The Determination of Reducing Sugars

In the course of an investigation, still in progress, of the physiological activity of a group of soil organisms, it became necessary to determine the quantity of dextrose in a series of solutions. It was desirable to have a method as accurate as the Fehling gravimetric one but which would yield results about as quickly as by direct titration. The reducing sugar present in the 150 cc. of each of these solutions varied from nearly zero to about 550 mg. A definite quantity of a copper salt had to be added to each solution. Direct titrat,ion could not be used unless this salt was first removed and the solutions of small sugar content were concentrated, a procedure which would make this method a very long one. There were no such difficulties if the gravimetric method was employed, as less copper sulphate solution could be used in mixing the Fehling’s reagents and the sugar solution could then be added directly. The new volumetric method was devised for the determination of the cuprous oxide in the Fehling’s solution. It consists in converting the oxide into cuprous chloride and then pipetting a definite volume into a known quantity of iodine solution. The iodine which is not reduced is t.it.rated with sodium thiosulphate solution. Since this method was completed Ihe author’s attention has been called to the work of Rupp and LehmanrP who also de-