PHYSIOLOGICAL CHEMISTKY 293 



the glycogen solution for inversion and bring it to a definite volume after 

 neutralising. 



For the estimation of the dextrose formed Pfliiger uses a special gravimetric 

 method (see Dictionnaire de physiologie, par C. Eichet, t. vii.), but Bang's 

 method, described in the following section, is of sufficient accuracy for most 

 purposes. 



Quantitative Estimation of Sugars. 



All the methods employed for this purpose consist in determining the 

 reduction brought about by a measured amount of sugar solution in a known 

 alkaline solution of cupric salts. They may be divided into two classes : 



(i) Methods in which sugar solution is added to a measured volume of the 

 cupric salt employed, until the reduction to cuprous salt or oxide is complete. 

 To this class belong the methods of Fehling, Pavy and Gerrard, which are 

 described in Chapter XX. For the estimation of sugar in urine they are usually 

 of sufficient accuracy, and are widely employed for this purpose. Their great 

 advantage is simplicity. 



(ii) Methods in which excess of cupric salt is employed. In Pfliiger's 

 method the precipitated cuprous oxide is weighed. In Bang's method the 

 excess of cupric salt is determined by titration. In both these methods it is 

 possible to keep the conditions of different titrations exactly similar except as 

 to the excess of cupric salt employed, which will naturally depend on the 

 amount of cupric salt reduced by the sugar in the determination. Now it is 

 found that the larger the excess of cnpric salt present the greater is the 

 reduction caused by a given amount of sugar. The reduction is therefore not 

 proportional to the amount of sugar employed in the determination, and it is 

 necessary to construct a table, representing the amount of reduction caused by 

 different known amounts of sugar, from which the results of any given 

 determination can be calculated. These methods are more accurate than 

 those of the first class. 



EXPERIMENT. Bang's Method. 



Principle. In the presence of carbonates and sulphocyanides cuprous oxide 

 forms cuprous sulphocyanide, which becomes dissolved to form a colourless 

 solution in the presence of excess of potassium sulphocyanide. A solution of 

 cupric carbonates containing potassium sulphocyanide is boiled for exactly 

 three minutes with an amount of the sugar solution, which is insufficient to 

 reduce all the cupric salt ; 'after rapidly cooling, the amount of cupric salt 

 which has not been reduced by the sugar is then determined by titration in 

 the cold with a standard solution of hydroxylamin (sulphate). This, like sugar, 

 reduces cupric salts to the cuprous state to form, as above explained, a 

 colourless solution. The hydroxylamin solution must not be run in too quickly, 

 else a precipitate forms and the results are vitiated. Shake sufficiently to 

 prevent the precipitate forming. At least 30 seconds should be taken in 

 titrating. The hydroxylamin solution is of such a strength that 1 c.c. of it 

 exactly decolourises 1 c.c. of the copper solution. From the amount of 

 hydroxylamin solution required to reach the decolourisation point, the sugar 

 content is obtained by using tables which have been constructed for this 

 purpose. 



