_Chloride of potassium_ is another impurity; it is detected as follows:--Precipitate the salt by an equal weight of nitrate of silver, and treat the yellow mass with solution of ammonia; if any chloride of silver is present, it dissolves in the ammonia, and after nitration is re-precipitated in white curds by the addition of an excess of pure nitric acid. If the nitric acid employed is not pure, but contains traces of free chlorine, the iodide of silver must be well washed with distilled water before treating it with ammonia, or the excess of free nitrate of silver dissolving in the ammonia would, on neutralizing, produce chloride of silver, and so cause an error. _Iodide of potash_ is a fourth impurity often found in iodide of potassium: to detect it, add a drop of dilute sulphuric acid, or a crystal of citric acid, to the solution of the iodide; when, if much iodate be present, the liquid will become yellow from liberation of free iodine. The rationale of this reaction is as follows:--The sulphuric acid unites with the base of the salt, and liberates hydriodic acid (HI), _a colorless compound_; but if iodic acid (IO{5}) be also present, it decomposes the hydriodic acid first formed, oxidizing the hydrogen into water (HO), and setting free the iodine. The immediate production of a yellow color on adding a weak acid to aqueous solution of iodide of potassium is, therefore, a proof of the presence of an iodate. As iodate of potash is thought to render collodion insensitive (?), this point should be attended to. Iodide of potassium may be rendered very pure by recrystallizing from spirit, or by dissolving in strong alcohol of sp. gr. ·823, in which sulphate, carbonate, and iodate of potash are insoluble. The proportion of iodide of potassium contained in saturated alcoholic solutions varies with the strength of the spirit. Solution of chloride of barium is commonly used to detect impurities in iodide of potassium; it forms a white precipitate if carbonate, iodate, or sulphate be present. In the two former cases the precipitate dissolves on the addition of _pure_ dilute nitric acid, but in the latter it is insoluble. The commercial iodide, however, is rarely so pure as to remain quite clear on the addition of chloride of barium, a _mere opalescence_, therefore, may be disregarded. Iodide of Silver. (_See_ Silver, Iodide of.) Iron, Protosulphate of. Symbol, FeO SO{3} + 7 HO. Atomic weight, 139. This salt, often termed _copperas_ or _green vitriol_, is a most abundant substance, and used for a variety of purposes in the arts. Commercial sulphate of iron, however, being prepared on a large scale, requires recrystallization to render it sufficiently pure for photographic purposes.