Tri- Bromide Equilibria. 107 



notwithstanding that its value is only about half* that given 

 by the univalent iodides. Herz, working in collaboration with 

 Bulla on the iodides of barium, strontium and calcium, again 

 employs this assumption with similar results. 



Abel and Hallaf point out the significant fact that Herz's 

 results for these three iodides give nearly the same value of 

 the equilibrium constant as that obtained by Jakowkin for the 

 iodides of barium and the alkalis, if calculated in the same 

 way. They add, however, the statement that no decision 

 between the two possibilities can be made on the basis of 

 measurements of distribution coefficients. 



This last statement is evidently based on a misapprehension. 

 Let us, following Herz's notation, represent by a the original 

 concentration in mols per liter of the barium iodide, by J the 

 molar concentration of titrable iodine in the water layer as 

 directly measured, and by x the concentration of free iodine as 

 found by dividing the observed concentration in the non- 

 aqueous layer by the distribution coefficient. 



Herz's assumption then obviously leads to the equilibrium 



expression : 



{a- ih — x)\x 



H ^-^- = constant (I) 



— X ^ ' 



while Jakowkin's point of view calls for: 



(2a- (b — x)) X ,-^^ 



-^ -, — constant (H) 



b — X ^ 



The ratio of these two expressions evidently varies with the 

 values of a, J, and x, that is, if one is constant the other can 

 not be. Hence, when applied to actual experimental results, 

 the one based on the assumption which is the more nearly 

 correct ought to show the better constancy. 



Table I. 



Barium Iodide. 



(From Experimental Data of Herz and Kurzer.) 



(assuming Bal4) (assuming Ba (13)2) 



0-00074 0-00148 



0-00070 0-00143 



0-00072 0-00149 



0-00071 0-00148 



0-00071 0-00150 



0-00069 0-00148 



*Herz expressed concentrations in millimols per 10 cm^ instead of the 

 customary mols per liter, and his constants must therefore be divided by ten 

 to render then comparable with those of Jakowkin and others. 



f '' Handbuch der anorganischen Chemie" (Abegg and Auerbach), vol. iv, 

 part 2, p. 443, 1913. 



