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

A new spectrophotometric method for measuring permanganate index (chemical oxygen demand using potassium permanganate (KMnO₄) 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 KMnO₄ 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 KMnO₄ and DPD was close to 1:5 (1:5.07). There was a well linear relationship (R² = 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 Fe³⁺. 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., KMnO₄ and H₂SO₄) and could be employed for online monitor.