Sodium dichromate or Na2Cr2O7 is an orange to red-coloured, crystalline, and inorganic compound which emits toxic chromium fumes on heating. Sodium dichromate is highly corrosive in nature and is a strong oxidizing agent. This substance is usually used to produce other chromium compounds but is also used for drilling muds, metal treatments, wood preservatives, production of dyes and organic chemicals, and as a corrosion inhibitor. In this article, we will learn about sodium dichromate in detail like the sodium dichromate reaction, sodium dichromate chemical formula, its uses, properties, density, and structure. Let us first learn about what is sodium dichromate or Na2Cr2O7 .
What is Sodium Dichromate?
Sodium dichromate or Na2Cr2O7 is an inorganic compound. It is also known as bichromate of soda, disodium dichromate, or sodium dichromate (VI). It is known to be a powerful oxidizing agent and is highly corrosive.
Sodium dichromate is a red to reddish-orange crystalline solid. It is odourless and dissolves in water, methanol and ethanol. It is commonly used as a corrosion inhibitor. On heating, it liberates chromium fumes that are toxic in nature.
Properties of Sodium Dichromate
Sodium Dichromate Structure
Sodium dichromate structure is shown below:
Production of Sodium Dichromate
On a larger scale, sodium dichromate is obtained from ores which contain chromium (III) oxides. Let us have a look at the Na2Cr2O7 reaction and how it is produced.
The first step is to fuse the ore with a base such as sodium carbonate.
Next, maintain a temperature around 1000 °C.
Then, conduct the above two steps in the presence of air that is the source of oxygen. The reaction is given below:
2Cr2O3 + 4Na2CO3 + 3O2 ---> 4Na2CrO4 + 4CO2
These steps solubilize chromium and allow it to extract in the hot water. The other components present in the ore are poorly soluble and therefore, acidification of the resulting aqueous extract by utilizing carbon dioxide or sulfuric acid helps to produce dichromate. The reaction is as follows:
2Na2CrO4 + 4CO + H2O ---> Na2CrO7 + 2NaHCO3
2Na2CrO4 + H2SO4 ---> Na2CrO7 + Na2So4 + H2O
Using the process of crystallization, dichromate is extracted as a dihydrate.
Sodium Dichromate Uses
Apart from its importance for the production of other chromium products, sodium dichromate also has many uses as an ingredient in the production of the following materials:
Metallic Finish - it helps to prevent corrosion, making it easier to clean the metallic surfaces and allows the paint to stick.
Organic products - used as oxidisers for the production of vitamin K and wax.
Pigments - used in making inorganic chromate pigments where it produces a range of light-stable colours. Some grades of chromate are used for the prevention of corrosion in undercoats and primers.
Ceramics - used to produce coloured glass and ceramic glaze.
Textiles - used as a mordant in the acidic dyes to improve colour-fast properties.
Used in the chromium sulphate production.
Health Hazards of Sodium Dichromate
Inhaling the dust or mist of sodium dichromate (VI) can cause respiratory irritation that resembles asthma and nasal septal perforation. Swallowing it leads to diarrhoea, and vomiting. When it is exposed to eyes and skin it results in dermatitis and local irritation.
Sodium dichromate is a non-combustible compound but increases the combustion of other compounds. It can explode and catch fire whenever it comes in contact with other combustible substances.
FAQs on Sodium Dichromate
1. What is sodium dichromate and what is its chemical formula?
Sodium dichromate is an inorganic chemical compound known for being a powerful oxidising agent. Its chemical formula is Na₂Cr₂O₇. It is most commonly handled and found in its dihydrate form, which is written as Na₂Cr₂O₇·2H₂O.
2. What are the main industrial and laboratory uses of sodium dichromate?
Sodium dichromate has several important applications across various fields. Its primary uses include:
- Chemical Intermediate: It serves as a precursor in the production of most other chromium compounds, such as chromic acid.
- Oxidising Agent: It is widely used in organic chemistry for the oxidation of primary and secondary alcohols.
- Corrosion Inhibitor: It is added to water cooling systems and other solutions to prevent the corrosion of metals.
- Pigment and Dye Manufacturing: It is used as a mordant in textile dyeing and in the production of inorganic chrome pigments.
- Wood Preservation: It is an ingredient in certain formulations used to protect wood from decay and insects.
3. What is the characteristic colour of sodium dichromate and why?
Sodium dichromate is a bright orange crystalline solid. This distinct colour is not due to d-d transitions, but rather to a phenomenon called ligand-to-metal charge transfer (LMCT). In the dichromate ion (Cr₂O₇²⁻), an electron temporarily transfers from an oxygen ligand to an empty d-orbital of the chromium atom. This process absorbs light from the blue-green part of the visible spectrum, causing the compound to appear orange.
4. How is sodium dichromate prepared on a large scale from chromite ore?
The industrial preparation of sodium dichromate from its chief ore, chromite (FeCr₂O₄), involves three key steps as per the NCERT curriculum:
- Formation of Sodium Chromate: The finely powdered chromite ore is roasted with sodium carbonate (soda ash) in a furnace with excess air. This process converts the chromium in the ore to soluble sodium chromate (Na₂CrO₄).
- Acidification to Sodium Dichromate: The resulting solution of sodium chromate is filtered to remove impurities and then acidified using concentrated sulphuric acid. This converts the yellow sodium chromate into orange sodium dichromate.
- Crystallisation: The solution is concentrated, causing the less soluble sodium sulphate to crystallise out first. The remaining solution is then further concentrated and cooled to obtain crystals of Na₂Cr₂O₇·2H₂O.
5. Why is sodium dichromate often preferred over potassium dichromate in certain applications?
Sodium dichromate is generally preferred over potassium dichromate, particularly for industrial-scale applications, mainly due to its much higher solubility in water. This property allows for the preparation of more concentrated solutions, which is often more efficient. Additionally, sodium dichromate is more economical to produce. However, for use as a primary standard in analytical chemistry, potassium dichromate is favoured because it is not hygroscopic (does not absorb moisture from the air), unlike sodium dichromate.
6. What happens when an acid or a base is added to a sodium dichromate solution? Explain the chromate-dichromate equilibrium.
In an aqueous solution, the dichromate ion (Cr₂O₇²⁻) exists in a dynamic equilibrium with the chromate ion (CrO₄²⁻). This equilibrium is highly sensitive to the pH of the solution.
- In acidic solutions (low pH), the equilibrium favours the formation of the orange-coloured dichromate ion (Cr₂O₇²⁻).
- In alkaline (basic) solutions (high pH), the equilibrium shifts to favour the formation of the yellow-coloured chromate ion (CrO₄²⁻).
This interconversion is a key concept in d-block chemistry and is represented by the equation: 2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O.
7. Explain the role of acidified sodium dichromate in the oxidation of alcohols. What colour change is observed?
Acidified sodium dichromate acts as a strong oxidising agent, capable of oxidising primary and secondary alcohols to aldehydes and ketones, respectively. During this reaction, the chromium in the dichromate ion is itself reduced. This reduction causes a distinct and observable colour change from the orange of the dichromate ion (Cr₂O₇²⁻), where chromium is in the +6 oxidation state, to the green of the chromium(III) ion (Cr³⁺). This colour change is a classic laboratory test for the presence of primary or secondary alcohols.
8. What is the significance of the Roman numeral (VI) in the name Sodium Dichromate (VI)?
The Roman numeral (VI) in the name Sodium Dichromate (VI) is part of the Stock notation system used in chemistry. It explicitly states the oxidation state of the chromium atom in the compound. In the dichromate ion (Cr₂O₇²⁻), each of the two chromium atoms has an oxidation state of +6. Using this notation is crucial for clarity, especially for transition metals like chromium that can exist in multiple stable oxidation states.
