Chromate (CrO42−) & Dichromate (CrO42−)
Table of Contents
The chromate and dichromate ions are closely related polyatomic oxyanions carrying a −2 charge and the chemical formulas CrO42– and Cr2O72–, respectively. Both ions form salts with cations such as sodium (Na+), potassium (K+), and ammonium (NH4+), including sodium chromate (Na2CrO4) and potassium dichromate (K2Cr2O7).
They are widely encountered in inorganic chemistry and have important industrial applications. However, due to the toxicity of chromium(VI), their use is now largely restricted to controlled applications.
Structure and Bonding
The chromate ion adopts a tetrahedral geometry around chromium, often described using an sp3 hybridization model for simplicity. The O–Cr–O bond angle is approximately 109.5°. [2]
The Lewis structure of the chromate ion is commonly represented using resonance structures. In these representations, chromium forms two single and two double bonds with oxygen atoms. The singly bonded oxygen atoms carry a −1 formal charge each, giving an overall charge of −2. However, the actual structure is a resonance hybrid in which π electrons are delocalized over all four Cr–O bonds, making them equivalent and distributing the negative charge evenly among the oxygen atoms.
The dichromate ion consists of two tetrahedral CrO4 units linked by a shared bridging oxygen atom. The negative charge is delocalized over the terminal oxygen atoms.
Properties
| Property [3] | Chromate (CrO42–) | Dichromate (Cr2O72–) |
| Molar mass of the ions | 116.00 g·mol–1 | 216.00 g·mol–1 |
| Oxidation state of Cr | +6 (hexavalent) | +6 (hexavalent) |
| Color of the ion* | Bright yellow | Orange |
| Solubility of salts in water | Alkali metal (e.g., Na2CrO4 and K2CrO4) and ammonium salts are soluble | Alkali metal and ammonium salts are soluble |
| Stability | Stable in alkaline medium | Stable in acidic medium |
| Toxicity | Highly toxic due to Cr(VI) | Highly toxic due to Cr(VI) |
*The characteristic colors arise from ligand-to-metal charge transfer (LMCT) transitions.
Preparation
Chromate salts are typically prepared by oxidizing chromium(III) compounds to chromium(VI) in an alkaline medium. [4,5]
1. Chromium(III) oxide is heated with potassium hydroxide and an oxidizing agent such as potassium nitrate to form potassium chromate and potassium nitrite:
Cr2O3 + 4 KOH + 3 KNO3 → 2 K2CrO4 + 3 KNO2 + 2 H2O
2. During industrial preparation, chromite ore is heated with sodium carbonate in the presence of air at high temperatures to produce sodium chromate:
4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
Dichromates are typically obtained by acidifying chromate solutions. The sodium chromate obtained from the above process is treated with concentrated sulfuric acid, resulting in sodium dichromate:
2 Na2CrO4 + H2SO4 ⇌ Na2Cr2O7 + Na2SO4 + H2O
Chemical Reactions
1. Chromate–Dichromate Equilibrium
Chromate and dichromate ions are interconvertible. Their relative abundance depends strongly on the pH of the medium, as described by the following pH-dependent reversible equilibrium: [6,7]
2 CrO42– + 2 H+ ⇌ Cr2O72– + H2O
The equilibrium shifts are as follows:
- In acidic medium: shifts toward orange dichromate
- In alkaline medium: shifts toward yellow chromate
This color change can serve as a qualitative indicator of pH.
2. Oxidation
Dichromate ions act as strong oxidizing agents, particularly in an acidic medium, where they are reduced to Cr3+. Other substances, such as iodide (I–) and iron(II) ions (Fe2+), are oxidized to iodine and iron(III), respectively:
Cr2O72– + 6 I– + 14 H+ → 2 Cr3+ + 3 I2 + 7 H2O
Cr2O72– + 6 Fe2+ + 14 H+ → 2 Cr3+ + 6 Fe3+ + 7 H2O
In neutral or alkaline conditions, chromates are weaker oxidizing agents.
3. Precipitation
Chromate and dichromate ions react with heavy metal ions, such as silver, to form insoluble precipitates:
2 AgNO3 (aq) + K2CrO4 (aq) → Ag2CrO4 (s) ↓ + 2 KNO3 (aq)
2 AgNO3 (aq) + K2Cr2O7 (aq) → Ag2Cr2O7 (s) ↓ + 2 KNO3 (aq)
Other examples include lead (Pb) chromate and dichromate, as well as barium (Ba) chromate/dichromate.
These reactions are widely used in qualitative analysis to identify metal ions.
Uses
- Analytical Chemistry: Act as oxidizing agents in redox titrations to determine the concentration of reducing agents. [8]
- Corrosion Protection: Used in chromate coatings and inhibitors to reduce metal corrosion, particularly in aerospace and specialized industrial applications under strict regulatory control.
Chromate and dichromate ions exemplify fundamental principles of equilibrium and redox chemistry while serving as important reference systems for developing safer oxidizing agents and corrosion inhibitors.
