Gooch and Heath — lodometric Determi?iation of Copper. 67 



from potassium iodide and cupric sulphate in neutral solution 

 is complete at "very high concentration of the solution; that 

 the completeness of the reaction is greatly affected by the 

 volume of liquid ; that the amount of potassium iodide em- 

 ployed is almost without influence either in neutral solution or 

 in acid solution ; and that the presence of free sulphuric acid 

 even in large amounts or of hydrochloric acid present in 

 amount equivalent to the cupric sulphate is advantageous. 

 Moser recommends, therefore, the addition of sulphuric acid 

 for the purpose of bringing the reaction to completion. To 

 the cupric sulphate (about 06 gm.) dissolved in 50 cm3 of water 



contained in a 300 cm3 stoppered flask are added 5 cmS of H 2 S0 4 , 



and 2 gm. of solid potassium iodide, the mixture is shaken 

 frequently for two minutes, and the free iodine is titrated by 

 sodium thiosulphate, with stirring, to the end-reaction of the 

 starch indicator. 



According to Fernekes and Koch,* an excess of acetic acid 

 does not influence titrations, while a certain amount of potas- 

 sium iodide — 1*5 grm. to 2 grin, for 0'003S grm. of copper, and 

 2*5 grm. for 0*0939 grm. of copper — must be added to bring 

 about complete action in a volume of 100 cm3 . 



Quite recently Cantoni and Rosensteinf have tested the 

 reaction between potassium iodide and a cupric salt under 

 various conditions ; but these investigators do not give the 

 absolute values of the amounts of copper taken and. found, 

 merely recording the relative effects of varying conditions. 

 From the record of their results it would appear that a five- 

 fold increase of the minimum amount of potassium iodide 

 added to portions of 100 cm3 of solution containing the same 

 amount of copper salt is without influence upon the result ; 

 that increase of volume from 100 cm3 to 350 =m3 , other conditions 

 being the same, may affect the results by as much as 5 per cent 

 of their value. The authors conclude that the method gives 

 good results under properly controlled conditions. 



So evidence and opinions as to the effects of various con- 

 ditions in the process are contradictory. 



The chief matters of difference concern the influence of an 

 excess of potassium iodide used as the precipitant, the dilution 

 at which the precipitation should take place, and the effects of 

 acids upon the formation of the cuprous iodide. We have 

 thought it desirable, therefore, to again study these points 

 experimentally. 



In the experiments detailed in Table I, small amounts of a 

 solution of pure copper sulphate, standardized by the electrolytic 



*Jour. Amer. Chein. Soc, xxvii, 1229. 

 fBull. Soc Chim. [3], xxxv, 1067-73 (1906). 



