Testing for anions in solution on the semimicro scale

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  

Sodium ions will accompany the anion A- in the solution. 

Sulphate test To 5 drops of the soda extract in a 4 ml test-tube add dilute hydrochloric acid until acid (test with litmus paper) and then add 3 drops in excess. Place in the hot water rack for 5 minutes to expel carbon dioxide completely, and then add 2-3 drops of barium chloride solution. A white precipitate (BaS04) shows the presence of sulphate. 

Test for reducing agents Acidify 5 drops of the soda extract with dilute sulphuric acid and add 3 drops of dilute sulphuric acid in excess. Add 2-3 drops of dilute potassium permanganate solution (prepared by dilution of 1 drop of 

Tests with silver nitrate solution -Sulphide, cyanide, and sulphite interfere in tests with silver nitrate solution, hence if any of these anions was detected in the preliminary test with dilute sulphuric acid, it must be removed first as follows. Acidify 1 ml of the soda extract with dilute acetic acid (use litmus paper) and boil gently in a small conical flask or crucible in the fume cupboard to expel H2S, HCN or S02 (1-2 minutes). It is important that the solution be acid throughout. Centrifuge, if necessary, and allow to cool. If the volume has been reduced appreciably, add water to restore the original volume. 

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