Determination of chloride by flow injection spectrophotometry with membrane reagent introduction Original Research Article

A single line manifold incorporating three Nafion® membrane reactors has been developed for the sequential introduction of components of the mercuric thiocyanate/iron(III)/acid reagent for the determination of chloride. Nitric acid was introduced at the first reactor to give sufficient ionic strength to allow cation exchange of FeSCN2+ at the second reactor, and Hg2+ at the third reactor. The composition of the reagent was adjusted by control of the flow rates of donor and acceptor streams, reagent concentration and reactor length. Calibration over the range 0.3–25 μg ml−1 was possible, though the calibration function was curved. The manifold was used to determine chloride in river water and in pond water at concentrations of 8 and 47 μg ml−1, respectively and gave results that were not significantly different from those obtained by an EPA method. The lower range sensitivity was 0.032 absorbance μg ml−1, which compares favorably with that of other flow injection methods, but the detection limit was 300 ng ml−1, considerably higher than that can be achieved by flow injection procedures. This was due to the presence of mixing noise caused by fluctuations in reagent concentration related to pulsations in the flow in both donor and acceptor streams. Valve switching and refractive index effects were absent.