Determination of reducing sugars by flow injection gravimetry Original Research Article

A flow injection procedure for gravimetric determination of reducing sugars was developed. The system was designed to permit the monitoring of an in-line formed suspension. In view of the high density of the precipitate, a product of oxidation of reducing sugars by Fehling reagent, the Archimedes principle was exploited, so that the precipitate was weighed inside the flowing stream. Sample and reagents were simultaneously injected into two convergent carrier streams flowing through a reaction coil immersed in boiling water under reflux. The formed precipitate was cooled, accumulated on a sintered glass mini-filter and weighed. An acidic solution was then injected into both carrier streams to promote in-line solubilization of the precipitate. Characteristics of the filtering device, geometry of the debubbler, flow rates, wash solution and possibilities of weighing with the filtering unit on the plate of the analytical balance or suspended under it were investigated. Furthermore, effects of reagent concentration, temperature, surfactant addition and available time for reaction were studied. The gravimetric flow injection system was very stable and required only 0.64 g Cu per determination. Precipitate drying was not needed. Precise results (r.s.d. usually <0.9%), in agreement with an official titrimetric procedure, were obtained within 0.20 and 1.00% w/v invert sugar. A new weighing was accomplished every sixth minute.