A REAGENT FOR THE COPPER-IODOMETRIC DETERMINATION OF VERY SMALL AMOUNTS OF SUGAR

Abstract

In sugar determinations based upon the reduction of cupric copper the cuprous oxide formed in the reaction is subject to partial reoxidation unless the operations are carried out in air-free vessels, as in one of Bang’s methods. There are several analytical methods, with carefully standardized technique, in which reoxidation does not impair the accuracy of the results. Only in the determination of very small quantities of sugar, a point is reached below which the amount of the cuprous oxide reoxidized is no longer negligible in relation to the total, and slight variations in the extent of reoxidation affect appreciably the accuracy and reproducibility of the results. Ferricyanide reagents, employed in the Hagedorn-Jensen method (1) and its several modifications, and alkaline mercuric potassium iodide solutions, used in Baudouin’s method (2), are free of this drawback, since the reduction products they yield with sugars are not readily reoxidized by atmospheric air. In view of this advantageous quality, we have made an extensive study of the use of these two groups of reagents in the determination of very small quantities of sugar. As regards ferricyanide solutions, we have reported some years ago, and our finding has been amply confirmed, that the ferric ion is considerably less specific in its reaction with sugars than copper, in that it oxidizes substances other than sugar to a far greater extent than the cupric ion. This fact is known seriously to affect blood sugar values, unless special precipitants are used to remove the non-sugar reducing materials. We find the non-specificity of ferricyanide to be even more disturbing in the analysis of polysaccharides, particularly in cases in which sugar is to be determined