Determination of the Permanganate Index in High-Chlorine Water Bodies

Despite the significant demand, accurate determination of the permanganate index in high-chloride bodies of water remains extremely limited. This study introduces an iodine-thiosulfate method that effectively eliminates the interference of chloride ions in the permanganate index determination. In this method, organic and inorganic substances in a water sample are oxidized with KMnO4 in an alkaline medium to mitigate chloride ion interference. To further minimize this interference during back-titration, the remaining KMnO4 is reduced using KI under adjusted acidic conditions. The released iodine is then titrated with standardized Na2S2O3 until the starch–iodine complex’s blue–black color disappears. The iodine-thiosulfate method demonstrated a method detection limit of 0.4 mg L–1 (n = 7) and a corresponding interlaboratory quantification limit of 1.6 mg L–1 in water samples with chloride concentrations of 5000 mg L–1. The method’s precision and accuracy ranged from 1.4% to 6.7% and −1.0% to 5.0%, respectively. The relative error in permanganate index determination remained below 20% even at chloride concentrations up to 60 g L–1, whereas the conventional oxalate-permanganate method exceeded a 20% relative error once the chloride concentration reached 10 g L–1 or higher. The sufficiently low detection limit along with excellent repeatability (intralaboratory precision), reproducibility (interlaboratory precision), and accuracy confirms its practical feasibility for routine analysis.