134 THE CARBOHYDRATES. [CH. V. 



solve, but a certain amount may remain in suspension until the 

 permanganate titration is nearing completion. 



Titration of the reduced iron. Run in the potassium permanga- 

 nate from a burette fitted with a glass stopcock, stirring the mixture 

 well. From time to time examine the beaker by holding it above 

 the head. Any lumps of undissolved cuprous oxide can thus be 

 detected. They must be broken up and brought into solution by 

 rubbing with the rod. It is most important that this should be done 

 before the titration is completed. The end point is reached when a 

 faint pink tinge persists for at least ten seconds. 



Calculation of results. One cc. of permanganate = 10 mg. Cu. 

 The amount of sugar corresponding to various amounts of copper are 

 obtained by plotting the results given below. The amount of sugar 

 corresponding to the exact amount of copper reduced is thus found. 

 The number of milligrammes of sugar in 10 cc. divided by 10 and 

 multiplied by the dilution employed gives the percentage of sugar. 



mg.Cu. mg.Glucose mg.Maltose mg.Cu. mg.Glucose mg. Maltose 

 anhydride anhydride anhydride anhydride 



25 7'3 



50 15 



75 22-4 



100 30 



125 37-8 



150 45*3 



NOTES. i. A rough approximation of the concentration of glucose in 

 the original solution can be made by use of Fehling's solution. The sugar 

 should be so diluted that 3 to 5 cc. reduce 3 cc. of Fehling's solution. 



2. Preparation of the asbestos. (See appendix.) 



161. The estimation of glucose by the method of Amos 

 Peters. 



Principle. A known volume of the sugar solution is 

 boiled with a measured amount of an alkaline solution of 

 copper sulphate. The cuprous oxide is filtered off and the 

 copper in the filtrate determined by treatment with 

 potassium iodide and titration of the iodine liberated by 



