potassium permanganateReading time:
At an industrial level, potassium permanganate is prepared from the manganese dioxide ore called pyrolusite by:
- wet or dry oxidation to produce manganate;
- electrolytic oxidation of manganate to produce permanganate.
Depending on the source of initial ore, the final product may contain some traces of metal impurities (arsenic, cadmium, chromium, mercury, nickel, lead, antimony, selenium). The minimum potassium permanganate content is standardised 98.5% by weight for the common product. It is normally available as fine, deep violet crystals. It can be stored at ambient temperature in closed, protected or plastic steel containers depending on the applicable codes. The product is dissolved in water to form a stock solution that has a concentration of approximately 3 g · L–1, injected in the water to be treated by means of a dispensing pump.
Permanganate is a relatively expensive reagent. It is mainly used in drinking water pre-treatment for removing soluble manganese and iron (see the oxidants and disinfectants, Iron removal and manganese removal). It is more effective on manganese than chlorine. There are other known applications:
- preventing algae from developing in the raw water tanks;
- pre-oxidation of natural waters in order to remove color, taste and odors or to minimise the formation of THM during a subsequent chlorination.
These applications for organic matter oxidation remain few because permanganate acts selectively on organic compounds. Therefore, the amount to be used must be adjusted according to the composition of the water to avoid overdosing, which would color the water in pink.