What is the Test for Chloride Ions

This article, ‘What is the test for chloride ions,’ explains five distinct tests that can be carried out to identify chloride ions. In some tests, the reaction mixture gives a precipitate with a characteristic colour, while some reaction mixtures evolve gases with a characteristic odour or colour. Chlorides have some characteristic features to distinguish from other halide ions. For example, its solubility and the properties of gases produced are different. For confirmation of the chlorides, further analyses should be carried out. Those steps are briefly explained in this article. Also, in some tests, before carrying out reaction process, we need to make sure that some of the ions are not present in the unknown mixture. This is because some ions produce harmful products during the reaction.

Test for Chloride Ions

Test 1. Solubility Test for Chloride Ions

Most of the metal chlorides ions are soluble in water. Some chlorides are sparingly soluble in water and a little number of Chlorides form insoluble precipitates in water. These metals ions can use to identify Chloride ions.

Mercury (I) chloride (Hg₂Cl₂), Silver chloride (AgCl), lead chloride (PbCl₂) are sparingly soluble in water but readily soluble in boiling water.
Copper chloride (CuCl), bismuth oxychloride (BiOCl), antimony oxychloride (SbOCl) and mercury(ll) oxychloride are insoluble in water.

Test 2. Reaction Test with Manganese Dioxide and Concentrated Sulphuric Acid

The solid chloride is mixed with an equal amount of manganese dioxide and then the concentrated sulphuric acid is added into the mixture. Finally, the mixture is gently warmed. This process releases chlorine gas, which can be identified by its suffocating odor. Chlorine is a yellowish-green coloured gas, and it bleaches moistened litmus paper and turns potassium iodide-starch paper into blue. In this reaction, hydrogen chloride forms first and then it converts to chlorine.

MnO₂ + 2H₂SO₄ + 2Cl^– —> Mn²⁺ + 2SO4^2- + H₂O +Cl₂ (g)

Test 3. Reaction Test with Silver Nitrate Solution

When silver nitrate is added to a chloride solution, a white coloured precipitate is formed. It is insoluble in water and dilute nitric acid, but soluble in dilute ammonia, potassium cyanide, and sodium thiosulphate solutions.

Cl^–+ Ag⁺ —> AgCl (s)

AgCl (s) + 2NH_³ —> [Ag(NH₃)₂]⁺ + Cl^–
[Ag(NH₃)₂]⁺ + Cl^– +2H⁺ —> AgCl(s) + 2NH₄⁺

Next, the formed precipitate can be used for a confirmatory test for Cl^– ions. The precipitate is filtered off and then washed with distilled water, followed by shaking with sodium arsenate solution. It gives a yellow silver arsenate precipitate which is distinct from other halides (Br^–, I^–).

3AgCl (s) + AsO²-3 —> Ag₃AsO₃(s)+ 3Cl^–

Test 4. Reaction Test with Lead Acetate

When lead acetate is added to a chloride solution, a white coloured precipitate of PbCl₂ is formed.

2Cl^– + Pb²⁺ —> PbCl₂(s)

Test 5. Chromyl Chloride Test – Potassium Dichromate and Dulphuric Acid

One part of the solid chloride is mixed with three parts of powdered potassium dichromate (1:3 w/w) in a small distilling flask. An equal amount of concentrated sulphuric acid is added to the mixture and then it is gently warmed.

Note: This test must not be carried out in the presence of chlorate ions (ClO3-), because the reaction forms explosive chlorine dioxide.
A deep red vapour of chromyl chloride (CrO2Cl2) is formed and it is passed into a test tube containing sodium hydroxide.

The resulting yellowish solution in the test tube contains sodium chromate. This is confirmed by acidifying with dilute sulphuric acid, and adding 1-2 ml of amyl alcohol followed by a little hydrogen peroxide solution. This will turn the organic layer to blue. Alternatively, diphenyclarbazide reagent test can be applied.

4Cl^– + Cr₂O₇^2- + 6H⁺ —> 2 CrO₂Cl₂ (g) + 3H₂O

CrO₂Cl₂ + 4OH^–—> CrO4^2- + 2Cl^– + 2H₂O

Some of chlorine may be liberated in the reaction mixture. This decreases the sensitivity of the test.

6Cl^– + Cr₂O₇²+14H⁺ 3Cl₂(g) + 2Cr³⁺ + 7H₂O

** The formation of chromate in the distillate indicates that a chloride was present in the solid substance since chromyl chloride (CrO₂Cl₂) is a readily volatile liquid (bp 116.50C). **

What is the Test for Chloride Ions – Summary

Most chlorides are soluble in water, but Mercury (I) chloride (Hg₂Cl₂), Silver chloride (AgCl), lead chloride (PbCl₂), copper chloride (CuCl), bismuth oxychloride (BiOCl), antimony oxychloride (SbOCl) and mercury (ll) oxychloride are insoluble in water. Silver chloride precipitate can be further tested with ammonia solution for confirmation. Chlorine gas has characteristic odour, colour and bleaching properties. Chromyl chloride (CrO₂Cl₂) test is a unique test to identify Chloride ions. None of the other halides behave like Cl^– in this reaction.

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