Gooch and Heath — Iodometrio Determination of Copper. 69 



reaction yields excellent indications of the amount of copper 

 handled when the amonnt of potassium iodide is considerably 

 more than the theoretical amount, varying from four to sixty 

 times the amount required by theory, the absolute excess varying 

 from 0*6 grm. to 12 - 7 grm. ; and this is true even though the 

 amount of potassium iodide is sufficient to dissolve completely 

 the cuprous iodide formed. 



So it is plain that the amount of potassium iodide used has 

 within limits an influence upon the result. In a volume of 

 about 50 cm3 an excess of 0*2 grm. of potassium iodide is not 

 enough, while an excess of 0*6 grm. appears to be sufficient. 

 Beyond this limit the addition of potassium iodide has no 

 appreciable effect. It is natural to suppose that at high dilu- 

 tions a larger excess of potassium iodide would be needed 

 to complete the reaction than is required at lower dilutions. 

 Table II contains the results of experiments made to test the 

 efficiency of potassium iodide in precipating 0*0010 grm. of 

 copper, taken as sulphate, in a volume of 100 cm3 . 



Table II. 



Effect of Potassium Iodide in Neutral Solutions at a Fixed 

 Volume o/100 om3 . 



Copper 



taken as 



CuS0 4 



KI 



used 



Acid 

 added 



Volume 



Copper 

 found 



Error 



grm. 



grm. 



cm 3 



cm 3 



grm. 



grm. 



o-ooio 



1 



none 



100 



0-0003 



— 0-0007 



a 



2 



a 



a 



0-0006 



— 0-0004 



U 



3 



a 



a 



0-0009 



— o-oooi 



u 



4 



(C 



it 



0-0009 



—o-oooi 



u 



3 



a 



u 



0-0013 



+ 0-0003 



From the results of these experiments it appears that while 

 the action upon a milligram of copper, taken as the sulphate in 

 100 cm3 of solution containing no free acid, is not completed by 

 1 grm. or 2 grm. of potassium iodide, it is practically complete 

 when an excess of 3 to 5 grm. of potassium iodide is present. 

 The fact is again emphasized that up to a certain proportion, 

 increasing with the dilution, the amount of potassium iodide 

 influences the completeness of the reaction in neutral solution. 

 An excess of potassium iodide amounting to 0*6 grm. to 1 

 grm. in a volume of 50 cm3 , and to from 3 gr. to 5 grm. in a 

 volume of 100 cm3 , will precipitate completely - 0020 grm. of 

 copper. In the practical application of these facts it must be 

 borne in mind that it is the excess of potassium iodide and not 

 the full amount added which is important. 



So we may very properly fix upon 2 grm. as the uniform 

 amount of potassium iodide suitable for the precipitation of 



