Detection & Identification of Anions

The method of detection of anions is not as systematic as that of basic radicals. It has not been possible to include acidic radicals (anions) into distinct groups as in the case of cations (basic radicals).

The process employed to detect an anion is divided into two classes:

1. Those involving the identification of volatile products obtained on treatment with acids.
   • Action of dilute sulphuric acid
   • Action of concentrated sulphuric acid
2. Those dependent on reaction in solution.

Before carrying out a confirmatory test in solution it is necessary to prepare a water solution or sodium carbonate extract of the salt.

Preparation of water solution for anion analysis

Take a pinch of given salt in a test tube. Add 2 to 3 mL of water and shake well.

Preparation of sodium carbonate of extract

If the salt is insoluble in water, preparation of sodium carbonate extract is necessary. Mix the salt with nearly twice its weight of sodium carbonate. Add sufficient distilled water in a dish, boil and filter. The filtrate is known as sodium extract.

Sodium carbonate extract always contains an excess amount of unreacted sodium carbonate which may interfere in the usual tests of acid radicals. It is absolutely necessary to destroy the excess of sodium carbonate. This is done by acidifying sodium carbonate extract with a suitable acid (choice depends upon the nature of the acid radical to be tested).