Chemical Elements - Analysis.

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Free chlorine may be recognized by its smell, its colour, and its characteristic reaction with mercury to produce white mercury(II) chloride. Tests for chloride ions are:
1. The formation of a white precipitate of silver chloride on addition of silver nitrate in dilute nitric acid. (This precipitate is soluble in the presence of ammonia.)
2. The formation of chromyl chloride, a red gas, by heating a solid sample with potassium dichromate and concentrated sulfuric acid. When chromyl chloride is passed into water, a yellow chromate solution forms (bromides and iodides do not form analogous compounds).
3. The evolution of free chlorine by heating the sample with manganese dioxide and concentrated sulfuric acid.
The following methods are available for the quantitative determination of free chlorine:
1. The chlorine-containing gas is shaken with an aqueous solution of potassium iodide, and the resulting iodine is determined by titration.
2. Chlorine is reduced in alkaline solution by an alkali arsenite. Back-titration of excess arsenite is carried out with potassium bromate.
3. In the presence of an alkali hydroxide, chlorine is reduced to the chloride ion by hydrogen peroxide, and the excess alkali hydroxide is back-titrated with acid.
4. With sulfur dioxide or sodium thiosulfate, chlorine is reduced to chloride, and the latter is analyzed as silver chloride (see below).
5. Colorimetric measurements are carried out in the presence of o-toluidine in hydrochloric acid.
For the determination of chloride ions, one of the following methods may be recommended: (1) gravimetric analysis (analysis by weight of a given product) as silver chloride; (2) titration of neutral chloride solution with silver nitrate in the presence of potassium chromate; and (3) potentiometric titration (measurement of voltage changes) with silver nitrate, a process that can be carried out in the presence of bromide and iodide ions.
Most insoluble chlorides can be melted with soda, and the resulting melt is then usually soluble in water. Organic compounds containing chlorine are heated with alkali peroxide, and the product is dissolved in water.

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Updated: 2/9/98
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