38 Gooch and Perkins — Determination of Free Iodine. 



the details of experiments in which the standard N/10 solution 

 of iodine in potassium iodide was shaken N/10 silver in flask 

 tilled with hydrogen and closed. 



These results make it plain that free iodine may be deter- 

 mined with accuracy in the presence of potassium iodide by 

 shaking the solution with metallic silver in a closed flask filled 

 with hydrogen and determining the increase in weight of the 

 silver. Silver reduced from a silver salt by zinc or from silver 

 sulphide by hydrogen may serve the purpose, provided it is 

 subjected to a preliminary treatment with potassium iodide, 

 and silver reduced from the oxide by hydrogen is also service- 

 able; but the best form of silver, and the one most easily prepared 

 in the pure state, is that deposited electrolytically upon a small 

 oscillating cathode of platinum from a solution of silver nitrate, 

 the platinum anode being enclosed in a porous cell. The 

 shaking of the silver may be done by hand or by some simple 

 form of mechanical shaker like that described in the figure. 

 The time required for the absorption of approximately 0*65 grm 

 of iodine in 50 cm3 of liquid was 15 to 25 minutes. The mean 

 error of the eleven determinations in which electrolytic silver 

 was employed proved to be — 0*0001 grm between extremes of 

 +0-0005 and -0'0004 grm . 



To test the accuracy of the process in alkaline solution 

 experiments similar to those above were made, in which the 







Table IV. 







The Action 



of Silver 



upon N/10 Iodine 



in an Alkaline Solution 







Increase in 







Silver 



Iodine 



weight of 



Error in 



Average 



taken 



taken 



silver 



iodine 



error 



grm. 



grm. 



grm. 



(A) 



grm. 



grm. 





The action of silver shaken in air with NaHC0 3 . 





2-0110 



0-3217 



0-3221 



+ 00004 





3-6684 



0-3217 



0-3219 



+ 0-0002 





3-0056 



0-3217 



0-3235 



+ 0-0018 





3-0093 



0-3217 



0-3245 



+ 0-0028 





3-0058 



0-3217 



0-3224 



+ 0-0007 





3-6686 



0-3217 



0*3243 



+ 0-0026 





2-9993 



0-3217 



0-3261 



+ 0*0044 





3-0013 



0-3217 



0-3235 



+ 0-0018 





3-0014 



0-6434 



0*6485 



+ 0-0051 



+ 0'002i 



(B) 

 The action of silver shaken in an atmosphere of hydrogen with NaHC0 3 . 



3-0014 0-3217 0-3216 — O'OOOl 



3-0169 0-3217 0*3216 —0*0001 



3-0083 06434 0'6433 —O'OOOl 



3-0016 0-2500 0-2503 +0'0003 



3-0069 0-3217 0-3219 +0'0002 +0*0001 



