Goock and Heath — Iodometric Determination of Copper. 65 



Akt. TIL — The Iodometric Determination of Copper • by 

 F. A. Gooch and F. H. Heath. 



[Contributions froni the Kent Chemical Laboratory of Yale University — clix.] 



When potassium iodide is added to a suitable solution of a 

 cupric salt, cuprous iodide is precipitated while iodine equiva- 

 lent to the amount of iodine fixed in the cuprous iodide is 

 liberated. This reaction has been made the basis of an iodo- 

 metric method for the determination of copper. The first 

 suggestion of such a method appears to have been made by 

 De Haen in 1854. In this process cupric sulphatawas treated 

 in solution with potassium iodide and the free iodine deter- 

 mined by sulphurous acid according to Bunsen. From the 

 amount of iodine thus found the copper was calculated, accord- 

 ing to the equation 



* 2CuS0 4 + 4KI >2K 2 S0 4 + Cu a I a + I a . 



This method was mentioned in the following year by Mohr,* 

 with the modification suggested by Schwarz that the free iodine 

 be determined by sodium thiosulphate instead of by sulphurous 

 acid. E. O. Brown,f apparently without knowledge of De 

 Haen's previous work, proposed, in 1857, similar procedure, 

 and in 1868 the method with slight modification was presented 

 again by Rumpler.;}; Concerning the utility of the method 

 oj3inions have varied. Mohr never favored it. So late as 

 1877 Mohr,§ after quoting Meidinger to the effect that cuprous 

 iodide freshly precipitated and washed is capable of taking up 

 iodine, and Carl Mohr's criticism that potassium iodide acts 

 upon cuprous iodide according to the concentration, states that 

 the method is not exact and has nowhere found practical appli- 

 cation. On the other hand, Fresenius|| recommended the 

 method for the determination of small amounts of copper, 

 noting that ferric salts and other substances capable of setting 

 free iodine from an acidified solution of potassium iodide must 

 not be present, and indicated the most favorable procedure. 

 The copper salt treated, he says, should be the sulphate, pre- 

 ferably in neutral solution, though a moderate amount of sul- 

 phuric acid is not objectionable. Much free sulphuric acid 

 and any free nitric acid should be neutralized by sodium carbon- 

 ate, and the precipitate dissolved in acetic acid, an excess of 

 which does no harm in the iodometric process. 



Of recent writers some have favored the method while 

 others have commented upon it unfavorably. Low^f has been 



* Titrirmethode, p. 387. \ Journ. Chem. Soc, x, 65. 



% Journ. prakt. Chem., cv, 193. § Titrirmethode, 5 Ann., 288. 



1 Quant. Anal., 6te AufL, 335, 1875. 

 *[ Journ. Amer. Chem. Soc, 18, 468 ; 24, 1083. 



Am, Jour. Sci.— Fourth Series, Vol. XXIV, No. 139.— July, 1907. 

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