EXPERIMENTAL METHODS. 35 



to have an excess of acid in order to obtain complete evolution of iodine, 

 about 1 c. c. of the concentrated acid for each 10 c. c. of Fehling's solution. 

 Where a copper solution other than Fehling's solution is used, the amount of 

 acid added is naturally regulated to the concentration of the alkali. Too 

 great an excess of acid is also to be avoided, as under such conditions iodine 

 is liberated from potassium iodide in the presence of air. The flasks are 

 then cooled to a temperature of not below 15 nor above 20. This is very 

 important. After acidification and cooling, a solution of potassium iodide 

 is added to each flask. 



The reaction between copper sulphate and potassium iodide runs to com- 

 pleteness only when there is present a considerable excess of the iodide; 

 hence it is essential that the iodide be high in comparison with the equivalent 

 of copper present and also in relation to the final volume at the end of titra- 

 tion. A saturated solution is therefore used, and for each cubic centimeter of 

 the original copper solution remaining, about 0.5 c. c. of the iodide solution 

 is used. A solution of sodium iodide may also be used. The final volume of 

 the titration mixture was usually 30 to 40 c. c. 



The iodine which is evolved is titrated immediately with sodium thio- 

 sulphate, using a fresh solution of soluble starch (Merck) as indicator, 

 which is added near the end of the titration. The concentration of the 

 thiosulphate used can be adjusted according to the amount of copper to be 

 determined, thus assuring the greatest accuracy under different conditions. 



N" N" 



The concentrations of the thiosulphate generally used were and . It 



2i(j 50 



is, of course, essential that the thiosulphate produce a definite end-point in 

 the titration. 



The presence of the slightly yellowish cuprous iodide makes the recog- 

 nition of the end-point a little more difficult than in a simple iodine titration. 

 Toward the end of the titration the mixture assumes a very light brown or 

 lavender color, which changes to a cream or very faint pink when the last 

 drop is added. The end-point can be verified best by means of back titration 

 with an iodine solution of the same concentration as the thiosulphate, which 

 is a good precaution for very accurate work. Where there are a number of 

 determinations to be made the various steps of measuring out the solutions, 

 heating, centrifuging, cooling the copper solutions, and titrating can be so 

 arranged that the operator is constantly busy and does not have to wait An 

 interval-timer with alarm attachment was found very helpful. 



THE STANDARDIZATION OF THE SOLUTIONS. 



The copper value of the thiosulphate solution is first determined by going 

 through the entire procedure, heating, centrifuging, etc., without the addi- 

 tion of any sugar solution. In this way any self-reduction of the alkaline 

 copper solution is also adjusted. The dilute solutions of sodium thiosulphate 

 change in strength with time, but a stock solution of normal concentration 

 changes only very slowly. The sugar value of the copper solution is deter- 

 mined by using a standard dextrose solution. The purest material was used 



