Chromyl Chloride Test

What Is Chromyl Chloride Test?

The chromyl chloride test is a qualitative analysis test used for the detection of Cl^– ions. Let’s take an example to get a better understanding of the test. A sample of chlorine-containing salt is heated with potassium chromate (K­₂Cr₂O₇) and concentrated sulphuric acid (H₂SO₄). If chloride is present, chromyl chloride is formed, and red fumes are given out. The chromyl chloride test reaction is given as follows:

K₂Cr₂O₇ +4NaCl+6H­₂SO₄ → 2CrO₂Cl₂ +2KHSO₄ + 4NaHSO₄ + 3H₂O

However, for salts such as chlorides of mercury and silver, chromyl chloride test is not applicable. This is because the chlorides of mercury and silver are covalent, and they do not generate Cl^– ions. The chromyl chloride test is applicable only for compounds having _­Cl^– ionic bonds. In essence, the chromyl chloride test is carried out if we suspect a sample having chloride or mainly for detecting the presence of Cl^– ions of ionic compounds.

Also Read: Qualitative Analysis of Organic Compounds

Chromyl Chloride and Its Properties

Chromyl chloride is a chemical compound, and its chemical formula is given as CrO₂Cl₂. They are dark red blood colour liquids where the molecules of chromyl chloride are tetrahedral. They are also mostly chromium (IV) derivatives (CrO₄)²⁺_.

Chemical Property

• It is often used as an oxidizing agent. Chromyl chloride can be used for the oxidation of toluene to benzaldehyde. This reaction is followed by two steps:

1. On reacting chromyl chloride with benzene, a chromyl compound is formed.

C₆H₅CH₃ + 2CrO₂Cl₂ → C₆H₅CH(CrO₂Cl₂)₂

2. This chromyl compound is hydrolyzed with water to give benzaldehyde.

C₆H₅CH(CrO₂Cl₂)₂ +2H₂O → C₆H₅CH=O + CrO₃ + 4HCl

This reaction is known as Etard’s Reaction, and the chromyl chloride that is used here is a mild oxidizing agent, which is excellent for making aldehydes.

• Chromyl chloride can react with water to form chromic acid and hydrochloric acid. This reaction is exothermic.

CrO₂Cl₂ + 2H₂O → H₂CrO₄ +2 HCl.

Physical Property

Chromyl chloride is a deep red viscous liquid which fumes in the air. It reacts with water and alcohol and is soluble in chlorinated carbons and carboxylic acids.

Chromyl Chloride Test Mechanism

The mechanism of the chromyl chloride test is simple. On reacting potassium dichromate with sulphuric acid, chromate trioxide (oxidation state = +6) is formed. The colour of chromate trioxide (CrO₃) varies from dark red to brown colour.

K₂Cr₂O₇ + H­₂SO₄ → CrO₃ + H₂O

Now, the salt-containing chloride (NaCl) is reacted with sulphuric acid that gives sodium bisulphate (NaHSO­₄), and hydrochloric acid (HCl) is also formed.

NaCl +H₂SO₄ → NaHSO­₄ +HCl

In the next step, chromate trioxide is reacted with hydrochloric acid, which produces chromyl chloride (CrO₂Cl₂ ), which gives out red fumes.

CrO₃ + HCl → CrO₂Cl₂

Confirmation for Chromyl Chloride Test

Now, for the confirmation of chromyl chloride, the red vapour needs to dissolve in a solution of sodium hydroxide (NaOH). The solution turns yellow (due to Na₂CrO₄).

CrO₂Cl₂ + NaOH → Na₂CrO₄ + NaCl +H₂O

Reacting this solution further with the lead acetate and diluted acetic acid (CH₃COOH) produces the yellow precipitate.

CrO₄^2- + Pb(CH₃COO)₂ → PbCrO₄ +CH₃COONa

PbCrO₄ is the yellow precipitate of chloride, and hence the test is accurate.

Experiment

Requirements: Orginal salt, powdered potassium dichromate, concentrated sulphuric acid, sodium hydroxide, dilute acetic acid, lead acetate, test tube, test holder, spatula, glass rod, dropper, bunsen burner.

Procedure

• A small quantity of salt is taken in a test tube. To this, add a small amount of powdered potassium dichromate using a spatula. Mix it with a glass rod.
• Add a small amount of concentrated sulphuric acid to the test tube dropper.
• Heat the test tube over the Bunsen burner and hold the test tube using the holder.
• Red fumes should be given out, indicating the formation of chromyl chloride.
• This red fume should be collected using another test tube.
• Next, add a small volume of sodium hydroxide to the test tube filled with red fumes using a dropper.
• Chromyl chloride reacts with sodium hydroxide to form the yellow solution of sodium chromate.
• Add a small amount of dilute acetic acid and the lead acetate using another dropper.
• They react to form a yellow precipitate.

Precautions: Chromyl chloride is extremely corrosive and will fume in contact with air, releasing a mist of hydrochloric and chromic acids. Proper protection must be worn when handling the compound. As it is a Cr (VI) compound, it is also carcinogenic. Chromyl chlorides are usually stored in glass containers.

To dispose of the chromyl chloride, safely neutralise chromyl chloride, add solvents like glacial acetic acid or chloroforms to the compound and stir the suspension/solution. Slowly add a reducing agent like ascorbic acid until the colour changes from red/orange to green. Avoid adding the aqueous solutions, especially if you’re using glacial acetic acid as solvent, as the neutralisation will generate heat.

Alternate Test for Chlorides

Prepare the soda extract of the given salt; this method is feasible for heavy metal chlorides too. They produce NaCl even for the covalent chlorides. Sometimes, we have to do this in the melt due to the insoluble chlorides. Then, dilute this extract using the water and filter it. Then, add AgNO₃ (silver nitrate) to the solution, which produces a white precipitate AgCl. To confirm the chloride, add sodium arsenite (Na₃AsO₃) to the solution and shake it well; a yellow precipitate is formed, thus, the presence of chloride is confirmed.