The mechanism of color reaction in the determination of residual chlorine by DPD as an indicator.

Abstract

In the present studies, the colour reaction mechanism of DPD (N, N-diethyl-p-phenylenediamine) method was studied to know the optimum condition for determination of residual chlorine in water. HOCl (hypochlorite) and KMnO4 (permanganate) were used as oxidizing agents. When the rate of the reaction mixture was more than 0.4 mol DPD per 2 equivalent HOCl and 0.8 mol DPD per 2 equivalent KMnO4, though the absorbance at 550 nm was low, the amount of oxidizing agents was able to be determined quantitatively by ferrous ammonium sulfate (FAS) titration method. When the rate of the reaction mixture was less than 0.4 mol DPD per 2 equivalent HOCl and the region of 0.3-0.8 mol DPD per 2 equivalent KMnO4, it was not able to be titrated by FAS, because no DPD color was developed in the reaction mixture. When 28.6 mol DPD was added subsequently to the mixture, there occurred colouration. The colouration by the subsequent addition of DPD is based on the formation of semiquinone from quinonediimine and DPD. The symproportionation constants to semiquinone were 3.02×108 K/M-1 (HOCl) and 3.07×108 K/M-1 (KMnO4). It is concluded from these results, in order to determine the amount residual chloride in water accurately by DPD ferrous titrimetric method or DPD colorimetric method, more than 5 mol DPD is required for 1 mol chlorine.