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protiodide of mercury, (e.) On the addition of starch and a few dropsof cither sulphuric or nitric acid, or of a solution of chlorine, (accordingto Devergie, a mixture of chlorine and nitric acid succeeds best) theblue iodide of starch is fonned. (f.) Chloride of platinum produces abrownish red solution of iodide of platinum.
That the base of this salt is potassium is shewn by the following cha-racters : — («.) Perchloric acid occasions a white precipitate of perchlo-rate of potash, while the supernatant liquid becomes yellowish brown.(b.) An excess of a strong solution of tartaric acid produces a white pre-cipitate of the bitartrate of potash, (e.) Carbazotic acid forms yellowneedle-like crystals of carbazotate of potash. ( d .) If a pack-thread besoaked in a solution of the iodide, and the wetted end dipped into meltedtallow, and applied to the exterior cone of the flame of a candle, thiscone assumes a pale or whitish violet colour.
Composition. —This salt consists, as its name indicates, of iodine and
Iodide Potassium ... 1.106.100.lOO'O
The crystals contain no water of crystallization.
Adulteration.— Iodide of potassium is frequently largely adulteratedwith carbonate of potash. In 182!) I analyzed a sample which contained77 per cent, of the latter salt {Med. and Phys. Jour. September, 182!).) Inone specimen Dr. Christison procured 74'5 per cent, of carbonate ofpotash, 16 of water, and only !)'5 of iodide of potassium ( Treatise onPoisons, 3d. edit. p. 182.) It is reported that this adulterated salt ismanufactured in the neighbourhood of Glasgow . The impure salt maybe distinguished, by its wanting any regular crystalline form ; by addinga few particles of it to lime-water a milky fluid is obtained, whereas theliquid remains transparent if the iodide be pure ; by its destroying thecolour of tincture of iodine, whereas the pure salt does not affect it ; andlastly, by alcohol, which dissolves iodide of potassium, but not carbonateof potash.
Traces of the chlorides and sulphates are not unfrequent in commer-cial iodide of potassium. To detect the chlorides add nitrate of silver,which precipitates the carbonates, chlorides, and iodides, and digest theprecipitate in ammonia, which re-dissolves the chloride, but not theiodide of silver. On the addition of nitric acid to the ammoniacal solu-tion, the chloride is thrown down, while the carbonate is converted intonitrate of silver. The sulphates may be detected by chloride of barium,which will occasion a white precipitate (sulphate of baryta) insoluble innitric acid.
I have met with a variety of iodide of potassium, which, by keeping,undergoes decomposition, evolves an odour of iodine, and becomes yellow.I have also samples of iodide of potassium (crystallized in octahedrons),containing traces of lead and tin, derived I presume from the vessels inwhich the salt has been made or crystallized.
Physiological Effects, (a.) On plants. —The effects of this salt onvegetables have not been ascertained.
{b.) On animals generally. —The experiments of Devergie on dogs(referred to in his Medecine Legale , t. ii. p. 536), as well as those of Dr.