Evaluation of a portable bare-electrode amperometric analyzer for determining free chlorine in potable and swimming-pool waters

Chlorine in the forms of HOCl or ClO⁻ was determined rapidly and precisely in the range from 0.10 to 3.0 ppm chlorine, without titration, using a bare-electrode portable amperometric analyzer. The instrument was calibrated with a 1.00 ppm chlorine standard solution or an equivalent permanganate solution which is stable for at least 6 months. The detection limit was 0.10 ppm chlorine, and relative standard deviations (N = 10) were <6.0% in potable or swimming-pool waters containing 0.25, 1.00 and 2.50 ppm or 0.35, 1.40 and 2.70 ppm chlorine respectively. Bromine-, triiodide ion-, and Mn (VII)-generated signals were stoichiometrically equivalent to that of hypochlorite, but MnO₂ suspensions (1.5, 5.0 or 15 ppm) did not produce detectable amperometric signals.Analysis of solutions of hypochlorite with ammonium chloride or selected organic nitrogen compounds indicated that various N-chloro compounds may interfere. In the presence of N-chloroglycine (2.70 ppm chlorine) the amperometric signal was about 5% of that for the equivalent concentration of hypochlorite, but higher relative responses were obtained with NH₂Cl, NHCl₂ or NCl₃ (19, 42 and 70% at 2.60, 1.20 or 1.00 ppm chlorine respectively). Chlorinated urea (2.2 ppm N, 2.90 ppm chlorine), chlorinated bovine albumin (10 ppm albumin, 2.00 ppm chlorine) or monochloroisocyanurate (1.30 ppm chlorine) produced amperometric signals (76, <5 and 50% respectively) which are lower than those obtained by the N,N-diethyl-p-phenylenediamine (DPD) method (100, 12 and 92%).Twenty potable water samples were analyzed for free chlorine by the DPD and amperometric procedures. Statistical analysis showed no significant differences between the two sets of results (P < 0.1). Swimming-pool-water samples were also analyzed by the two methods in the field (22 samples) and in the laboratory (24 samples). In each set of results the mean free-chlorine value by the DPD procedure was significantly higher than that obtained amperometrically (P > 0.005). This discrepancy was associated with the probable presence of chlorinated urea, whose signal by amperometry is lower than that by the DPD method.The advantages of this simple procedure must be weighed against possible inaccurate results in the presence of NH₂Cl, NHCl₂ or NCl₃.