Potassium Dichromate (K2Cr2O7): Properties, Reactions and Uses

Potassium Dichromate is a hexavalent chromium compound. Its oxidising qualities are less potent than potassium permanganate. The inorganic chemical compound is considered a primary standard (in volumetric analysis). It is red in colour and highly toxic. Its solutions have shown long term stability and resistance to acid, light, most organic matter and chloride ions. Potassium Dichromate is often used in laboratories and industries as an oxidising agent. 

Keyterms: Potassium, Chromium, Compound, potassium permanganate, Solutions, Resistance, Oxidising agent, Solutions, Water, Alcohol

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What is Potassium Dichromate?

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Potassium Dichromate is an oxidising agent used for industrial applications. It is less potent than potassium permanganate. The chemical formula of this crystallised ionic solid is K₂Cr₂O₇. Produced by adding acid to potassium chromate it is soluble in water and insoluble in alcohol. It is a hexavalent compound meaning the chromium in it has a +6 oxidisation state. This makes it toxic to humans. Its popularity in lab experiments comes from its inability to absorb air moisture and dissolve in it.

Potassium dichromate Structure

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Important Formulae and Structure

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Potassium Dichromate can be obtained two ways:

• Through a reaction of potassium chloride with sodium dichromate. A heated sodium dichromate solution is mixed with a specified amount of potassium chloride and allowed to cool until red-orange potassium dichromate crystals form.

Na₂Cr₂O7 + 2KCl → K₂Cr₂O7 + 2NaCl

Potassium Dichromate

• Alternatively, it can be obtained from potassium chromate through roasting chromite ore with potassium hydroxide. 

K₂Cr₂O₇ → 2K⁺ + Cr₂O₇²⁻

Cr₂O₇²⁻ + H₂O ⇔ 2CrO₄²⁻ + 2H⁺

Potassium dichromate or K₂Cr₂O₇ has 294.185 g/mol molar mass. It has a density of 2.68g/cm². Its boiling and melting points are 500 °C and 398 °C respectively. The ionic compound is made up of two positively charged potassium ions (K⁺) and the negatively charged dichromate ion (Cr₂O₇^-). The two hexavalent chromium atoms are found attached to three oxygen atoms and also to a bridging oxygen atom. 

Crystalline Potassium Dichromate

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Chemical Properties of Potassium Dichromate

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Reaction with heat: Introducing heat to K₂Cr₂O₇ decomposes it into potassium chromate (K₂CrO₄) and produces O₂ gas.

4K₂Cr₂O₇ → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

Reaction with alkalis: When the solution of K₂Cr₂O₇ reacts with an alkali (ionic salt), a yellow solution is obtained because of the potassium chromate formation. 

K₂Cr₂O₇ (Orange-red) + 2KOH → 2K₂CrO₄(Yellow) + H₂O

The addition of acid changes the yellow colour of K₂CrO₄ back to orange-red due to the reformation of K₂Cr₂O₇.

2K₂CrO₂(Yellow) + H₂SO₄ → K₂Cr₂O₇(Orange-red) + K₂SO₄ + H₂O

Reaction on oxidization: K₂Cr₂O₇ is used as a powerful oxidizing agent. In the presence of dilute H₂SO₄, the following reaction occurs.

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

Reaction on Sulphuric acid or Chromyl Chloride Test: Potassium dichromate on reaction with concentrated sulphuric acid gives reddish-brown vapours of chromyl chloride. This reaction is also called Chromyl Chloride Test. The reaction is as follows:

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

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Physical Properties of Potassium Dichromate

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The physical properties of Potassium dichromate can be described as follows:

• It is a red-orange ionic compound when at room temperature.
• It is odourless.
• It is soluble in water and not soluble in alcohol.
• It is highly toxic.
• It is highly corrosive.

Solution of Potassium Dichromate

• It has a refractive index of 1.738.
• It is non combustible in nature.
• Its crystalline structure is triclinic (7 crystal system) and coordination geometry for the central atom chromium is tetrahedral.

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Uses of Potassium Dichromate

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The uses of potassium dichromate are as follows:

• The chromic acid prepared from potassium dichromate is used for cleaning glassware and etching materials. But this practice is no longer used primarily because of health and safety concerns around hexavalent chromium.
• It is used in cement to slow the setting of the mixture and increase the density and texture of the finished product.
• It is used in the estimation of volumetric analysis.
• Potassium dichromate is a common reagent in traditional “wet tests”. It is used in chrome tanning in the leather industry.
• A black titration with acidified potassium dichromate is used to determine the concentration of ethanol in a sample. 

Potassium dichromate in the titration process

• It is used to prepare chrome alum and several other industrially important chromium compounds.
• The mixture of potassium dichromate with approximately 35% nitric acid solution is known as Schwerter’s Solution. This solution is used to test the purity of the silver.
• Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. 

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Health Hazards of Potassium Dichromate

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• Potassium dichromate has a high potential for causing cancer. Prolonged exposure can cause severe respiratory problems and in some cases even death.
• A dose of 40g or less can prove fatal and cause heavy side effects.
• It burns the inner lining of the stomach and intestines if consumed.
• Exposure and consumption can cause symptoms like excess urination and vomiting and loose motion. 

Health hazards of Potassium dichromate

• The Potassium ion present in this compound has been known for the cause of heart diseases such as low blood pressure, heartbeat malfunction, heart attack, choking off blood veins.
• Skin also gets burnt by its corrosive nature.
• The compound is also corrosive and exposure may produce severe eye damage or blindness.
• Consequences of prolonged exposure include reproductive disease, loss of fertility in males, genetic disorders and harm to unborn children.

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Things to Remember

• Potassium Dichromate is a hexavalent chromium compound.
• Its solutions have shown long term stability and resistance to acid, light, most organic matter and chloride ions. 
• Potassium Dichromate is often used in laboratories and industries as an oxidising agent. 
•  The chemical formula of this crystallised ionic solid is K₂Cr₂O₇.
• Potassium Dichromate is produced through a reaction of potassium chloride with sodium dichromate.
• The chromic acid prepared from potassium dichromate is used for cleaning glassware and etching materials. But this practice is no longer used primarily because of health and safety concerns around hexavalent chromium.

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