36 Gooch and Perkins — Determination of Free Iodine. 



The experiments next described were made with silver 

 deposited electro-lytically from a solution of silver nitrate upon 

 a platinum cathode, the anode being enclosed within a porous 

 cell to prevent admixture of the silver dioxide formed at the 

 anode with the metallic silver at the cathode. Experience 

 showed that, while the bright and crystalline deposit which 

 formed upon a stationary cathode lacked in absorptive power, 

 the product obtained by continually oscillating the cathode 

 during the deposition of the metal, broken and dark when 

 formed, proved to be sensitive to iodine as well as pure. The 

 results of experiments with electrolytic silver thus prepared 

 are given in Table II. 





Table II. 







The Action of Electrolytic Silver. 







Increase in 





Silver 



Iodine weight of 



Error in 



taken 



taken silver 



iodine 



grm. 



grm. grm. 



grm. 



2-8184 



0-6461 0-6494 



+ 0-0033 



3-2130 



06461 0-6490 



+ 0-0029 



2-0514 



0-6461 0-6491 



+ 0-0030 



3-0102 



0-6461 0-6490 



+ 0-0029 



7-5943 



(cryst) 0-6479 0'6513 



+ 0-0034 



Though the silver used in this process was pure, the errors 

 observed are positive and high ; and this fact emphasizes an 

 obvious inference from the previous work that the excess in 

 weight is due to the absorption by the silver of an extra amount 

 of iodine liberated from the potassium iodide by prolonged 

 agitation in contact with the air. In harmony < with this idea 

 is the fact, observed throughout the entire series of experiments 

 with silver reduced by chemical processes and subsequently 

 treated with potassium iodide, that the error is greatest when 

 the time used to accomplish the absorption is the longest. 

 This was especially marked in the experiments with silver 

 reduced by hydrogen, in which the largest amount of time was 

 needed, on account of the less sensitive character of the glisten- 

 ing and filamentary metal. 



Moreover, direct experiments in which the silver was shaken 

 with 50 cm8 of a solution of potassium iodide, 20 grm to the liter, 

 fully confirmed the idea that the action of air must be prevented 

 during the agitation of the solution of the iodide in contact 

 with silver ; for in these experiments it was found, that from 

 the solution of potassium iodide shaken in contact with air 

 finally divided electrolytic silver absorbed 0'0010 grm of iodine 

 in fifteen minutes, that silver reduced by zinc from silver 

 iodide absorbed 0-()012 grra of iodine in fifteen minutes, that 

 silver reduced from the sulphide by hydrogen took up 0-0032 srm 



