How Excess Permanganate Index Changes Water Colour

The permanganate index (CODMn) is a measure of organic matter and certain reducing inorganic substances in water. An elevated index does not directly colour the water, but the pollutants that cause the high index often impart distinct hues. Recognising these colour changes helps field staff quickly diagnose the type of contamination.

1. Humic Substances – Yellow to Brown‑Black

When water contains high levels of humic and fulvic acids (from decaying vegetation, agricultural runoff, or peatland drainage), the permanganate index rises. In low concentrations, the water appears pale yellow or tea‑coloured; as the concentration increases, it turns yellow‑brown, dark brown, or even blackish brown. The colour intensifies at lower pH because humic acids become less soluble and form colloidal suspensions.

2. Iron and Manganese – Red‑Brown to Black

Groundwater or mining effluents often contain ferrous iron (Fe²⁺, pale green) and soluble manganese (Mn²⁺). Both are oxidised by permanganate and thus contribute to CODMn. Once exposed to air or oxidised, iron forms reddish‑brown ferric hydroxide, turning the water rusty‑red or yellow‑brown. Manganese oxidises to black manganese dioxide (MnO₂), producing dark grey or blackish water, sometimes accompanied by black flocs.

3. Industrial Organic Pollutants – Variable Artificial Colours

Discharges from textile dyeing, papermaking, or coking plants contain synthetic organic compounds that are themselves coloured and drive up CODMn. Such wastewater can appear red, blue, green, purple, dark brown, or even ink‑black, depending on the pollutant. These colours are often chemically stable and difficult to remove by simple settling.

4. Algal Blooms – Green to Dark Olive

Eutrophic waters with dense algal growth show high permanganate index because algae contain large amounts of organic matter. During a bloom, the water is bright green, blue‑green, or yellowish‑green. After the algae die and decompose, the water turns grey‑green, dark olive, or black (under anoxia), accompanied by a foul odour.