A spectrophotometric method for measuring permanganate index (CODMn) by N,N-diethyl-p-phenylenediamine (DPD)

A new spectrophotometric method for measuring permanganate index (chemical oxygen demand using potassium permanganate (KMnO 4 ) as oxidant, COD Mn ) in water was established. The method was based on the rapid oxidation of N,N -diethyl- p -phenylenediamine (DPD) by residual KMnO 4 in digestion solution under neutral pH condition to form the stable pink radical (DPD ●+ ). Only 20 s were enough to form the pink DPD ●+ . The generated DPD ●+ could be quantitatively measured by a visible spectrophotometer at 551 nm. Stoichiometric coefficient of the reaction between KMnO 4 and DPD was close to 1:5 (1:5.07). There was a well linear relationship (R 2  = 0.999) between the change of the absorbance of DPD ●+ at 551 nm and the concentration of COD Mn in the range of 0–4.46 mg L −1 . Limit of detection of the DPD method was as low as 0.02 mg L −1 COD Mn . The DPD method was highly accurate for measuring COD Mn in standard solutions with well recovery rates of 99.17%–102.22%, and was well tolerant to the interference of coexistent Cl − and Fe 3+ . The DPD method was successfully applied for measuring COD Mn in real water samples, including surface water, underground water and drinking water . In comparison to the traditional titration method, the proposed DPD method was more convenient to operate, required less samples and digestion reagents (i.e., KMnO 4 and H 2 SO 4 ) and could be employed for online monitor.