Chromyl Chloride test

The detection of Chloride ions in qualitative analysis is facilitated by the specialized Chromyl Chloride test.

The Chromyl Chloride test identifies chloride ions (Cl⁻) by reacting the sample with potassium dichromate (K_2​Cr₂O₇) and concentrated sulfuric acid (H₂SO₄). The presence of chloride ions is indicated by the formation of chromyl chloride (CrO₂Cl₂), a reddish-brown vapor.

1.0Definition of Chromyl Chloride Test

A chemical test that involves the formation of chromyl chloride vapors from a test sample containing chloride ions when heated with potassium dichromate and concentrated sulfuric acid. Let’s learn more about the Chromyl Chloride test.

2.0Mechanism of Chromyl Chloride Test

1. Sample Preparation:

• Begin by taking a small amount of the salt suspected of containing chloride ions.
• Add an equal quantity of solid potassium dichromate (K₂Cr₂O₇) to the salt. Potassium dichromate acts as a source of chromate in the reaction.

2. Acid Addition:

• To the mixture in the test tube, carefully add a few drops of concentrated sulfuric acid (H₂SO₄).
• The acid serves to catalyze the reaction, facilitating the release of chromyl chloride gas if chloride ions are present.

3. Heating the Mixture:

• Heat the mixture gently over a Bunsen burner or a hot plate. The heat accelerates the chemical reaction between the potassium dichromate, sulfuric acid, and chloride ions.
• This reaction produces chromyl chloride (CrO₂Cl₂) gas, which is distinctively characterized by its reddish-brown color.

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

4. Gas Collection and Absorption:

• Direct the evolved gas through a setup where it bubbles into a solution of sodium hydroxide (NaOH).
• Sodium hydroxide captures and reacts with chromyl chloride to form sodium chromate (Na₂CrO₄), which is yellow, indicating the presence of chromate ions.

CrO₂Cl₂  +  4NaOH → Na₂CrO₄  +  2NaCl  +  2H₂O

5. Observation and Splitting:

• If the sodium hydroxide solution turns yellow, it confirms the transformation of chromyl chloride into chromate ions.
• Divide this yellow solution into two separate portions for further testing.

6. Acidification and Confirmation:

• Acidify the first portion with acetic acid. This step neutralizes excess base and prepares the solution for a precipitation reaction.
• Add lead acetate solution to the acidified sample. Lead acetate reacts with chromate ions to form lead chromate (PbCrO₄), a yellow precipitate.

(CH₃COO)₂Pb    +        Na₂CrO₄         →       PbCrO₄     +       2CH₃COONa

                         (Sodium Chromate)        (Lead Chromate, 

                                                                       Yellow ppt.)

7. Interpretation of Results:

• The formation of a yellow precipitate of lead chromate is a positive confirmation of chromate ions in the solution, which indirectly confirms the presence of chloride ions in the original sample.

3.0Confirmation of Chromyl Chloride Test

After conducting the initial chromyl chloride test, which indicates the presence of chloride ions through the formation of chromyl chloride, the test can be extended to include reactions with hydrogen peroxide (H₂O₂) for further analysis of chromium's chemical behavior.

Neutralization of Acidic Medium: 

Following the initial reaction where chromyl chloride vapors are passed through a sodium hydroxide (NaOH) solution, forming sodium chromate (Na₂CrO₄), the mixture is typically basic. It is then acidified with dilute sulfuric acid (H₂SO₄) to adjust the pH, making it conducive for subsequent reactions.

Addition of Amyl Alcohol: A small amount of amyl alcohol is added to the solution. Amyl alcohol serves as an organic solvent that can extract certain chromium species from the aqueous phase.

Formation of Chromium Pentoxide (CrO₅)

• Reaction with Hydrogen Peroxide: To the acidified solution containing amyl alcohol, 1 mL of 10% hydrogen peroxide solution is added. This leads to the oxidation of chromium (VI) in sodium chromate to form chromium pentoxide (CrO₅). The reaction is facilitated by the oxidative ability of hydrogen peroxide, which reacts with the chromate ions:

                         H₂O₂  +     CrO₄²⁻            →    CrO₅     +   H₂O

Observation of Color Change:

• Solubility in Amyl Alcohol: Chromium pentoxide is soluble in amyl alcohol, and upon dissolving, it imparts a deep blue color to the organic layer.
• Gentle Shaking: On gentle shaking of the mixture, the organic layer turns blue, confirming the presence and formation of the CrO₅ complex.

                CrO₄²⁻  +  2H^⊕ +  2H₂O₂    →   CrO₅   +   3H₂O

4.0Properties of Chromyl Chloride Test

Chemical Property:

• The test is specific to chloride ions among the common halides, as bromides and iodides under similar conditions do not form analogous stable compounds.
• Chromyl chloride (CrO₂Cl₂) is formed through the oxidation of chloride ions by dichromate in the presence of sulfuric acid.

Physical Property:

• Vapor Color: The formation of dark reddish-brown vapors of chromyl chloride is the key physical property used to indicate the presence of chloride ions.