Special tests for mixtures of anions on the semimicro scale

7 SPECIAL TESTS FOR MIXTURES OF ANIONS ON THE SEMIMICRO SCALE The subject is treated fully in Section IV.45, but on a macro scale. The student should be able to adapt these to semimicro work. Some typical semimicro separations of mixtures of anions are given below. The quantities are for guidance only and can be reduced, if desired. If the mixture is insoluble in water it is often convenient to employ the soda extract (Section VI.5) or the neutralized soda extract (Section VI.5, test 5). The special gas testing apparatus of Fig. 11.30 will, of necessity, find application here. 

Carbonate in the presence of sulphite T reat 20 mg of the mixture with 20 mg of finely powdered K2Cr207 and 0-5 ml dilute H2S04 place the apparatus (Fig. II.30b or c) in the hot water rack, and test for C02 with lime or baryta water. (The addition of a few milligrams finely divided zinc will generate H2 and assist in driving the C02 out of the tube.) 

Nitrate in the presence of nitrite Dissolve 10 mg of the solid in 1 ml of water. Remove 1 drop with a glass stirring rod and spot on Kl-starch paper moistened with very dilute H2S04 a blue colour is obtained, due to the iodine liberated by the nitrous acid. Add 20-30 mg sulphamic acid and stir until effervescence ceases (Section IV.7, 6). Test for absence of nitrite by spotting on Kl-starch paper. Then apply the brown ring test (Section VI.6). 

Nitrate in the presence ofbromide and iodide Treat 10-15 mg of the substance with 1 ml sodium hydroxide solution and 10-15 mg of Devarda s alloy (or of aluminium as powder or as thin foil), place in the hot water rack, and test for ammonia with red litmus paper or mercury nitrate paper (Fig. II.30a). 

Another procedure is to just acidify 5 drops of the soda extract with dilute acetic acid, and then add the ammoniacal silver sulphate reagent (see Section 

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