Abstract: A new spectrophotometric method for the determination of chlorine dioxide (ClO2) in tap water was developed based on methyl red (MR) oxidation and discoloration. Wavelength scanning showed that MR had maximal absorption at 518 nm. Under the experiment conditions of c(MR)/c(ClO2) = 6.4, pH 4.0－4.5, 25 ℃ and reaction time of 40 min, a linear equation was obtained as δA518 = －1.7845c(ClO2) +1.5228 (r = 0.992) in the range of 0.0004－0.80 mg/L, the apparent molar absorptivityε = 1.2 × 105 L/(mol ·cm), and the detection of limit 0.0036 mg/L. MR discoloration by ClO2 oxidation was a first-order reaction. The dynamic equation at 25 ℃ was ln[c∞/(c∞－c)] =－0.000689t ＋ 2.263669 with the activation energy Ea = 87.3 kJ/mol and the half-time t1/2 = 16.8 min. The average recoveries for ClO2 in tap water and ClO2 disinfectant were 98.2%－103.2% (n = 5) with a relative standard deviation of 2.43%－3.83%. This method and the recommended standard N,N-diethyl-p-phenylendiamin (DPD) method showed no significant difference (P = 0.05) according to F-test and t-test. Furthermore, satisfying determination results were obtained from this method.

Key words: tap water, disinfectant, chlorine dioxide, methyl red, discoloration, spectrophotometry