Potassium iodide, KI

The Potassium iodide, KI, is obtained by neutralizing the carbonate with hydriodic acid, and also by the interaction of potassium hydroxide and iodine, the iodate simultaneously produced being converted into iodide by heat or by the action of a reducer such as carbon:

6KOH + 3I₂ = 5KI + KIO₃ + 3H₂O.

It is manufactured by the action of iron-filings on iodine in presence of water, the soluble iodide Fe₃I₈ formed being decomposed by potassium hydroxide, a process accompanied by precipitation of the oxide Fe₃O₄ and formation of a solution of potassium iodide. The salt crystallizes on concentration of the aqueous solution. It can also be prepared from the ashes of seaweed, and from the iodate present in Chile saltpetre.

Potassium iodide forms crystals belonging to the cubic system, melting at 677.3° C., 680° C., 684.1° C., 705° C., or 722.7° C. Its density is given by various investigators as 3.079, 3.043 at 24.3° C., and 3.115 at 25° C. The specific heat is given as 0.0766, and 0.0819 between 20° and 99° C. The heat of formation from the solid elements is 80.1 Cal.

The molecular electric conductivity of potassium iodide between 691.5° and 814° C. is given by the formula

μ_t = 85.41 + 0.1564(t - 700).

The salt dissolves in water to form a neutral solution, readily decomposed by atmospheric oxygen and carbon dioxide with separation of iodine.

Solubility of Potassium iodide in water

Temperature. °C.	-10	0	10	20	30	40	50	60	70	80	90	100	110	120
Solubility of KI in 100 gr of water	115.1	127.5	136.0	144	152	160	168	176	184	192	200	208	215	223

The boiling-point of a solution in contact with excess of the salt is 118.4° C., and the liquid contains 222.6 grams of dissolved salt per 100 grams of water.

At 25° C. 100 grams of ethyl alcohol dissolve 2.16 grams of potassium iodide.

Potassium iodide finds application in medicine. The chief impurities present in the " pure " commercial product are iodate, carbonate, and chloride of potassium.

On heating potassium iodide with ammonium bromide in a long narrow tube of hard glass, iodine is liberated. By the aid of this test one-tenth of a milligram of the iodide can be detected.

Various polyiodides of potassium have been mentioned as formed by addition of iodine to solutions of potassium iodide in water and in organic solvents. There seems to be much uncertainty as to whether they are true poly-derivatives or only mixtures of potassium iodide and iodine, but Foote and Chalker state that the compounds KI₃ and KI₇ have a definite existence.