Determination of persulphates using N,N-diethyl-p-phenylenediamine as colorimetric reagent: Oxidative coloration and degradation of the reagent without bactericidal effect in water

Determination of persulphate is important in persulphate based advanced oxidation process (AOP). Potassium iodide (KI) based iodometry has been the conventional method for determination of persulphate. Recently, determination of persulphate by decolorization of organic pollutants via cobalt ion activated persulphate was reported. Here, we have studied the oxidative coloration of N,N-diethyl-p-phenylenediamine (DPD) by persulphates such as peroxydisulphate (PDS) and peroxymonosulphate (PMS). At room temperature, oxidation of DPD by these persulphates generated pink colored solution that showed absorption maximum at 510 and 551 nm. Importantly, the absorption at 510 and 551 nm increased linearly on increasing the concentration of persulphate, and linear calibration graphs were obtained with PDS and PMS concentrations in the range between 10 μM and 100 μM for quantitative analysis of persulphate. Determination of persulphates by the oxidative coloration of DPD was in good agreement with those determined by KI based conventional iodometric titration. Importantly, after analysis, the pink colored oxidized DPD waste solution was rapidly degraded by heat (80 °C) activated persulphate. Additionally, DPD, oxidized pink colored DPD waste solution left over after analysis, showed no bactericidal effect towards Escherichia coli, a common bacterium. Moreover, the colorless solution obtained after the degradation of pink oxidized DPD wastes by thermal activation of persulphate also showed no bactericidal effect towards E. coli. Degradation of DPD by persulphate activation, as well as the absence of bactericidal effect by DPD and its wastes, supports that the proposed DPD method for persulphate determination is environment friendly. Thus, the study highlights DPD as a potential colorimetric reagent for the spectrophotometric determination of both PDS and PMS that are commonly used for persulphate based AOP.