Chromate (CrO4) – Structure, Properties, Molecular Mass & Uses

Nov 8, 2022

An atom of chromium and four of oxygen combine to give chromate. It is a conjugate of chromic acid. It is a chromium oxoanion that is formed by the removal of two protons from chromic acid. As an oxidizing agent, it finds application in various industries. Chromate conversion coating imparts brightness and enhances the corrosion resistance of bare metals. It also acts as a substrate to ensure improved paint adherence on nonferrous surfaces.

chromate

What is Chromate?

The chromium oxoanion formed from chromic acid is called chromate. It is a divalent inorganic anion. The salts of chromic acid containing the chromate anion usually have an intense yellow colour. The most widely used chromate salts are potassium and sodium salts.

IUPAC name of chromate: dioxido(dioxo)chromium
Chromate chemical formula/ chromate symbol: CrO₄^2-

Chromate Structure

This structure has one chromium atom and four oxygen atoms. The anion has a minus two negative charge.

Chromate structure

Chromate Ion

The Lewis structure of the chromate ion shows its bonding and non-bonding electrons. The anion has two single covalent bonds and two double covalent bonds. It has ten lone pairs of electrons. There are four oxygen atoms and a chromium atom. Two oxygen atoms carry a -1 charge on them. The following image depicts the Lewis structure of a chromate ion.

Lewis structure of Chromate

What is the Difference between Chromate and Dichromate?

Chromate and dichromate are two different anions containing the same elements– chromium and oxygen. Therefore, both are oxoanions of chromium. However, the two are quite different from each other.

There is a marked difference in their colour. While it appears bright yellow, dichromate has a bright orange colour. Usually, you can find a high amount of chromate at high pH values, i.e., in an alkaline medium, while at low pH values or an acidic medium, you are likely to find more dichromate ions.

The following table enumerates the key differences between chromate and dichromate:

Characteristics	Chromate	Dichromate
Chemical formula	This is anion’s chemical formula is CrO₄^2-.	The dichromate anion chemical formula is Cr₂O₇^2-.
Molar Mass	115.99 g/mol	215.99 g/mol
Chromium Atoms	It has one chromium atom per ion.	It has two chromium atoms per ion.
Colour	Bright yellow colour	Bright orange colour
Occurrence	It is commonly found at high pH values, i.e., above 6.5.	It occurs at low pH values,i.e., lower than 6.5.
Structure

Physical Properties of Chromate

Odour: Odourless
Molecular Weight/ Molar Mass: 194.1896 g/mol
Appearance: Yellow powder
Valency: -2
Solubility: insoluble in water
Density: 2.73 g/cm³
Conjugate acid‎: ‎Chromic acid (H₂CrO₄)
Oxidation State: +6

Chemical Properties of Chromate

Here are some of the important chemical reactions in which chromate participates:

Reaction with Water

Chromate ion is a strong oxidising agent when present in an acid solution. However, in an alkaline solution, it combines with water to give chromium(III) hydroxide.

CrO₄^2- + 4 H₂O + 3 e− → Cr(OH)₃ + 5 OH−

Reaction with Barium Nitrate

Its potassium salt: potassium chromate, reacts with barium nitrate. The balanced chemical equation for the reaction is as follows:

K₂CrO₄ + Ba(NO₃)₂ ↔ BaCrO₄ + 2 KNO₃

Reaction with Silver Nitrate

Potassium chromate reacts with silver nitrate to give a silver chromate precipitate in the aqueous reaction mixture. The precipitate has a brick red colour. The balanced chemical equation for the reaction is as follows:

2 AgNO₃(aq) + K2CrO₄(aq) → 2 KNO₃(aq) + Ag₂CrO₄(s)

Reaction with Hydrochloric Acid

Potassium chromate undergoes a strong reaction with hydrochloric acid to give potassium chloride, potassium dichromate, and water. The following chemical equation represents the reaction:

2 K₂CrO₄ +2 HCl→K₂Cr₂O₇ +2 KCl + H₂O

Reaction with Nitric Acid

Dilute nitric acid is made to react with potassium chromate to give potassium dichromate and potassium nitrate. Water is liberated in the process. The following chemical equation represents the reaction:

2 K₂CrO₄ +2 HNO₃→ K₂Cr₂O₇ +2 KNO₃+ H₂O

Preparation of Potassium Chromate

Potassium Chromate is an inorganic salt containing potassium cation and chromate anion. It has the following chemical formula: K₂CrO₄. An ionic bond exists between potassium and chromate ions, and thus it is an ionic compound. It is a strong oxidising agent. Potassium chromate is a yellow crystalline solid widely used in laboratories.

It can be prepared using the following methods:

Potassium Dichromate and Potassium Hydroxide Reaction

When two moles of potassium hydroxide react with one mole of potassium dichromate, it gives two moles of potassium chromate and water. The chemical equation for the reaction is as follows:

K₂Cr₂O₇(aq) +2 KOH→2 K₂CrO₄ + H₂O

Chromium Oxide and Potassium Hydroxide Reaction

Potassium chromate is obtained when potassium hydroxide reacts with chromium oxide. Water is liberated in the process, and the chemical equation can be represented as follows:

2 KOH + Cr₂O₃→2 K₂CrO₄ + H₂O

Uses of Chromate

Chromate anion and its salts are highly corrosive and find application in various industries.

Sodium and potassium salts of these are used in enamels.
It is also used in finishing leather.
Suitable for rust-proofing metals
Potassium chromate crystals that are bright yellow are used as a pigment in dyes and inks.
Industrial crayons consist of lead chromate.
When used in chrome plating, chromate ions help prevent corrosion. It is a good inhibitor of corrosion.
It acts as a primer.
It helps impart a decorative finish.
Chromate ions help maintain electrical conductance.
Potassium chromate is used as a fungicide.
Why is potassium chromate used as an indicator?
Potassium chromate is used as an indicator in precipitation titrations as it turns red in the presence of excess silver ions.

Is Chromate Toxic?

It is hexavalent chromium. Hexavalent chromium is any chemical compound with chromium in a +6 oxidation state (thus hexavalent). All hexavalent chromium compounds are toxic, and thus this is toxic too. The oxidising power is responsible for its toxicity.

It is carcinogenic, especially if airborne. So, if a person inhales chromate, it can lead to lung cancer. Furthermore, several positive associations have been found between hexavalent chromium compound exposure and cancer of the nose and nasal sinuses.

Important Chromate Compounds

It combines with several cations to form several useful compounds, such as

Lead Chromate

Chemical Formula of Lead Chromate: PbCrO₄
Molecular Mass of Lead Chromate: 323.1937 g/mol
Applications of Lead Chromate: It is used in paints and as a pigment in oil and watercolours. It is also used in ceramics, rubber and plastics.

Potassium Chromate

Chemical Formula of Potassium Chromate: K₂CrO₄
Molecular Mass of Potassium Chromate:1896 g/mol
Applications of Potassium Chromate: It is used as an anticorrosive agent in primers and coatings.

Strontium Chromate

Chemical Formula of Strontium Chromate: SrCrO₄
Molecular Mass of Strontium Chromate: 614 g/mol
Applications of Strontium Chromate: It is an excellent corrosion inhibitor for metal substrates.

Sodium Chromate

Chemical Formula of Sodium Chromate: Na₂CrO₄
Molecular Mass of Sodium Chromate: 161.97 g/mol
Applications of Sodium Chromate: It is primarily used to produce chromic acid and chromium pigments used in the leather industry.

Zinc Chromate

Chemical Formula of Zinc Chromate: ZnCrO₄
Molecular Mass of Zinc Chromate: 181.403 g/mol
Applications of Zinc Chromate: They are used in priming paints used for metals. It is also popular in spray paints, artists’ paints, and varnishes. It is employed in making linoleum.

Barium Chromate

Chemical Formula of Barium Chromate: BaCrO₄
Molecular Mass of Barium Chromate: 253.37 g/mol
Applications of Barium Chromate: It is used as a pigment in anticorrosion jointing pastes.

Calcium Chromate

Chemical Formula of Calcium Chromate: CaCrO₄
Molecular Mass of Calcium Chromate:072 g/mol
Applications of Calcium Chromate: It is widely used as an oxidising agent and a corrosion inhibitor.

Caesium Chromate

Chemical Formula of Caesium Chromate: Cs₂CrO₄
Molecular Mass of Caesium Chromate:8 g/mol
Applications of Caesium Chromate: It was earlier used in creating vacuum tubes.

Chromium Chromate

Chemical Formula of Chromium Chromate: Cr₂(CrO₄)₃
Molecular Mass of Chromium Chromate:99 g/mol
Applications of Chromium Chromate: It is used in aircraft components, zinc castings, galvanised steel passivation, and magnesium castings.

Conclusion

It is an oxoanion containing four oxygen and one chromium atom. It results from the removal of two protons from chromic acid. It carries a minus two negative charge. Its salts of potassium and sodium are widely used in dyeing industries as they have a distinct yellow colour. It is a strong oxidising agent. It should not be confused with dichromate, although both are oxoanions of chromium.

Frequently Asked Questions

1. Is chromate acidic or basic?

A. It is the predominant species in alkaline solutions. However, dichromate can become the predominant ion in acidic solutions.

2. Are chromate and chromium the same?

A. No, chromium is an element with an atomic number of 24 represented by Cr. It is grey in colour. At the same time, It is an anion containing chromium and oxygen atoms, represented by CrO₄^-2. Its salts have a yellow colour.

3. How is chrome different from chromate?

A. While this is an anion comprising chromium and oxygen atoms, chrome is the short form of the element chromium. The name is widely used in the plating industry. Chrome plating is overlaid on metals, and it consists solely of chromium. It is typically polished and has a very shiny coating.