This is made according to different processes. Our officinal code directs that iodine and potassa should be dissolved together in water, the solution evaporated, charcoal incorporated towards the close of the evaporation, the resulting dry mass heated to dull redness, the saline matter afterwards dissolved out of it by water, and the solution, finally, filtered, evaporated, and set aside to crystallize. The iodine and potassa react on each other, a part of the latter yielding its oxygen to a part of the former to produce iodic acid, which unites with the undecomposed potassa forming iodate of potassa, while the separated potassium and the remaining iodine unite to form iodide of potassium. By the heat subsequently applied, with the aid of the charcoal, the iodate of potassa is deprived of its oxygen, and is converted into iodide of potassium; so that the matter extracted by the water consists exclusively of the latter salt, which is obtained by evaporation.

Properties

iodide of potassium is in white or translucent crystals, inodorous, of a saline acrid taste, very freely soluble in water cold or hot, somewhat less so in alcohol, decrepitating with heat, and volatiliza-ble at a high temperature without change. When quite pure, it is said to be permanent in the air whether dry or moist; but, as ordinarily found, it is slightly deliquescent in a moist air, and sometimes very much so, in consequence of the presence of carbonate of potassa as an impurity. its great importance and liability to adulteration render it desirable that every one should be able to recognize it, and judge of its purity. it is known to be an iodide by the production of a blue compound, decolorizable at a boiling heat, when to its solution in water, a cold solution of starch and a few drops of strong nitric acid, or chlorine water, or a mixture of the two are added. That it is a compound of potassium is proved by a copious precipitation of white bitartrate of potassa, when a strong solution of tartaric acid is added in excess to a strong solution of the iodide. The most common adulteration is probably carbonate of potassa. This is detected by producing a milky turbidness with lime-water, and by being left behind when the salt is dissolved by alcohol. iodate of potassa is often accidentally present, from its incomplete decomposition in the preparation of the salt. it is an injurious impurity, in consequence of the facility with which its acid parts with oxygen, and the iodine is liberated. This change will gradually take place even by time alone : but it is much accelerated by various chemical reagents; so that the iodate could scarcely be introduced into the stomach, without undergoing decomposition and evolving iodine. As the iodide is sometimes given largely, it will be readily understood why it should now and then irritate the stomach unexpectedly. if it contain a considerable proportion of iodate, this is what might be expected. The absence of this salt should always, therefore, be determined before large doses of the iodide are exhibited. it may be detected by adding concentrated acetic acid to the solution of the suspected salt. if the iodate is present, the fact will be shown by the liberation of iodine, which will produce a blue colour on the addition of starch. {Mialhe.) The iodate also, like the carbonate, is left behind when the salt is dissolved in alcohol. Other occasional impurities are the chlorides of potassium and sodium, and the bromide of potassium; but, for the means of detecting these, I must content myself with referring the reader to the U. S. Dispensatory.

Incompatibles. iodide of potassium is decomposed, with precipitation, by the soluble compounds of mercury, lead, and silver, and by the bitartrate of potassa, and, with the evolution of hydriodic acid, or iodine, by sulphuric, nitric, and muriatic acids, and by chlorine. By prolonged contact, it renders the insoluble compounds of mercury, and even mercury itself soluble.

Effects on the System

These have been already sufficiently described under the general head of iodine. in its local operation, iodide of potassium differs from iodine in being much less irritant; and large quantities may generally be taken into the stomach with impunity. Sometimes, however, it irritates, even in moderate quantities, producing nausea, colicky pains, and looseness of the bowels. This is ascribed by M. Mialhe to the presence of iodate of potassa. it may sometimes be owing to individual peculiarity. if freely given, it may result from its having been administered in too concentrated a solution. iodide of potassium quickly passes unchanged out of the system, by the various emunctories, especially the salivary glands and the kidneys. From the experiments of M. Cl. Bernard upon the lower animals, it appears to be discoverable in the saliva very soon after having been swallowed; considerably earlier, indeed, than in the urine. (Med. Times and Gaz., Feb. 1860, p. 160.) in the latter secretion it was found by Dr. E. Hardy from the seventh to the eighth minute after having been taken. (Journ. de Pharm. et de Chim., 3e sÚr., xliv. 158, a.d. 1853.) The results obtained by Bernard, in reference to the earlier appearance of the salt in the saliva than in the urine, in the lower animals, have been invalidated, as regards man, by an experiment of Dr. Geo. Harley upon himself. Having swallowed five grains of the iodide, he detected it in ten minutes in his saliva, and one minute afterwards in the urine; so that the elimination by the parotids and the kidneys must have been simultaneous or very nearly so. in regard to the length of its detention in the system, Dr. Harley states that, at the end of 24 hours it could still be detected in both secretions, of 48 hours only in the urine, and of 72 hours but doubtfully in the latter. (B. and F. Medico-chir. Rev., Jan. 1860, Am. ed., p. 151.) it probably continues to remain longer in the system in consequence of the reabsorp-tion from the stomach of that portion swallowed with the saliva; and this circle of action must be kept up until the last particle at length escapes by the kidneys or other emunctories. Another interesting result of its copious secretion with the saliva, is its continued local application to the mouth and fauces, thus in some measure accounting for its remarkable influence in the cure of obstinate ulcers in the throat, as also for its tendency, when long used, to excite irritation if not inflammation in the mucous membrane of these parts.