Chromyl Chloride Test

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What is Chromyl Chloride Test?

Chromyl chloride test is done for the qualitative analysis for the confirmation of chloride ions. We can explain it with an example where a sample of chlorine-containing salt is heated with potassium chromate and concentrated sulphuric acid. In this test, red fumes appear because of the formation of chromyl chloride and it highlights the presence of chloride. This reaction can be represented in the following manner:

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

What is Chromyl Chloride?

It is a reddish-brown compound of chromium present with oxygen and chlorine. This compound is extremely corrosive in nature and can readily generate reddish-brown fumes in the air. Like most of the chromium compounds, chromyl chloride is also highly carcinogenic. It is prepared by reacting potassium chromate or potassium dichromate with HCl (Hydrogen chloride) in the presence of sulphuric acid. Further, a gentle distillation is done.

K₂Cr₂O₇ + 6 HCl → 2 CrO₂Cl₂ + 2 KCl + 3 H₂O

Here, the dehydration agent is sulfuric acid. Chromyl chloride can also be prepared by direct exposure of chromium trioxide to anhydrous hydrogen chloride gas.

CrO₃ + 2 HCl ⇌ Cr₂Cl₂ + H₂O

Method of preparation of chromyl chloride can be used for qualitative tests for chloride. It is done by heating a sample suspected of containing chloride with a mixture of potassium dichromate and concentrated sulphuric acid. Chromyl chloride formation takes place which highlights the presence of chloride with red fumes of CrO₂Cl₂. We do not find the formation of these analogous compounds with bromides, fluorides, iodides and cyanides. This way we can conduct the chromyl chloride test for chloride. 

Properties of Chromyl Chloride

The chemical formula of chromyl chloride, a chemical compound, is CrO₂Cl₂. It has a dark red blood colour in liquid form where the chromyl chloride molecules are tetrahedral shaped. These are mostly chromium (IV) derivatives. 

Chemical Properties: Chromyl chloride is often used as an oxidizing agent and it is used for the oxidation of toluene to form Benzaldehyde. The following reactions showcase the same:

  1. When chromyl chloride reacts with benzene, it leads to the formation of a chromyl compound.

C₆H₅CH₃ + 2CrO₂Cl₂ → C₆H₅CH(CrO₂Cl₂)₂

  1. Further, this chromyl compound gets hydrolyzed with water to form Benzaldehyde.

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

It is called the Etard’s reaction and the chromyl chloride used here is a mild oxidizing agent which is beneficial in making aldehydes.

Chromyl chloride reacts with water to form chromic acid and hydrochloric acid and this reaction is exothermic.

CrO₂Cl₂ + 2H₂O → H₂CrO₄ +2 HCl

Physical Property of Chromyl Chloride: It is found in a liquid state which is deep red and viscous in appearance and it fumes in the air. It reacts with alcohols, water and is soluble in carboxylic acids and chlorinated carbons.

After knowing chromyl chloride and its properties, let’s know about the mechanism of chromyl chloride test and how various reactions are involved in the confirmation of chromyl chloride test. 

Mechanism of Chromyl Chloride Test

The Chromyl Chloride test mechanism is simple; on its reaction with potassium dichromate with sulphuric acid, chromate trioxide is formed. The colour of chromate trioxide varies from dark red to brown colour.

K₂Cr₂O₇ + H­₂SO₄ → CrO₃ + H₂O

When salt-containing chloride (NaCl) is reacted with sulphuric acid, it gives sodium bisulphate (NaHSO­₄) and also hydrochloric acid (HCl).

NaCl +H₂SO₄ → NaHSO₄­ +HCl

Further, when chromate trioxide is reacted with hydrochloric acid, it produces chromyl chloride (CrO₂Cl₂ ) giving out red fumes.

CrO₃ + HCl → CrO₂Cl₂


Confirmation for Chromyl Chloride Test

Now for confirming chromyl chloride, the red vapour is dissolved in a solution of sodium hydroxide (NaOH). The solution formed turns yellow (due to the presence of Na₂CrO₄).

CrO₂Cl₂ + NaOH → Na₂CrO₄ + NaCl +H₂O

Further, on the reaction of this solution with the lead acetate and diluted acetic acid (CH₃COOH), a yellow precipitate is formed.

CrO₄²⁻ + Pb(CH₃COO)₂ → PbCrO₄ +CH₃COONa

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

FAQ (Frequently Asked Questions)

Q1. What is an Alternative Test for Chlorides Other than Chromyl Chloride Test?

Ans: Here is an alternate test for chlorides which is feasible for heavy metal chlorides too. We have to prepare soda extract of the given salt. NaCl is produced even for the covalent chlorides. Sometimes we need to melt them due to the insoluble chlorides. Then we need to follow with the dilution of the extract using the water and filter it. Further, add AgNO₃ (silver nitrate) to the solution and it produces a white precipitate AgCl. For confirming chloride, we can add sodium arsenite (Na₃AsO₃) to the solution and shake it well until a yellow precipitate is formed. This confirms the presence of chloride.

Q2. What are the Risks Associated with the Handling of Chromyl Chloride Tests?

Ans: We should take safety precautions while handling the chromyl chloride tests and Chromyl Chloride hydrolyses to release HCl and hexavalent chromium. The chromyl chloride vapour is known to irritate the respiratory system and eyes. Also burning of skin and eyes takes place when these come in contact with the liquid. Its ingestion can also cause severe internal damage. In chronic conditions, it can generate chromosomal aberrations and acts as a human carcinogen via inhalation. Ulceration may also be caused due to frequent exposure of the skin to chromyl chloride.

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