108 



Van Naw.e and Brown — Tri-lodide and 



This inference is tested by Table I. In the first column are 

 Herz and Kurzer's constants for barium iodide as calculated by 

 them, but divided by 10 to reduce to mols per liter. The 

 second column gives the constants calculated according to 

 Jakowkin's method from the same data. The superior con- 

 stancy of the second column is apparent in the smaller total 

 variation (4*9^ as compared with T'O^), and in the less sys- 

 tematic character of the variations. Another and more 

 striking example is furnished by Table II which contains a 

 similar comparison based on some of our own results for 

 strontium iodide (Table III, Exp. 2). These tables make it 

 clear that a decision between the two possibilities can be 

 reached in this way, and prove that the assumption made by 

 Jakowkin is unquestionably more correct than that of Herz. 





Table 



II. 







Strontinm 



Iodide. 





Ki 







Ki 



(assuming Sri*) 







(assuming Sr (13)2) 



0-000382 







0-00134 



0-000516 







0-00141 



0-000587 







0-00141 



0-000622 







0-00140 



0-000642 







0-00142 



0-000651 







0-00138 



0-000696 







0-00143 



Moreover, it may be inferred by analogy, that in the case of 

 tri- and polyvalent iodides each iodine atom of the original iodide 

 should be active in uniting with iodine, and this we have 

 actually proved to be true in the case of a trivalent iodide, Lalg. 



We have therefore, throughout this investigation, followed 

 Jakowkin in our method of calculating the constants, the 

 general expression which we have used for iodides and bromides 

 both of uni- and polyvalent metals being : 



K 



(iia — h + x) X 



(III) 



in which n is the valence of the metal, a the original molar 

 concentration of the halide, h the molar concentration of the 

 titrable halogen, and x that of the free halogen, all referring 

 to the water layer. 



If, for an iodide, (21) and (21)3 represent the total concen- 

 trations of the iodide and tri-iodide radicals respectively, equa- 

 tion III takes the form : 



