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.
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: CrO42-
Chromate Structure
This structure has one chromium atom and four oxygen atoms. The anion has a minus two negative charge.
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.
Xem thêm : Why Does It Take So Long To Recover From Anxiety Disorder?
The following table enumerates the key differences between chromate and dichromate:
CharacteristicsChromateDichromateChemical formulaThis is anion’s chemical formula is CrO42-.The dichromate anion chemical formula is Cr2O72-.Molar Mass115.99 g/mol215.99 g/molChromium AtomsIt has one chromium atom per ion.It has two chromium atoms per ion.ColourBright yellow colourBright orange colourOccurrenceIt 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.
CrO42- + 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
Xem thêm : How to Make the Best Gluten-Free Shortbread
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.
Nguồn: https://buycookiesonline.eu
Danh mục: Info
