Determination of Acidity in Fruit Juices and Soft Drinks by Cyclic Flow Injection Analysis

Abstract

A cyclic flow injection analysis is proposed for the determination of acidity (expressed as citric acid content) in fruit juices and soft drinks. The method is based on acid-base neutralization for detecting the color change of a pH indicator. A reagent carrier solution consisting of 1.5 × 10-4 M Xylenol Orange (XO) and 12% (w/v) sucrose in 0.01 M phosphate buffer (pH 8.0) was continuously circulated in a single-line cyclic flow system. Into the stream, a 5 μL sample solution was injected, and the color change of XO was monitored spectrophotometrically at 400 nm. Sucrose is added to the reagent carrier solution in order to compensate both the differences in the refractive index and the viscosity between the carrier and the sample solutions, which make the sample injection direct without any dilution process. Several parameters were optimized, resulting in a simple, fast, effective and economical acidity monitor. The procedure has been applied to determine acidity in fruit juices and soft drinks.