Determination of free and combined residual chlorine by flow-injection spectrophotometry

Abstract

A new reaction scheme for the determination of free residual chlorine in the presence of other chlorine species, viz. chlorite, chlorate, combined chlorine and chlorine dioxide, has been evaluated. It is based on the oxidation of 4-nitrophenylhydrazine by free chlorine to the 4-nitrophenyldiazo cation and its electrophilic coupling with N-(1-naphthyl)ethylenediamine dihydrochloride to give an azo dye which absorbs maximally at 532 nm. Flow-injection systems are proposed for the determination of 0.05–10 μg ml −1 free chlorine by the reagent-injection technique, and 1–40 μg ml −1 free chlorine by sample injection. The limits of detection of free chlorine and mean R.S.D. (range) were 0.03 μg ml −1 and 0.9% (0.6–1.2%), and 0.4 μg ml −1 and 0.6% (0.4–0.8%), respectively. Only chlorine dioxide interferes. The results were in good agreement with those obtained by flow-injection N, N-diethyl- p-phenylenediamine spectrophotometric and classical iodimetric titration methods. Since chloramines do not interfere in free chlorine determination, a method was developed for the determination of total chlorine (and hence of combined chlorine by difference after determination of free chlorine) utilizing a secondary reaction of both free and combined chlorine with bromide to yield bromine which then acts as oxidizing agent. The sample throughput was 110 h −1 in both methods.