EXPERIMENTAL METHODS. 31 



The reducing power of these sugars is not equal, but the reduced copper 

 was apportioned as follows : 100 c. c. standarized Fehling's solution = 1.00 

 gram d-glucose, 0.95 gram invert sugar, 0.90 gram 1-xylose, 0.90 gram 

 hexose polysaccharides, and 0.85 gram pentosan. With these figures the 

 amount of the various sugars per gram of dry material and in some cases 

 of original fresh material was calculated. 



THE ANALYSIS OF THE SUGAR SOLUTIONS. 



On account of the large number of estimations of sugar which had to be 

 made, it was necessary to employ a method of sugar determination which 

 was rapid as well as accurate. The application of the reduction of alkaline 

 copper solutions by sugars that is, some modification of the Fehling 

 method is best suited to this purpose. The examination of the various 

 current processes showed that these are open to serious error. 



In the following method an attempt has been made to modify the usual 

 procedures of sugar determinations by means of copper in such a manner 

 as to avoid most of the sources of error inherent to these, especially as 

 applied to the sugar solutions under consideration. It will not be necessary 

 to enter into a detailed discussion of the theory and practice of sugar 

 determinations by means of alkaline copper solutions. Such critical and 

 experimental studies have been carried out by several workers, especially 

 by Pflueger 1 and by Peters/ which have yielded valuable results, but 

 unfortunately have not been as universally applied as they deserve. 



The results of these studies indicate clearly that where rapidity is of 

 importance only volumetric methods should be considered. Any doubt 

 regarding the accuracy of the volumetric procedure can pertain only to the 

 measurement of the amount of copper reduced by the sugar ; the conditions 

 under which the reduction is carried out must in any case be carefully 

 standardized. The majority of the methods require a filtration of the 

 cuprous oxide which is either weighed as such or dissolved, and then deter- 

 mined by some suitable method. 



It will be necessary to consider but a few of the most important factors 

 affecting the accuracy of the procedure. The filtration of the cuprous oxide 

 formed by the reduction of the alkaline copper solution, through an asbestos 

 filter or some similar device, is open to several sources of error. Thus, it 

 has been found by several investigators that the weight of the asbestos filters 

 is not constant because of solution of the asbestos and the self -reduction of 

 the reagents, and requires a correction which can not always be applied 

 safely. In determining the sugars in solutions obtained from plant material 

 such as the cacti, the precipitated cuprous oxide contains contaminating 

 substances which are undeterminable and liable to affect the subsequent 



1 PFLUEGER, E. Untersuchungen ueber die quantitative Analyse des Traubenzuckers. 



Arch. Ges. Physiol., 69, 399-471, 1898. 

 * PETEES, A. W. A critical study of sugar analysis by copper reduction methods. 



Jour. Amer. Chem. Soc., 34, 928-954, 1912. 



