74 Gooch and Heath — lodometric Determination of Copper* 



increases very materially with the dilution of the solution. It 

 appears also that the noted small amounts of sulphuric acid, 

 hydrochloric acid, and nitric acid (free from nitrogen oxides) 

 exert no appreciable influence upon the indications of the pro- 

 cess carried out at a volume approximately 100 cm3 ; and that 

 acetic acid may be present in amount equivalent to at least 25 

 per cent of the absolute acid. 



We find no ground for the inference of Moser* that the 

 presence of acid, best sulphuric acid, is necessary to the attain- 

 ment of good results at all volumes excepting the most con- 

 centrated : and there appears to be no reason why the addition 

 of small amounts of acid should increase the amount of 

 iodine liberated if the potassium iodide is free from ioclate 

 or other oxidizer. We are wholly unable to offer any ex- 

 planation for Moser's extraordinary observation, quite con- 

 trary to our own, that variation in the amounts of — — H 2 S0 4 , 



from l om3 to 100 cm3 (0-49 grm. to 49 grm.) for 50 cmS of a solution 

 of copper sulphate, is practically without effect in the treat- 

 ment by potassium iodide. 



The best general procedure in determining by the iodometric 

 method amounts of copper not exceeding about 0*3 grm. seems 

 to us to be covered by the following directions : — The solu- 

 tion of the cupric salt, containing no more than 3 cm3 of 

 concentrated sulphuric acid, hydrochloric acid or nitric acid 

 (free from nitrogen oxides), or 25 ctn3 of 50 per cent acetic acid, 

 is to be made up to a volume of 100 cm3 , 5 grm. of iodate-free 

 potassium iodide are to be added, and the titration of the free 

 iodine is to be made by sodium thiosulphate in the usual man- 

 ner with the use of the starch indicator at the end. In case the 

 end reaction has not appeared when 25 cm3 of the thiosulphate 

 have been added, 2 grm. to 3 grm. more of potassium iodide 

 are to be added before continuing the titration. 



The error of the process, properly conducted, should not 

 exceed a few tenths of a milligram in terms of copper. 



*Zeitschr. anal. Chem., xliii, 597, 1904. 



