Cyclic Flow Injection Analysis with Spectrophotometric Detection for Assay of Acidity in Fruit Juices and Soft Drinks

Abstract

A single-line flow system, through which a reagent carrier stream is circulated, is proposed for the spectrophotometric determination of acidity (expressed as citric acid content) in fruit juices and soft drinks. The reagent carrier solution consists of pH indicator (Methyl Orange, MO) and sugar (sucrose) in an acetate buffer solution. The method is based on the acid-base neutralization detecting the color change of MO. The resulting solution, after passing through a flow through cell, returns to the reservoir and reused repeatedly. Sucrose is added in the reagent carrier solution to compensate both the differences in refractive index and viscosity between the reagent carrier solution and the sample solution, which make the sample injection directly without any dilution process. Several parameters were optimized, resulting in a simple, fast, and ready constructed acidity analyzer. Fruits juices and soft drinks could be analyzed without any prior chemical treatment. Among several organic acids, citric acid is the major acid in fruit juices. It is frequently added to the industrial drinking derivatives to keep quality and flavor of products constant. The acidity is determined routinely during the processing of drinking products. It is expressed usually as citric acid content. Commonly used techniques for this analysis is titration with the standard base(1). Normally all batch titrations involve quite high volume of reagents consumption and manually operations which are subjected to time consuming and personal error. Therefore, an on-line, near real time acidity analyzer is very desirable. Flow injection analysis (FIA) is widely accepted as an analytical technique capable to perform rapid analysis, and offers many advantages over batch titration (2-4). For the assay of acidity in fruit juices, several procedures (5-7) based on the flow injection method have been proposed employing mixing chamber or variation in reagent flow rate, using spectrophotometry or potentiometry as detection techniques. Imato and Ishibashi reported the spectrophotometric FIA determination of strong acids and bases using a pH indicator and buffer solution (8,9). This method is excellent because the estimation of end point is not necessary. The calibration graph can be constructed from the peak heights (absorbances) of the standard solutions of various concentrations. We developed this method into a cyclic FIA for the determination of strong acids and bases, where the reagent carrier solution was circulated and reused repeatedly (10). Recently we found this cyclic FIA was applicable to citric acid determination in a similar manner as above. In quality control, actual sample analysis without any prior chemical treatment should be desirable in order to decrease sample handling steps while also saving time of analysis. These facilities can be provided by exploiting the direct determination with the simplest manifold. This paper therefore describes the