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:
When chromyl chloride reacts with benzene, it leads to the formation of a chromyl compound.
C₆H₅CH₃ + 2CrO₂Cl₂ → C₆H₅CH(CrO₂Cl₂)₂
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.