

Tri- Bromide Equilibria. 119 



of the lialide. It occurs, so far as jet proved, only with 

 cadmium and mercury. 



Cadmium iodide and mercuric chloride are conspicuous among 

 inorganic salts for their low ionization, and for a number of 

 other peculiarities of behavior which have long been ascribed 

 to the presence in their solutions of complex molecules and 

 ions. These peculiarities are known to be shared to a greater 

 or less extent by the other halide salts of the same metals. 

 Putting these facts together, the most obvious explanation for 

 the abnormality which these salts display toward the trihalide 

 equilibrium is to assume that the power to unite with the 

 halogen is possessed only by the normal molecules and ions and 

 not by the complex molecules and ions.''^ This hypothesis 

 seems to afford a satisfactory explanation of the observed facts, 

 and will be assumed hereafter to be approximately true. 



An important result of this point of view is that it permits 

 the calculation of the extent to which the simple molecules 

 have united to form complexes. We shall hereafter designate 

 the fraction of the total halide concentration which is in the 

 form of simple molecules, whether ionized or not, as the 

 " active fraction." Let us consider, for example, the case of 

 cadmium iodide. The apparent value of K^ as calculated from 

 the equation K^^i^l) {1^)/{^1^ is abnormally high, but plainly 

 the observed values of {1^ and (SIg) are independent of the 

 presence of complexes, and only subject to the normal experi- 

 mental error. Assuming the correctness of (Ij and (SIg) we 

 can calculate the actual value of (21) in the equilibrium mix- 

 ture from the equation 



(SI) = ^^i^ (VI) 



in which (SIg) and (I^) have their observed values in the given 

 cadmium iodide solution, and K^ = 0*0014, the value of that 

 constant for a normal iodide in moderately dilute solution. 



The "active fraction" is then obtained by dividing 

 (^I) + (SIg) by the equivalent concentration of cadmium iodide, 

 the value of (21) used here being, of course, the one calculated 

 from equation YL The same method applies in the case of 

 any other iodide or mixture of iodides, and also for bromides, 

 though the value of K^ at 25° in the latter case must be taken 

 as 0-()623 instead of 0-0014. 



*It miglit, at first sight, seem possible to explain the variation in Ki by 

 assuming that the degree of ionization of the tri-halide in the abnormal cases 

 was very much greater than that of the halide ; or in other words, by ascrib- 

 ing this variation to the failure of the relationship, y/y' = const., on which the 

 constancy of Ki depends. This, however, is contradicted by the comparative 

 constancy of Ki in such cases when the iodine concentration is varied, and 

 fails to explain the rapid rise in Ki when the halide concentration is 

 increased. 



Am. Jour. Sci.— Fouuth Series, Vol. XLIV. No. 260.— August, 1917. 

 9 



