are brought together, rapid combination takes place,with a considerable rise of temperature and theevolution of a violet light. The union of the twoelements is often attended with a violent explosion.The result is iodide of potassium.
This salt is one of the most valuable compoundsof iodine ; it is of great importance in medicine; andas a reagent in the laboratory and also in photo-graphy it is in general use. There are severalmethods adopted in its preparation, of which thefollowing are most generally employed :—
Iodine is added to a solution of caustic potassa aslong as it dissolves. The point of saturation isknown by the liquid acquiring a brownish tint. Theexcess of iodine is redissolved by a proportionatequantity of the alkaline lye. By evaporation thereis obtained a white residuary mass, consisting ofiodide and iodate ; the latter is decomposed by fusingthe mass with a little charcoal, when the iodate isdecomposed, and iodide of potassium only remains.The temperature must not be too elevated, or someloss of iodide of potassium by volatilization is sus-tained. No iodine is liberated, and the whole ofthe oxygen of the iodate is taken up by the carbon,ivhich is thereby converted into carbonic acid.After heating, the mass is exhausted with water,which dissolves out the iodide. Alcohol may besubstituted with advantage when a very pure pro-duct is desired, for this takes up only the iodide andleaves the carbonate of potassium, which is nearlyalways present. By distillation the spirit may beremoved, and the iodide crystallized from the con-centrated alcoholic solution or from water.
An excellent method is to saturate a solution ofcarbonate of potassa with hydriodic acid, preparedby the action of sulphydric acid on iodine, andevaporate the solution to crystallization ; or it maybe made from zinc iodide. Zinc turnings, water,and iodine are agitated together in a close vessel,and the metallic salt which is thus produced is after-rvards decomposed by addition of carbonate of potassato the liquid as long as a precipitate appears. Thecarbonate of zinc is filtered off, and the solutionevaporated to the crystallizing point. The objectionto this method is the extreme difficulty experiencedin washing out the last traces of iodide of zinc fromthe precipitated carbonate. In order to obtain apure salt by this process, Girault gradually poursthe cold solution of the iodide of zinc into a boilinghot one of carbonate of potassium as long as thereis any effervescence observed; cold water is thenadded to the mixture, and the whole filtered, andthe carbonate of zinc washed twice with water. Thetrace of zinc still held in solution is removed by thecautious addition of carbonate of potassium, theprecipitate filtered, and the liquid concentrated andset by to crystallize. Iodide of iron is more commonlyused: 2 parts of iodine, 1 of iron, and 10 ofwater are agitated together in a close vessel till theliquid loses its brown colour; as soon as thisoccurs the liquid is filtered, and precipitated by aboiling solution of carbonate of potassium. Caremust be taken that the air is as much as possible
excluded during the production of the iodide of iron,so as to prevent the formation of a basic iodide ofthis metal, which could be decomposed only by anexcess of alkali.
This process is troublesome on account of thepartial solubility of the carbonate of iron in theliquid ; this becomes oxidised, and precipitates outduring the evaporation.
Iodide of potassium crystallizes from its aqueoussolution in the form of cubes, parallelopipeds, pyra-mids, or octahedrons; and, according to Kane,sometimes in forms belonging to the square pris-matic system. The crystals are soluble in aboutthree-fourths of their weight of water at ordinarytemperatures, reducing the temperature about 75°'2Fahr. (24° C.). The boiling point of the saturatedsolution is about 248° Fahr. (120° C.). It is solu-ble in less than half its weight of boiling water.It is soluble in 5'5 parts of alcohol of '850 specificgravity. The salt melts at a temperature belowredness, and forms pearly crystals on cooling ; if airbe admitted to it whilst in a state of fusion, a por-tion volatilizes undecomposed. Schindler found,when performing the experiment in glass tubes,that the volatilization does not take place below thefusing point of the glass. Chlorine, sulphurousacid, sulphuric and nitric acids, decompose iodide ofpotassium, yielding iodine in the free state ; or in thecase of sulphuric acid when diluted, hydriodic acid,as noticed under the manufacture of iodine.
Iodide of potassium may be adulterated with car-bonate, chloride, sulphate, and iodate of potassium,and, under particular circumstances, traces ofxanthate of potassium. Carbonate of potassium,derived from the imperfect neutralization of theiodide or hydriodic acid, in the first instance, maybe discovered by the effervescence which succeedsthe addition of hydrochloric acid to a portion of thesalt, and also by its solution giving a precipitate withlime water; this impurity remains undissolved whenthe salt is treated with alcohol. Traces of alkalinechlorides and sulphates may be derived from the car-bonate used to prepare the salt. The presence of achloride is detected by its giving a precipitate withnitrate of silver, soluble in ammonia, and reprecipitatednitric acid. Sulphuric acid is detected by addingto a weak acid solution a few drops of chloride ofbarium. Alkaline sulphates are also left undissolvedby alcohol. Xanthate of potassium is only producedwhen the iodide is prepared by the action of sulphideof potassium upon iodine dissolved in alcohol.When this body is present the taste of the salt is likethat of assafoetida; it turns greyish-brown on beingheated, and evolves sulphurous acid; when theheated sesidue is treated with water, a substanceremains undissolved which is composed chiefly ofcarbon, but traces of sulphates are found in thesolution. Iodate of potassium is detected by addingtartaric acid, by which hydriodic acid is liberated;this reacts upon the iodate, its hydrogen combiningwith the oxygen of the iodic acid of the latter, andsetting the iodine free. Thus—
I110 3 + 5HI = 3I1 2 0 + 61.