Effect of potassium iodide concentration on measurement of combined available chlorine.

Abstract

In the chlorination of miscellaneous nitrogenous compounds contained in water, N-chloro compounds are formed from amine or imine in the compounds. Although the chlorine in N-chloro compounds has property of combined available chlorine, reactivity of each N-chloro compound is different each other. In the fractional determination of available chlorines by the N, N-diethyl-p-phenylene diamine (DPD) method or amperometric titration etc. different amounts of potassium iodide are added to differentiate each class of N-chloro compound. The velocity of color reaction of DPD depends mainly on the concentration of residual available chlorine. Consequently, the color development can be approximated by a first-order reaction. From the recorded curve of color development at 550 nm, periods to reach 85% and 95% value of the maximum absorbance are measured respectively. The rate constants of the first-order reaction K are calculated from these values. The rate constants increase in proportion to KI concentrations, and all experimental data points are essentially on a straight line. The time required to finish DPD color development and reciprocal of KI concentration are also correlated. In the case of added 30 μM KI, the DPD color development is completed within a period from 19 to 38 s, except chloropiperazine (over 80 s). The relationship between rate constant K and pKa of each N-compound has a good correlation.