K2Cr2O7 — Potassium Dichromate
Potassium dichromate (K₂Cr₂O₇) is an orange-red crystalline inorganic compound used as a powerful oxidizing agent in laboratories, leather tanning, and dyeing industries.
Interactive 3D Molecular Structure — K2Cr2O7
Properties
| Chemical Formula | K₂Cr₂O₇ |
|---|---|
| Molecular Mass | 294.18 g/mol |
| Physical State | Solid (Crystalline) |
| Melting Point | 398°C |
| Boiling Point | Decomposes before boiling |
| Density | 2.68 g/cm³ |
| pH | Acidic in aqueous solution |
| Odor | Odorless |
| Color | Bright orange-red |
| Taste | Bitter and metallic (toxic, not for tasting) |
| Polarity | Polar ionic compound |
| Type of Bond | Ionic (K⁺ and Cr₂O₇²⁻) |
Introduction to Potassium Dichromate
Potassium dichromate (K₂Cr₂O₇) is one of the most common and important compounds of chromium in its hexavalent state. It appears as bright orange-red triclinic crystals that are highly soluble in water. Known for its strong oxidizing properties, it is widely used in analytical chemistry, organic synthesis, and laboratory titrations.
Potassium dichromate belongs to the family of dichromates which contain the dichromate ion \(Cr_2O_7^{2-}\). It is a powerful oxidizer and must be handled with care due to its toxic and carcinogenic nature. The compound is often used as a cleaning agent for laboratory glassware (chromic acid solution), in leather tanning, and in photographic processing.
Structure and Bonding
The chemical structure of potassium dichromate consists of two chromium atoms connected via a bridging oxygen atom, forming the dichromate ion \(Cr_2O_7^{2-}\). Each chromium atom exhibits an oxidation state of +6. The structure can be visualized as two tetrahedral \(CrO_4\) units sharing one oxygen atom.
\(Cr_2O_7^{2-} \leftrightarrow 2CrO_4^{2-} + 2H^+\)
This equilibrium shows how dichromate and chromate ions interconvert depending on the pH of the solution. Under acidic conditions, dichromate ions dominate, whereas chromate ions predominate under basic conditions.
Preparation of Potassium Dichromate
Potassium dichromate is commercially prepared from chromite ore (FeCr₂O₄). The preparation involves several steps:
- Oxidation of chromite ore: The ore is fused with sodium carbonate and lime in the presence of air to form sodium chromate.
- Conversion to sodium dichromate: Sodium chromate is acidified with sulfuric acid to yield sodium dichromate.
- Crystallization: Potassium chloride is added to sodium dichromate solution, and potassium dichromate crystallizes out due to lower solubility.
\(Na_2Cr_2O_7 + 2KCl → K_2Cr_2O_7 + 2NaCl\)
Chemical Properties
Potassium dichromate is a strong oxidizing agent and reacts with reducing agents like sulfur dioxide, hydrogen sulfide, ferrous salts, and organic compounds.
- It gets reduced to chromium(III) ion (Cr³⁺) during redox reactions.
- It acts as an acidified oxidizing agent in volumetric analysis, particularly in redox titrations.
- In acidic medium, it oxidizes ethanol to acetic acid, and Fe²⁺ to Fe³⁺.
\(K_2Cr_2O_7 + 14H^+ + 6e^- → 2Cr^{3+} + 7H_2O\)
Uses and Applications
- Analytical Chemistry: Used as a standard oxidizing agent in redox titrations such as the estimation of Fe²⁺ ions.
- Organic Chemistry: Oxidizes alcohols, aldehydes, and other organic substances to their corresponding acids or ketones.
- Leather Industry: Employed in the tanning of leather to enhance durability and resistance.
- Photography and Printing: Utilized in photographic processing and in printing inks for its color and reactivity.
- Cleaning Agent: Chromic acid solution (made from K₂Cr₂O₇ and H₂SO₄) is used to clean glassware of organic residues.
Health Hazards and Environmental Impact
Potassium dichromate is highly toxic and carcinogenic due to the presence of hexavalent chromium (Cr⁶⁺). Direct exposure may cause skin ulcers, respiratory issues, and damage to internal organs. It is a strong irritant to the eyes and mucous membranes.
In the environment, K₂Cr₂O₇ poses serious risks to aquatic life as it can accumulate and interfere with biological processes. Hence, proper disposal and handling under safety protocols are essential.
Key Reactions of Potassium Dichromate
Reduction of Potassium Dichromate in Acidic Medium
In an acidic medium, potassium dichromate acts as a powerful oxidizing agent and is reduced to chromium(III) ion:
\(Cr_2O_7^{2-} + 14H^+ + 6e^- → 2Cr^{3+} + 7H_2O\)
This reaction forms the basis of its use in redox titrations, such as the estimation of Fe²⁺ ions in Mohr’s salt.
Oxidation of Ethanol to Acetic Acid
Potassium dichromate oxidizes ethanol to acetic acid when heated in acidic conditions:
\(3CH_3CH_2OH + 2Cr_2O_7^{2-} + 16H^+ → 3CH_3COOH + 4Cr^{3+} + 11H_2O\)
This reaction illustrates its oxidizing strength and is used in qualitative organic analysis.
Reaction with Hydrogen Sulfide
When hydrogen sulfide gas is passed through an acidified solution of potassium dichromate, it gets oxidized to sulfur:
\(Cr_2O_7^{2-} + 3H_2S + 8H^+ → 2Cr^{3+} + 7H_2O + 3S\)