Polymerization in Cadmium Iodide Solutions. 465 



tions tlie agreement is fairly good, but in the 0*25 molar 

 solution the difference is not in the expected direction, and in 

 the strongest solution the discrepancy is surprisingly large, far 

 exceeding the probable experimental error. For this result 

 there is no evident explanation, but it is significant that it 





Table Y. 







(1') at 0° 



(I') at 25° 



(Cdls) 



From freezing point 



From E. M. F. 



500 



u 



30-4 



250 



36 



31-2 



125 



24 



27-5 



10 



9-6 



10-5 



coincides with a marked irregularity in the freezing point 

 lowering. As was noted on page 463, the results of all cryo- 

 scopic measurements with cadmium iodide show that above 

 about 0"3 molar the value of ^ apparently rises with the con- 

 centration, although the attendant decrease in ionization and 

 increase in polymerization would both tend to lower it. 



Mature and Concentration of the Complexes. — Thus far the 

 question of the nature of the complex ions and molecules has 

 been left open. Of the various complex ions which may be 

 present in the solutions, Cdlg^, as McBain"^ has shown, is 

 apparently the most probable one. The high transport num- 

 ber of tlie anion, which approaches 1'25 in the most concen- 

 trated solutions, cannot be explained by assuming the pre- 

 dominance of Cdl/ without assigning to that ion an improbably 

 high velocity. This objection, however, would not apply to such 

 ions as CdJ/ or Cdgl/', though there would be less reason to 

 expect their presence than that of the less complex ones just 

 mentioned. 



McBain has calculated the approximate composition of a 0*1 

 molar solution of cadmium iodide upon the assumption that 

 Cdlg^ is the only complex ion present in appreciable amount. 

 This calculation, which is based entirely upon freezing point, 

 transference, and conductivity data, gives the values (CdlgO = 

 0-0084 and (F) = 0-0126. This would make (Cd++) = 0-0105, 

 thus accounting for about 19 per cent of the total iodide. 

 McBain concludes that the remaining four-fifths is present in 

 the form of simple, non-ionized Cdl^ molecules, and that the 

 proportion of complex molecules is negligible. 



There are several serious objections to these figures : {a) If 

 so large a part of the salt is in the form of simple Cdl^ mole- 

 cules the power of the salt to combine with iodine should be 

 but slightly lower than normal, while in reality the " active 



*Loc. cit. 



