Certain Metals in Dissolved Iodine. 



209 



)veci in many cases to be wholly negligible, and in only one 

 experiment out of about thirty did it amount to five units in 

 the second decimal place (about 07 per cent), thus showing 

 that for most purposes the simpler method was sufficiently 

 accurate. 



Experiments with Cadmium. 



A preliminary series of experiments showed that cadmium 

 would be a more suitable metal for the purposes in view than 

 mercury, the metal used by Van Name and Edgar in the 

 majority of their experiments. It is worthy of mention that 

 the rates of solution of these two metals were found to be the 

 same within the error of experiment, thus confirming the con- 



clusion 

 were as 



reached in 

 follows : 



the earlier paper.* The values obtained 



Mercury (Disk of gold, heavily amalgamated) 



with n/l KI, (4 exps.) 7'41 7'36 7-46 



" n/l !NaI, (3 exps.) 6-86 6'56 6"72 



Cadmium 



with n/l KI, (2 exps.) 7"46 7 54 



" n/l Nal, (1 exp. ) 



7-47 



Mean, 7-43 

 Mean, 671 



Mean, 7-50 

 6-83 



Substitution of sodium for potassium iodide lowers the rate of 

 solution of both metals, and by the same amount. Details of 

 these experiments are omitted both because the results are infe- 

 rior in accuracy to those later obtained with the improved appa- 

 ratus and method, and because the constants are not directly 

 comparable with the later ones on account of subsequent 

 changes in the dimensions of the apparatus. 



Except in the above preliminary work, the iodide solutions 

 used were invariably 0*5 equivalent normal with respect to 

 iodide (1/2 KI, 1/4 JBaI 2 , etc.). The change to a more dilute 

 iodide solution brought with it the same phenomenon observed 

 with zinc in the earlier work,f — a tendency for the metal disk 

 to become coated with hydroxide owing to hydrolysis of its 

 iodide. To avoid this the solutions were in all cases made 

 slightly acid, generally O02 equivalent normal, with sulphuric 

 or hydrochloric acid. It will be shown later that up to over 

 - l normal the degree of acidity has no appreciable effect upon 

 the reaction velocity. The fact that hydrolysis of the cadmium 

 iodide did not occur when the concentration of alkali iodide 

 was normal or above is no doubt due to the extensive formation 

 of the double salt of the type K 2 CdI 4 in the stronger solutions. 



The metallic cadmium used was of Kahlbaum's preparation. 

 To ascertain whether possible impurities in the metal were 



* This Journal, xxix, 237. \ This Journal, xxix, 245. 



