IODIDE OF POTASSIUM ; KI=165.
Iodine and potassium unite energetically, giving out heat and light ; and potassium burns in the vapour of iodine. The result of this combination is the white anhydrous salt—iodide of potassium. The mode of producing this salt commercially is to add iodine to a solution of potass, until it assumes a brown colour ; then evaporate to dryness, add a little charcoal, and fuse the residue at a red heat, which decomposes any iodate of potass that may be formed ; dissolve, filter, and recrystallize the salt.
Sometimes iodide of potassium is made by passing sulphuretted hydrogen through a brown mixture of liquor potassEe and iodine, till it becomes colourless ; expelling any excess of sulphuretted hydrogen by heat, filtering, neutralizing with potass, and crystalliz ing. Another way is, to decompose either iodide of zinc or iodide of iron by carbonate of potass.
Iodide of potassium crystallizes both in cubes and prisms. The crystals are anhydrous, and very slightly deliquescent in damp air. They are extremely soluble in water, 100 parts of which dissolve at 65° 143 parts of the salt, with production of cold ; hut much less soluble in alcohol; absolute alcohol S. G. 794, dissolving about 8 grains to the ounce ; S. G. 823, about 25 grains ; S. G. 835, about 60 grains.
The impurities contained in commercial iodide of potassium are iodate of potass, carbonate of potass, sulphate of potass, chloride of potassium, and sometimes of zinc, and iron.
Iodate of potass may be detected by adding a little tartaric acid to the solution, and also a little starch. If iodate be present the
tartaric acid decomposes it, and the liberated iodine forms purple iodide of starch. Fusion, at a red heat, decomposes iodate into iodide.
Carbonate and sulphate of potass are detected by adding chloride of barium to the solution, which throws down white insoluble car bonate, or sulphate of baryta. Another test for carbonate of potass is to expose the alcoholic solution of the iodide to sunshine ; if it soon becomes slightly discoloured it is pure, if not, it contains car bonate. The presence of much carbonate of potass renders iodide of potassium highly deliquescent.
Aqueous solution of iodide of potassium is immediately discoloured by the addition of chlorine water, or nitric acid, but not by the weaker acids in the dark ; in the light the discoloration proceeds more rapidly.
This salt is much used as the iodizer for the negative processes on collodion or waxed paper. The white spots sometimes seen in negatives, and which are square when viewed in the microscope, are supposed to be due to undecomposed crystals of iodide of potassium. These peculiar spots do not occur with other iodizers. No iodide produces a more sensitive collodion than iodide of potassium, and it is on the whole a very good one to employ. The alcoholic solution is stable in the dark, but collodion iodized with it becomes gradually reddened by time, and rendered insensitive.