INTRODUCTION. 11 



pure metallic copper, which procedure may then be substituted for electrolytic 

 determination on the stock solution. The standardization may appropriately 

 be made on 20 c.c. of the stock solution, which is the same volume as will be 

 used in sugar determinations, and with a final volume at the end of titration in 

 no case exceeding 120 c.c. This volume should contain 20 c.c. of concentrated 

 acetic acid and about 7 c.c. of saturated solution of pure potassium iodide. 

 The thiosulphate value of 20 c.c. of the stock solution of copper sulphate deter- 

 mined in simple acetic-acid solution is designated as the "basis titration," and 

 is of fundamental importance for subsequently testing the constancy of solu- 

 tions and of conditions even when it is not used for the purpose of standardi- 

 zation. A solution of pure copper sulphate will remain at a constant standard 

 value, but the thiosulphate will change somewhat if much exposed to air and 

 especially to light. 



(2) The alkaline filtrates from sugar determinations, whose volume must 

 not exceed 75 c.c, should be acidified with concentrated sulphuric acid. The 

 amount of acid used should be such that the thiosulphate value of a blank 

 determination, in which the unheated reduction mixture of 60 c.c. previously 

 described contains all the reagents but no sugar, will be identical with the 

 "basis titration." This amount has been found by experiment, as described 

 in the previous references, to be 4 c.c. of concentrated sulphuric acid, but should 

 be verified or changed if necessary. 



After the mixture has been acidified and cooled to 20 C, potassium iodide 

 is added to the amount of 6 to 7 c.c. of an aqueous solution that is saturated 

 at room-temperature. The iodine which is evolved is titrated at once with 

 thiosulphate, using a solution of soluble starch as an indicator near the close of 

 the titration. The end point should be determined by the "spot test," espe- 

 cially in the presence of urinary or other coloring matter, and may be verified 

 by immediate back titration with a drop of iodine solution of about the same 

 concentration as the thiosulphate. 



(3) For the residual method a blank determination is always necessary. 

 The assumption that there is no self-reduction of reagents may lead to error, 

 although by the present method pure and especially freshly prepared reagents 

 show a negligibly small self-reduction. The blank value is obtained with the 

 reduction mixture of 60 c.c. previously described but without the presence of 

 sugar. This is heated and filtered and treated in all respects as when a deter- 

 mination of sugar is made. From the thiosulphate value of the copper thus 

 found unreduced is subtracted the value found in the same way in a subsequent 

 sugar analysis. This thiosulphate difference evidently shows the amount of 

 reduced copper due to the action of the sugar. The difference may be con- 

 verted into milligrams of copper by means of the standard value of the thiosul- 

 phate previously obtained, and the corresponding amount of sugar may be 

 found from a table of reduction values. 



The Control of All of the Conditions. 



All of the conditions of this method and the technique of the operator can 

 be controlled by the use of a standard solution of pure dextrose. The results 



