CH. V.] METHOD OF FOLTN AND McELLROY. 129 



Calculation of Results. 



25 cc. of Benedict's solution are reduced by 0-05 gram, of glucose. 



0-053 gram, fructose. 

 0-074 gram, maltose. 

 0-0676 gram, lactose. 

 Example. First titration required 2-4 cc. 



Solution diluted i in 4 (10 cc. of sugar diluted with 30 cc. water). 



Second titration required 9-7 cc. 



So 9-7 cc. diluted solution contain 0-05 gram, glucose. 



10 cc. diluted solution contain '5 x IO 



9-7 



, . O-O5 X TOO X 4 



100 cc. original solution contain 



9-7 



Percentage of glucose = 2-06. 



159. The method of Folin and McEllroy.* 



Principle. Five cc. of a 6 per cent, solution of crystalline 

 copper sulphate are treated in a large test-tube with a mixture of 

 sodium phosphate, sodium carbonate and sodium or potassium 

 thiocyanate in the solid form. A clear blue solution is obtained on 

 boiling. The sugar solution is run in from a burette. A white 

 precipitate of cuprous thiocyanate is formed. The sugar is run in 

 very slowly until the blue copper colour is just discharged. Owing 

 to the reduction of the alkalinity of the solution, the amount of 

 copper reduced by a given amount of sugar is considerably greater 

 than in Fehling's or Benedict's methods. Also the reoxidation 

 of the cuprous salts to the cupric condition is relatively very slow. 

 The main objection to the method is the relatively slow rate of 

 reduction. 



Reagents required : 



1. Copper sulphate. Dissolve 60 grams, of the best air-dried crystalline 

 copper sulphate in distilled water, add 2 or 3 cc. of pure sulphuric acid, and 

 make the volume up to i litre. The acid is to prevent the slow deposition of 

 copper hydroxide and silicate due to the action on the glass. 



2. A Ikaline phosphate powder. Mix together in a large mortar, 100 grams, 

 of disodium phosphate crystals (HNa 2 PO 4 i2H 2 O), 60 grams, of anhydrous 

 sodium carbonate and 30 grams, of sodium or potassium thiocyanate. Accord- 

 ing to the authors this mixture keeps indefinitely, but in England it is apt 

 to absorb moisture and resolve itself into a somewhat pasty mass. 



* Journ. of Biological Chemistry, xxxiii., p. 516 (1918). 



