Testing for anions in solution

5 TESTING FOR ANIONS IN SOLUTION Tests for anions should be carried out in the order as given below. 

Sulphate test To 2 ml of the solution add dilute hydrochloric acid until acid (test with litmus paper) and then add 2 ml in excess. Boil for 1 -2 minutes to expel carbon dioxide completely, and then add about 1 ml barium chloride solution. A white precipitate (BaS04) shows the presence of sulphate. Confirm by test 1 in Section IV.24 (charcoal reduction of precipitate). 

Tests with silver nitrate solution Sulphide and sulphite interfere in the tests with silver nitrate solution, hence must be removed first as follows. Acidify 

AgCl - white. AgBr - pale yellow. Agl - yellow. Add 1 ml AgN03 solution, then 5 per cent NaNOz solution (prepared from the A.R. solid) dropwise and with stirring until precipitation is complete. [If no precipitate forms, do not add more than 0-5 ml NaNOz solution.] Filter, if necessary, and wash with 2-3 ml of 2m nitric acid (2). 

AgCl derived from AgClOj. Add NaOH solution dropwise (use a dropper) and with vigorous stirring until neutral to litmus (3), then add 0 5 ml dilute acetic acid, followed by 1 ml AgN03 solution, and heat to about 80°C (4). If a permanent precipitate forms, add more AgN03 solution until precipitation is complete. Filter and wash with hot water. 

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Testing for anions in solution This includes the preparation of a soda extract , followed by systematic tests. These, in turn, can be made conclusive by confirmatory tests. 

VI.5 TESTING FOR ANIONS IN SOLUTION ON THE SEMIMICRO SCALE The preliminary tests 7 and 8 with dilute sulphuric acid and with concentrated sulphuric acid will have provided useful information as to many anions present. For more detailed information, it is necessary to have a solution containing all (or most) anions free from heavy metal ions. This is best prepared by boiling the substance with concentrated sodium carbonate solution double decomposition occurs (either partially or completely) with the production of the insoluble carbonates of the metals (other than alkali metals) and the soluble sodium salts of the anions, which pass into solution. Thus, if the unknown substance is the salt of a bivalent metal M and an acid HA, the following reaction will occur ... 

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