Identification Tests Iodide

The selectivity of the test is quite limited, even compared to the specificity seen in the identification test for chlorides. In the identification three criteria have to be fulfilled to qualify for a positive reaction. The unknown should give a white (curdled) precipitate formed upon addition of silver nitrate, which is insoluble in dilute nitric acid but redissolves in ammonia. In the limit test 2.4.4. Chlorides any substance capable of giving a white or weakly colored precipitate in dilute nitric acid will give a response like chloride, and this should be remembered in case of an xmexpected result. For the sake of example the following ions and substances are capable of giving a false positive reaction bromide, iodide, bromate, iodate, sulfite, chlorate, oxalate, and benzoate. In addition to this a variety of more complex organic substances are likely to precipitate, for example, alkaloids. 

Identification Cautiously dissolve 250 mg of sample in 5 mL of glacial acetic acid, and add a few drops of a saturated solution of potassium iodide. Iodine is liberated. Add 20 mL of water and sufficient sodium thiosulfate TS to remove the iodine color. The resulting solution gives positive tests for Calcium, Appendix IIIA. 

Identification Cautiously pass a few milliliters of sample gas through 10 mL of 1 N sodium hydroxide that has previously been chilled in an ice bath. The resulting solution gives positive tests for Chloride, Appendix IIIA, and it darkens starch iodide paper. 

Qualitatively, these may be quickly detected by the Beilstein test for halogens a copper wire is heated in a gas burner until no color can be seen and the coil plunged into the acetic acid, then brought into the gas flame again. Any trace of green or blue-green flame shows the presence of halogen. The lower identification threshold is about 0.7 ppm for chloride, about 0.65 ppm for bromide, and about 0.55 for iodide. 

The substance to be examined is changed to a colored compoxmd by the action of a reagent that does not become a part of the new entity formed. One example is test (b) for 3.19. Iodides, where identification is brought about by the violet or violet-red color produced when iodide is oxidized by dichromate to iodine. 

In the test, chloride is precipitated as silver chloride, and the precipitates reaction toward nitric acid and ammonia is used to rule out other silver precipitates. Since a very high number of elements and also organic substances give a precipitate with silver nitrate, the description below of selec-fivify is nof meanf to be conclusive. It mainly deals wito the other halogens and a few inorganic cations. Precipitation with silver nitrate is also used for the identification of iodide and bromide, and the difference between the tests of these is therefore dealf with in more detail. 

Antimony pentoxide and tetroxide are dissolved by strong hydrochloric acid in the presence of excess alkali iodide. Iodine is set free and Sblg or H[Sbl4] is formed. This complex acid gives a red-violet water-insoluble salt with the basic dyestuff Rhodamine B. The chemistry of this sensitive test and the procedure, which has an identification limit of 0.6 y antimony when conducted as a spot test, is discussed on page 108. 

When very small amounts of iodide are to be detected (quantities below the limit of identification of test (2)), the iodide may be oxidized toiodate. The iodine can then be liberated by the addition of iodide and acid ... 

The Sb+ ions formed combine with I--ions in acid solution to form H[Sbl4] or [Sbl4]" ions, provided an excess of potassium iodide is employed. The addition of a solution of Rhodamine B immediately produces a violet precipitate in solutions containing the complex acid. Even traces of SbaOg in mixtures with other oxides, or antimony oxide in residues after ignition of sulfidic minerals etc., can be sensitively detected. In this manner all the necessary steps for the identification of antimony with Rhodamine B may be conducted by means of the spot test technique. 

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