128 THE CARBOHYDRATES 



case of relatively pure solutions of suitable strength, 1-5 

 drops of basic acetate followed by 3-5 c.c. of alumina cream 

 should suffice. After the addition of these reagents, the 

 mixture is thoroughly shaken and the precipitate allowed to 

 settle after making up nearly to the graduation mark with 

 water ; any froth which may have formed is broken by ad- 

 dition of a drop of alcohol and water is then added up to the 

 mark. After once more shaking the liquid is allowed to 

 settle and then filtered through a dry filter paper. Such a 

 solution is then ready for direct measurement by a polari- 

 scope, but if required for titration by Fehling's solution it 

 must be freed from lead by hydrogen sulphide, and after 

 filtering off the lead sulphide, the hydrogen sulphide must be 

 removed. 



Estimation of Pentoses. 



When pentoses alone are present they may be estimated 

 by determining their reducing power of Fehling's solution 

 as in the case of glucose. The values for arabinose and xylose 

 in terms of copper oxide have been determined by Daish,* 

 working under the standard conditions laid down by Brown, 

 Morris, and Millar f ; the tables may be consulted in Daish's 

 paper. If pentoses are mixed with other carbohydrates, or 

 are present in the form of pentosanes, other methods must 

 be used (see p. 137). 



Estimation of Reducing Sugars. 



The following precautions must be taken in estimating 

 reducing sugars by these and similar methods involving the 

 reduction of metallic salts : — 



1. Any substances such as tannins which may have the 

 power of reducing the salts used in titration must be removed. 



2. The strength of the sugar solution must be weak, 

 because the reducing power of sugar varies with the concen- 

 tration, hence it is best to titrate a solution of about the 

 same strength as that used for the standardizing of Fehling's 



* Daish: " J. Agric. Sci.," 1914.6,225. 



tBrown, Morris, and Millar ; "J. Chem. Soc," 1897,71, 105. 



