DETERMINATION OF TITRATABLE ACIDITY AND ASCORBIC-ACID IN FRUIT JUICESIN CONTINUOUS-FLOW SYSTEMS

Two continuous-flow systems for the determination of titratable acidit y and ascorbic acid in fruit juice samples are described. The assembli es permit on-line dialysis of analytes prior to the reaction step, thu s improving selectivity and performing sample dilution. Flow systems a re built with a channel carrying the donor phase (sample in both deter minations) and another channel carrying an acceptor phase, both of the m entering the dialyser. The outcoming stream transporting the dialyse d sample fills the valve loop, permitting its injection into a carrier stream which continuously passes through the spectrophotometric detec tor. For the titratable acidity, acceptor phase and carrier are distil led water, the reagent merged with the carrier channel being a buffere d solution of bromothymol blue (pH 7). The analytical signal obtained is then monitored at 616 nm. For ascorbic acid, the acceptor phase was a Fe(III) solution, which reacts with the dialysed analyte to form Fe (II). A buffered solution of o-phenanthroline (pH 4.5) is used as carr ier, reacting with Fe(II) to give the analytical signal, which is moni tored at 510 nm. Chemical and physical parameters are optimized for bo th systems. The analytical features of the determination are establish ed. Finally, the proposed procedures are compared with the official vo lumetric AOAC methods for both parameters. The FIA methods turn out to be suitable for a rapid and accurate control of fruit juice samples, compared with the reference methods; additionally they compete advanta geously with the volumetric methods in the case of turbid and highly c oloured samples.