Potassium polyiodides

Iodine is only slightly soluble in water and no hydrates form upon dissolution. The solubiHty increases with temperature, as shown in Table 2 (36). Iodine is soluble in aqueous iodide solutions owing to the formation of polyiodide ions. For example, an equiHbrium solution of soHd iodine and KI H2O at 25°C is highly concentrated and contains 67.8% iodine, 25.6% potassium iodide, and 6.6% water. However, if large cations such as cesium, substituted ammonium, and iodonium are present, the increased solubiHty may be limited, owing to precipitation of sparingly soluble polyiodides. Iodine is also more... 

A soln. of twenty-two grains of iodine and thirty-three grains of iodide of potassium, in one ounce of distilled water forms the liquor iodi of the British Pharmacopoeia. The effects produced by the ammonium salts are attributed to their hydrolysis into ammonium hydroxide, and the consequent formation of ammonium iodide or polyiodide. The effects produced by soln. of the halide salts are doubtless due to the formation of poly iodides as in the analogous case with bromine and potassium bromide. A. A. Jakowkin allowed carbon disulphide to remain in contact with aq. soln. of iodine and potassium iodide until equilibrium was attained and... 

When iodine is dissolved in hydriodic acid or a soln. of a metallic iodide, there is much evidence of chemical combination, with the formation of a periodide. A. Baudrimont objected to the polyiodide hypothesis of the increased solubility of iodine in soln. of potassium iodide, because he found that an extraction with carbon disulphide removed the iodine from the soln. but S. M. Jorgensen showed that this solvent failed to remove the iodine from an alcoholic soln. of potassium iodide and iodine in the proportion KI I2, and an alcoholic soln. of potassium iodide decolorized a soln. of iodine in carbon disulphide. The hypothesis seemed more probable when, in 1877, G. S. Johnson isolated cubic crystals of a substance with the empirical formula KI3 by the slow evaporation of an aqueous-alcoholic soln. of iodine and potassium iodide over sulphuric acid. There is also evidence of the formation of analogous compounds with the other halides. The perhalides or poly halides—usually polyiodides—are products of the additive combination of the metal halides, or the halides of other radicles with the halogen, so. that the positive acidic radicle consists of several halogen atoms. The polyiodides have been investigated more than the other polyhalides. The additive products have often a definite physical form, and definite physical properties. J. J. Berzelius appears to have made the first polyiodide—which he called ammonium bin-iodide A. Geuther called these compounds poly-iodides and S. M. Jorgensen, super-iodides. They have been classified 1 as... 

A. A. Noyes 5 to the belief that the number of molecules in the soln. is not changed by the addition of more iodine because polyiodides are formed MI+mI2=MI2 +i. Y. Osaka showed that the rise of the f.p. which occurs when iodine is added to hydriodic acid or to aq. soln. of potassium iodide is proportional to the amount of iodine added, and is greater for hydriodic acid than for the potassium salt. Hence, the total concentration of the ions and of unionized molecules is decreased by the addition of iodine. A. A. Jakowkin inferred from the partition coeff. of iodine in dil. soln. that potassium tri-iodide was formed, and that with more cone. soln. still more complex polyiodides are produced. Still further, the change in the partition coeff. of iodine between aq. soln. of potassium iodide and nitrobenzene led H. M. Dawson and R. Gawler to infer that polyiodides as high as potassium ennea-iodide, KI9, are probably present in soln., although no such compound has been obtained in the solid state. H. L. Wells and H. L. Wheeler and others, however, have prepared several solid alkali polyiodides for example ... 

The polyiodides, and the chloro- and bromo-iodides.—An aq. soln. of one of the alkali iodides dissolves considerable quantities of iodine,18 and this the more, the greater the amount of iodine in soln. Thus, at 7°, soln. of potassium iodide dissolve the following proportions of iodine ... 

The increased solubility is due to the formation of polyiodides, and evidence of the existence of iodides as high as the enna-salt, KIfl, has been previously discussed. The tri-iodides of potassium, rubidium, and caesium have all been isolated. The m.p., temp, of whitening, and the axial ratios of the crystals of potassium, rubidium, and caesium tri-iodides are indicated—Table XXXI—vide Fig. 19, 2. 19, 16. 

Other properties of aqueous solutions investigated are density,3 refractive index,4 molecular elevation of the boiling-point,6 vapour-pressure,6 specific heat,7 and electric conductivity.8 References are also appended to work on the compressibility,9 the solubility in organic solvents10 and sulphurous acid,11 the molecular weight in liquid sulphur dioxide,11 the electric conductivity in acetone12 and dilute alcohol,13 the non-existence of polyiodides,14 isomorphism with potassium iodide,15 and the formation of a double salt with silver iodide.16... 

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 Chalker8 state that the compounds KI3 and KI7 have a definite existence. 

Tebbe, K.F. and Dombrowski, L, Studies of polyhalides. XXXVQI. On potassium-benzo-18-crown-6-polyiodides [K(benzo-18-crown-6)]In withn = 3, 4, 6 Preparation and crystal structures of a triiodide [K(benzo-18-crown-6)]l3, an octaiodide [K(benzo-18-crown-6)]2l8 I/2C2H5OH and a dodecaiodide [K(benzo-18-crown-6)]2li2, Z. Anorg. Mlg. Chem. 625 (1), 167-174 (1999). 

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