.
• Acute exposure to this substance can lead to vomiting, and excess urination while long-term exposure can cause infertility in men and can also be lethal to unborn children.
• It can also lead to severe damage to your respiratory organs and probably death.
• It can also cause cancer when mixed with other compounds from the Cr(VI) group.
• It is also highly corrosive and non-combustible.

In addition, due to its potassium ion, it can cause heart attack, low blood pressure, and other heart diseases.

Aquatic animals, rodents, and bunnies aren’t left out either as this compound can also harm them.