126 THE CARBOHYDRATES. [cH. V. 



the sugar solution is added. I have abandoned the method for 

 routine class work, owing to the considerable errors made by a 

 large proportion of the students. 



4. Benedict's. See p. 127. 



5. Folin and McEllroy's. See p. 129. 



B. Indirect Volumetric Methods. 



6. Amos Peters'. See p. 134. 



7. Bertrand's. The sugar is heated with an excess of Fehling's 

 solution. The cuprous oxide formed is filtered off through asbestos 

 and dissolved in an acid solution of ferric sulphate, some of which is 

 reduced to ferrous sulphate. The amount of the latter is determined 

 by titration with standard permanganate. By reference to tables 

 or a curve the amount of sugar present is readily calculated. 



8. Wood-Ost's. Seep. 131. This is very similar to the above, 

 except that a solution of copper bicarbonate is used instead of 

 Fehling's. The amount of copper reduced by a given weight of 

 sugar is thus nearly doubled, and, owing to the reduced alkalinity, 

 other substances do not so readily affect the results. 



9. Bang's. See p. 253, Ex. 312, note 2. An alkaline solution 

 of copper in potassium chloride is boiled with the sugar and a 

 measured amount of potassium iodate. On acidifying the solution 

 with sulphuric, the iodic acid oxidises the cuprous oxide to cupric 

 oxide. The loss of iodic acid is determined by adding potassium 

 iodide and titrating with thiocyanate. The method has been adapted 

 for the estimation of sugar in very small quantities of blood. 



C. Colorimetric Methods. 



10. Benedict's. See p. 251. The sugar solution is heated 

 with sodium carbonate and sodium picrate. The picrate is reduced 

 to picramate, which is estimated colorimetrically against picramic 

 acid,, either a solution of the pure substance, or one prepared from a 

 standard solution of glucose. 



