Tri- Bromide Equilibria, 121 



necessarilj shift the eqiiilibrium between simple molecules and 

 complex molecules in favor of the former. 



What is of the greatest interest and importance, however, is 

 not the value of the active fraction in solutions containing 

 free iodine, but its magnitude in a pure solution of cadmium 

 iodide. This value can easily be found bj plotting the values 

 of the active fraction against the iodine concentrations, and 

 extrapolating graphically back to zero iodine concentration. 

 When several concordant determinations are available, especi- 

 ally in the region of low iodine concentration, this extrapola- 

 tion can be carried out with a good deal of certainty, and 

 furnishes what, so far as known to the writers, is an entirely 

 new point of attack, for solving the problem of the composi- 

 tion of iodide and bromide solutions which contain complex 

 molecules. It should be noted that this process, unlike the 

 E.M.F. method and the catalysis method of Walton,* does not 

 give the concentration of the iodide ions, but the total con- 

 centration of simple iodide molecules, ionized and un-ionized. 



By this method we find, as shown in Table XY, that the 

 active fraction in a pure solution of cadmium iodide increases 

 with the dilution as would naturally be expected, its values in 

 0-5, 0-25, 0-125, and 0*01 molar Cdl, being 6-0 per cent, 10-6 

 per cent, 16-8 per cent, and 55-0 per cent, respectively. 



(CdBi-O 

 155-6 



61-0 



38-3 



Similar calculations, using the data in Table XI, give the 

 values recorded in Table XYI, which indicate that in cadmium 

 bromide solutions the active fraction is much larger. The 

 active fraction in 0-125 molar CdBr^, as found by interpola 

 tion from the data in Table XYI, is about 40 per cent, as com- 

 pared with 16-8 per cent for Cdl^ of the same concentration. 



For the other abnormal iodides and bromides our experi- 

 mental data are less complete and less accurate, making the 



■^ Zeitschr. phys. Chem., xlvii, 185, 1904. 

 f Found by extrapolation. 





Table XYI. 







(2Br3) 



(Br,) 



(2Br) 

 (calculated) 



(2Br) + (2Br 



3) Active 

 Fraction %. 



52-1 



31-5 



102-9 



155-0 



49-8 



0-39 



0-208 



117-2 



117-6 



37-8 



0-0 



0-0 



119-5 



119-5t 



38-4 



22-68 



29-05 



48-6 



71-3 



58-4 



0-22'75 



0-241 



58-8 



59-0 



48-4 







0-0 



59-0 



59-Of 



48-4 



21-27 



40-01 



33-14 



54-4 



71-1 



0-665 



0-991 



41-75 



42-4 



55-4 



0-0 



0-0 



42-0 



42-Ot 



54-9 



