Photometric Determination of Reducing Sugar in Vegetables Based on the Color Reaction System of K3Fe(CN)6-FeCl3

In an alkaline solution of NaOH, the reagent solution of K3Fe(CN)6 was reacted with the reducing sugar in the sample solution by heating for 10 min in a boiling water bath, to give K4Fe(CN)6 quantitatively. The solution was cooled to room temperature and acidified with 0.1 mol·L-1 HCl to pH 3.5-4.0, and then reacted with difinite amount of FeCl3 solution to give Prussian blue in 5 min. Absorbance of the blue color was measured at the wavelength of 680 nm with 1 cm absorption cell (vs. reagent blank). Linear relationship between values of absorbance and mass concentration of reducing sugar was kept in the range of 0.8-4.0 mg·L-1, with its molar absorptivity of 3.4×104L·mol-1·cm-1 . Samples of 3 kinds of vegetables were analyzed by this method, giving results of reducing sugar checked quite well with results obtained by the Lane-Eynon′s method, with RSD′s (n=5) less than 2.5%. Results of recovery test were found in the range of 99.7% to 100.1%.