The quantitative determination of free chlorine based on its oxidation current was studied using cyclic voltammetry (CV) with highly boron-doped diamond (BDD) electrodes. The voltammograms exhibited well-defined anodic peaks and a good linear relationship in the concentration range of 20-100 mg Cl L-1 using BDD electrodes without any pretreatment. Excellent reproducibility was demonstrated by an RSD of 2.56%, (n = 10). The electrochemical behavior of free chlorine at different pH values suggests the optimum condition to be pH 8. Observations with the BDD electrodes terminated with either hydrogen or oxygen suggest that the active species in the anodic reaction was CIO-. Interference of limited concentration of chloride ion was not observed, whereas exceeded concentration was found to accelerate the electron transfer on chlorine oxidation. Flow injection analysis (FIA) with amperometric detection was used to estimate lower concentrations of free chlorine. A linear relationship (r(2) = 0.96) was found for the concentration range of 0.1-2 mg L-1 and the low detection limit (S/N = 3) was estimated to be 8.3 mu g L-1. The results suggest that BDD electrodes are more than adequate for determining the concentration of free chlorine. (c) 2007 Elsevier B.V. All rights reserved.