Special tests for mixtures of anions

45 SPECIAL TESTS FOR MIXTURES OF ANIONS When the reactions of individual anions have been studied, the student should try his skill in carrying out these special tests. All of these are of great practical importance, as samples often contain mixtures of ions which might interfere with each other s tests. 

Full experimental details of the various tests referred to in this section will be found under the reactions of the anions. Reference will be made to the soda extract and to the neutralized soda extract (see 20 below) as these are normally used in systematic analysis. A solution of the sodium salts of the various anions may, of course, be used when trying out these procedures. 

Carbonate in the presence of sulphite Sulphites, on treatment with dilute sulphuric acid, liberate sulphur dioxide which, like carbon dioxide, produces a turbidity with lime or baryta water. The dichromate test for sulphites is, however, not influenced by the presence of carbonates. To detect carbonates in the presence of sulphites, treat the solid mixture with dilute sulphuric acid and pass the evolved gases through a small wash-bottle or boiling tube containing potassium dichromate solution and dilute sulphuric acid. The solution will be turned green and the sulphur dioxide will, at the same time, be completely removed the residual gas is then tested with lime or baryta water in the usual manner. 

An alternative procedure is to add a little solid potassium dichromate or a small volume of 3 per cent hydrogen peroxide solution to the mixture and then to warm with dilute sulphuric acid the evolved gas is then passed through lime or baryta water. 

Nitrate in the presence of nitrite The nitrite is readily identified in the presence of a nitrate by treatment with dilute mineral acid, potassium iodide, and starch paste (or potassium iodide-starch paper). The nitrate cannot, however, be detected in the presence of a nitrite since the latter gives the brown ring test with iron(II) sulphate solution and dilute sulphuric acid. The nitrite is therefore completely decomposed first by one of the following methods  

It is not possible to devise a comprehensive separation scheme for anions however, it is possible to detect them individually in most cases, after perhaps a 1-2 stage separation. It is advantageous to remove all heavy metals from the sample by extracting the anions through boiling with sodium carbonate solution heavy metal ions are precipitated out in the form of carbonates, while the anions remain in solution accompanied by sodium ions. The selected tests are listed in order of increasing complexity it is advisable to do them in the order they are described. 

Preparation of the soda extract Intimately mix the solid sample with 3-4 times (in volume) of anhydrous sodium carbonate, add just sufficient water to dissolve the sodium carbonate. In the case of dissolved samples, add 5g anhydrous sodium carbonate to 10 ml solution. Heat the mixture under reflux for 15 minutes and filter. Wash and discard the residue. Acidify the combined filtrate and washings gently with 8m nitric acid, boil gently to expel carbon dioxide and allow to cool. Add 2m ammonia solution portionwise and with stirring, until the solution just turns alkaline. Boil gently for 1-2 minutes to remove excess ammonia. Cool, and filter again if necessary. The filtrate will be termed here the neutralized soda extract . 

Carbonate in the presence of sulphite sulphites, on treatment with dilute sulphuric acid, liberate sulphur dioxide which, like carbon dioxide, produces a turbidity with lime or baryta water. The dichromate test for sulphites is, 

NO3 + NO2 2. Nitrate in the presence of nitrite the nitrite is readily identified in the 

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VI.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. 

VIII.7 SPECIAL TESTS FOR MIXTURES OF ANIONS The tests described in this section are recommended even for beginners, as the chemistry involved in these is of great educational value. The tests will only briefly be described full experimental details of the various tests are described in Chapter IV. 

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