Flow injection conductimetric or spectrophotometric analysis for acidity in fruit juice

Two flow injection (FI) procedures for the determination of acidity (expressed as citric acid content) in fruit juices are proposed: conductimetric involving injection into ammonia followed by gaseous diffusion into acetic acid; spectrophotometric FI titration in which the sample is mixed with sodium hydroxide in the presence of phenolphthalein. These procedures have been applied to the analysis of Thai fruit juices having acidity at a level of about 1% (w/v) citric acid. The relative standard deviations (n=12) were found to be within the ranges 3.6–6.2% and 0.7–2.2%, respectively, for the conductimetric and spectrophotometric procedures, the latter showing a negative bias due to the presence of colored matter in the samples. Sodium chloride and sucrose, when present in concentrations of up to 2% and 10% (w/v) respectively, do not interfere. Results obtained by either procedures are consistent with those obtained by a standard classical titrimetric method.