Confirmatory tests for anions

19 CONFIRMATORY TESTS FOR ANIONS In every case where the presence of any anion has been indicated, it must be confirmed by at least one distinctive confirmatory test. Conclusive tests for anions (halides, sulphate, oxy-halides, thiocyanate, phosphate, silicate, fluoride, cyanide and chromate) have already been given in Section V.18, it will, of course, be unnecessary to confirm these further. The following list, which, for the sake of completeness includes those anions already referred to, will assist the student in the choice of suitable tests. Full experimental details will be found in Chapter IV under the reactions of the anions the reference to these will be abbreviated as follows thus (IV.2, 3) is to be interpreted as Section IV.2, reaction 3. It is assumed, of course, that interfering acids are absent or have been removed. 

Chloride, (i) Heat solid with concentrated H2S04 and Mn02 Cl2 evolved (bleaches litmus paper and also turns potassium iodide-starch paper blue) (IV.14, 2). (ii) Chromyl chloride test (IV.14, 5). (iii) Silver chloride-sodium arsenite test (IV.14, 3). 

Bromide, (i) Heat solid with concentrated H2S04 and Mn02 Br2 evolved 

Iodide, (i) NaOCl-HCl (or chlorine water) and CHC13 or CC14 test violet colouration (IV.16, 4). (ii) NaOCl-HCl and starch paste test blue colouration (IV.16, 4). 

Fluoride, (i) Etching test (IV.17, 2). (ii) Silicon tetrafluoride test heat with concentrated sulphuric acid in a test-tube (IV.17, 1) better, test 8 in VII.16. 

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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. 

If a mixture of halides, or of phosphate, arsenate, and arsenite, is suspected, use the methods of separation given in Section V.18, 4. The confirmatory tests for the individual anions are collected in Section VI.6. 

If a mixture of the halides, or of phosphate, arsenate or arsenite, is suspected, use Table V.30 or V.31 given in Section V.18. The confirmatory tests for individual anions are collected in Section V.19. The reactions with silver nitrate solution are intended to act as a guide to the presence of groups of anions, and the table must be interpreted in conjunction with the observations made in the preliminary tests. Arsenite, arsenate, and chromate will also be found in the analysis for cations (Sections V.12 and V.14). 

The detection of anions in the original sample does not proceed in the same way in that the material is usually subjected to a series of preliminary tests. Moreover, the cation analysis described above can often be assessed to infer the presence of certain anions. A sample solution is separately treated with AgNOs and BaCh, and the various precipitates of silver and barium are used to verify the presence of anionic components. This procedure is then followed by the addition of concentrated cold H2SO4 to the unknown solid, which results in the liberation of characteristic gases (e.g., 1 gives the odor of H2S and violet fumes of I2), and by a series of confirmatory tests for each anion. 

Colorimetric methods are most common and widely employed in environmental wet analysis. Most anions, all metals, and many physical and aggregate properties can be determined by colorimetric technique, which is fast and cost-effective. The method may, however, be unreliable for dirty and colored samples. Often, the presence of certain substances in samples can interfere with the test. In addition, if the color formation involves a weak color such as yellow, additional confirmatory tests should be performed. Despite these drawbacks, colorimetry is often the method of choice for a number of wet analyses. 

This test method comprises a continuously operating activated sludge test which is recommended by OECD as a Confirmatory Test [60] for anionic surfactants and has been included in the relevant EEC guideline [58] in the form of a reference test. 

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