Differences of reactivity of N-chloro compounds with N,N-diethyl-p-phenylenediamine.

Abstract

The differences of reactivity with N, N-diethyl-p-phenylenediamine (DPD) indicator reagent among N-chlorinated nitrogenous compounds were investigated. The DPD color development with N-chlorosuccinimide started immediately after addition, but it took several minutes to complete. Consequently, if titration with ferrous ammonium sulfate (FAS) is begun too quickly, it would reach an apparent end point (quenching the DPD color) before the coloration of DPD with N-chlorosuccinimide had reached its end-point. Subsequently, coloration would continue until all of the remaining N-chlorosuccinimide had taken part in the DPD coloration. Thus, when chlorosuccinimide solution is titrated by the normal DPD-FAS titrimetric procedure, an erroneous result would be obtained, suggesting the presence of both free and combined residual chlorine. Similar time courses of DPD color development were observed with mixtures of hypochlorite and imides and other nitrogenous compounds. When water containing succinimide, phthalimide, or other such compounds is chlorinated, and residual chlorine is measured by the normal DPD-FAS titrimetric procedure, erroneously high values for free available chlorine and low values for combined chlorine would be obtained, and the values obtained would be dependent on the time taken to perform the titration. A catalytic amount of KI is used in the DPD-FAS titrimetric method for differentiation of residual chlorine. The relation between the rate of DPD color development with some N-chloro compounds and the amount of KI added to the mixture was studied. The rate of color development in the presence of KI and the amount of KI needed to evoke a rapid coloration differed from N-chloro compound to N-chloro compound.