Tri-Bromide Equilibria. 123 



solving in a mixture of acetic acid and water, but tins would 

 only be a parallel if it could be shown, for example, that a 

 number of different esters of acetic acid, in equivalent concen- 

 tration, increased the solubility of iodine (or camphor) in 

 water by exactly the same amount, or if the same thing were 

 proved for a series of substituted acetic acids. 



The experimentally established fact which calls for explana- 

 tion is that in dilute water solution equivalent concentrations 

 of all metallic iodides, excepting only, so far as yet known, 

 those of cadmium and mercury, take up, at the same tempera- 

 ture, the same concentration of titrable iodine, when brought 

 into equilibrium with solid iodine or with a carbon bisulphide 

 layer containing iodine in a fixed and constant concentration. 

 That this relation should become inaccurate with increasing 

 concentration and finally fail altogether, is only what would be 

 expected from the deviations from the law of mass action 

 which are normally observed in concentrated solutions. Its 

 failure for cadmium and mercury is wholly in keeping with 

 the other abnormalities of the halides of these two metals. 



The existence of such clear-cut stoichiometric relations 

 between the concentrations of iodide and of iodine is the 

 strongest possible proof that the plienomenon is not due to 

 solubility in the ordinary sense, but rather, to the formation of 

 a definite compound. Application of the law of mass action 

 leaves no doubt that this compound is the tri-iodide. 



Summary. 



1. The iodine-iodide equilibrium has been studied at 25° in 

 a number of cases not previously investigated, and the value of 

 A" = (21) ([,) / (2I3) determined. 



2. For the iodides of strontium, zinc, nickel, and lanthanum, 

 the value of K^ in dilute solution is normal. For cadmium 

 iodide, and double iodides containing cadmium or mercury, the 

 value of K^ is much higher, and rises rapidly with increasing 

 iodide concentration. 



3. Similar abnormality in the corresponding constant for the 

 bromine-bromide equilibrium was found in the case of cadmium^ 

 bromide, mercuric bromide, and double salts containing either 

 of these. 



4. The measurement of K^ in abnormal cases furnishes a 

 useful method for determining the percentage of complex 

 molecules and ions, not only in the equilibrium mixture, but 

 also in solutions containing no free haK)gen. This method is 

 based on the very probable assumption that the complex mole- 

 cules and ions do not combine with the halogen. 



