INDIRECT FLOW-INJECTION DETERMINATION OF ASCORBIC-ACID BY FLAME ATOMIC-ABSORPTION SPECTROMETRY

A continuous-flow procedure is proposed for the indirect determination of ascorbic acid, based on its reducing properties because of the oxi dation of its 1,2-enediol group. Iron(III) was injected into a 1,10-ph enanthroline stream, which was mixed with a sample carrier and then wi th a sodium picrate solution stream. In these conditions the iron(III) was reduced to iron(lI) by the ascorbic acid. Thus, the iron(II) form ed reacts with 1,10-phenanthroline to form a charged red complex, whic h with picrate ion forms a stable red-orange uncharged ion-association complex that is adsorbed ''on-line'' on a non-ionic polymeric adsorbe nt (Amberlite XAD-4), proportionally to the ascorbic acid in the sampl e. The unadsorbed iron was determined by flame atomic absorption spect rometry. The proposed method allows the determination of ascorbic acid in the range 0.5-25 mu g ml(-1) with a relative standard deviation of 2.9% at a rate of ca. 90 samples h(-1). This method has been applied to the determination of ascorbic acid in pharmaceutical preparations, fruit juices and sweets. The results obtained in the analysis are comp ared with those provided by the 2,6-dichloroindophenol method.