Potassium DiChromate

Potassium dichromate (K₂Cr₂O₇) is a bright red-orange crystalline compound widely used in laboratories and industries as an oxidizing agent. Its non-deliquescent nature makes it preferable to other dichromates. However, as a hexavalent chromium compound, it is highly toxic and harmful to health.

1.0Introduction

Potassium dichromate, an inorganic compound with a bright orange colour, is widely used as an oxidizing agent in laboratories and industries. It is produced by reacting chromates with sodium or potassium carbonate. Although it is less potent than potassium permanganate, it is valued for its stability in acid and resistance to light and organic matter. Potassium dichromate is used in cleaning glassware, etching, and photographic screen printing. However, it is highly toxic and must be handled carefully due to its health hazards. As a dichromic acid dipotassium salt, it is one of the most significant and widely used chromium compounds in inorganic chemistry.

2.0Structure of Potassium Dichromate

The structure of potassium dichromate (K₂Cr₂O₇) consists of two potassium ions (K^-) and one negatively charged dichromate ion (Cr₂O₇^2-). In this ionic compound, each of the two hexavalent chromium atoms (oxidation state +6) are  bonded to three oxygen atoms, along with a bridging oxygen atom. The molar mass of potassium dichromate is 294.185 g/mol.

3.0Preparation of Potassium Dichromate

Potassium dichromate is typically prepared from chromate, derived by fusing chromite ore  (FeCr₂O₄) with sodium carbonate in the presence of oxygen. The reaction with sodium carbonate is as follows:

1. Conversion of Chromite Ore to Sodium Chromate: Chromite ore (FeCr₂O₄) is fused with sodium carbonate (Na₂CO₃) in the presence of oxygen, producing sodium chromate (Na₂CrO₄). The chemical reaction is as follows:

4FeCr₂O₄​(s) + 8Na₂​CO₃​(s) + 7O_2​(g) → 8Na₂​CrO₄(s) + 2Fe₂​O_3​(s) + 8CO₂​(g)

2. Conversion of Sodium Chromate to Sodium Dichromate: The sodium chromate solution is filtered and acidified with sulfuric acid (H₂SO₄), resulting in the formation of sodium dichromate (Na₂Cr₂O₇·2H₂O). The reaction is:

2Na₂​CrO_4​(s)+2H⁺(aq)→ Na_2​Cr₂​O₇​(s) + 2Na⁺(aq) + H₂​O(l)

3. Conversion of Sodium Dichromate to Potassium Dichromate: Potassium dichromate (K₂Cr₂O₇) is then prepared by treating the sodium dichromate solution with potassium chloride (KCl). This produces potassium dichromate, which is less soluble and crystallizes out:

Na_2​Cr₂​O₇ (s) + 2KCl(s) → K₂Cr₂O₇​(s) + 2NaCl(s)

4.0Physical Properties of Potassium Dichromate

• Appearance: Solid at room temperature with bright orange crystals.
• Odor and Taste: Odorless with a bitter taste.
• Toxicity: It is toxic and can irritate the eyes.
• Combustibility: Non-combustible but prone to rusting.
• Melting and Boiling Points: Melts at 398°C and decomposes upon boiling at 500°C.
• Solubility: Solubility increases in water at higher temperatures but is insoluble in alcohol and acetone.
• Refractive Index: 1.738.
• Structure: The chromium ion in potassium dichromate has a tetrahedral coordinate geometry. The crystalline structure is triclinic.

5.0Chemical Properties of Potassium Dichromate (K₂Cr₂O₇)

1. Oxidation Reactions:

• Mild Oxidizing Agent: Potassium dichromate is a milder oxidizer than potassium permanganate (KMnO4) and effectively produces aldehydes from alcohol. It releases three atoms of oxygen when reacted with dilute sulfuric acid: 

K_2​Cr₂​O_{7 ​}+ 4H₂​SO₄ ​→ K₂​SO_4​ + Cr₂​(SO_4​)₃​ + 4H₂​O + 3O

• Iodine Liberation: It liberates iodine from potassium iodide (KI): 

K_2​Cr₂​O_{7 ​}+ 7H₂​SO₄ ​​+ 6KI → 4K₂​SO₄​ + Cr₂​(SO_4​)₃​​ + 3I₂​+ 7H₂​O

• Sulfur Oxidation: It oxidizes hydrogen sulfide (H₂S) to sulfur:

K_2​Cr₂​O₇ + 4H₂​SO₄ ​+ 3H₂​S → K₂​SO₄ + Cr₂​(SO_4​)₃​​​ + 7H₂​O +3S

• Alcohol Testing: It oxidizes secondary alcohols to ketones, while tertiary alcohols are not oxidized. Potassium dichromate can also be used to test for aldehydes or ketones, as the colour change due to the reduction of dichromate ion indicates the presence of these compounds.
• Oxidation of Ferrous Sulfate: It oxidizes ferrous sulfate (FeSO₄) to ferric sulfate (Fe₂(SO₄)₃): 

K_2​Cr₂​O₇ ​+ 7H₂​SO₄​​ + 6FeSO₄ ​→ K_2​SO₄​+ Cr₂​(SO₄​)_{3 ​}+ 3Fe_2​(SO₄​)₃ ​+ 2H₂​O

2. Decomposition on Heating: When heated strongly, potassium dichromate decomposes to form potassium chromate (K₂CrO₄), chromium(III) oxide (Cr₂O₃), and releases oxygen gas: 

4K_2​Cr₂​O₇ → 4K₂​CrO_4​ + 2Cr₂​O₃​ + 3O_2​

3. Reaction with Alkalis:

• With Carbonate: Reacts with potassium carbonate (K₂CO₃) to form potassium chromate (K₂CrO₄), changing color from orange to yellow: 

K_2​Cr₂​O₇​ + K_2​CO₃​ → 2K₂​CrO₄​ + CO₂​

• With Hydroxide: Reacts with potassium hydroxide (KOH) to also form potassium chromate, resulting in a yellow solution: 

K_2​Cr₂​O₇ + 2KOH → 2K₂​CrO₄​ + H₂​O

• Acidification: When potassium chromate is acidified, it converts back to potassium dichromate, changing the solution color from yellow to orange: 

2K_2​CrO_4​ + H₂​SO₄ ​​→ K_2​Cr₂​O₇ ​+ K₂​SO₄ ​+ H_2​O

4. Reaction with Sulfuric Acid:

• Cold Sulfuric Acid: Reacts with cold sulfuric acid to form chromium trioxide (CrO₃), a red crystalline substance:

K_2​Cr₂​O₇ ​+ 2H₂​SO₄ ​​→ 2CrO_{3 ​}+ 2KHSO₄​ + H_2​O

• Hot Sulfuric Acid: When heated with concentrated sulfuric acid, it releases oxygen gas:

K_2​Cr₂​O₇​ + 8H₂​SO₄​​​ → 2K2​SO4 ​+ 2Cr_2​(SO₄​)₃​ + 8H_2​O + 3O₂​

• Chromyl Chloride Test: When potassium dichromate reacts with chloride salts, sulfuric acid, and potassium chloride, a reddish-brown vapor of chromyl chloride (CrO₂Cl₂) is produced. This test is used to detect chloride ions:

K_2​Cr₂​O₇​ + 4KCl + 8H₂​SO₄​​​ → 2CrO₂​Cl₂​ + 6KHSO₄​ +3H₂​O

6.0Uses of Potassium Dichromate 

The uses of potassium dichromate include:

• Cleaning: Potassium dichromate is a milder oxidizer than potassium permanganate (KMnO4) and effectively produces aldehydes from alcohol. It releases three atoms of oxygen when reacted with dilute sulfuric acid: 
• Construction: In cement, potassium dichromate slows the setting time and enhances the density and texture of the final product.
• Volumetric Analysis: It is used to estimate certain substances in volumetric analysis.
• Analytical Reagent: Commonly used in traditional “wet tests” in analytical chemistry due to its non-hygroscopic nature.
• Chrome Tanning: Employed in the leather industry for chrome tanning.
• Ethanol Determination: Used in a black titration method with acidified potassium dichromate to determine ethanol concentration. Ethanol reacts with excess acidified potassium dichromate to form acetic acid.
• Preparation of Chromium Compounds: Utilized to produce chrome alum and other important chromium compounds..
• Purity Testing: To test the purity of silver, a mixture of potassium dichromate with approximately 35% nitric acid, known as Schwerter’s Solution, is used..
• Wood Staining: Used to stain wood by darkening its tannins, producing a deep, rich brown colour that is difficult to achieve with modern dyes.