Novel approach for mono-segmented flow micro-titration with sequential injection using a lab-on-valve system: a model study for the assay of acidity in fruit juices

A new concept for micro-titration using a “lab-on-valve” (LOV) system with sequential injection of mono-segmented flow is proposed. The performance of the system was demonstrated by the assay of acidity in fruit juices which is based on acid–base neutralization. A standard/sample solution containing citric acid, indicator, sodium hydroxide, were sandwiched between air segments and were aspirated in microliter volumes through a selection valve into a holding coil. The acid, indicator, and base were mixed by flow reversal. After removing air segments, the solution was pushed to the detector for monitoring of the change in absorbance of the indicator color, which depended on the concentration of the remaining base. With LOV, microliter volumes of the solution can be detected without dispersion of the color zone. A calibration graph (plot of absorbance vs. acidity value) in the range of 0.2–1.2% (w/v) as citric acid was established. Sample throughput of 30 sample h−1 and good reproducibility (RSD = 1.2%, n = 11 for 0.6% acidity) were achieved. The procedure has been applied to determine acidity in fruit juices.