In the test, an excess of manganese(II) salt, iodide (I−) and hydroxide (OH−) ions are added to a water sample causing a white precipitate of Mn(OH)2 to form.
[1] Today, the method is effectively used as its colorimetric modification, where the trivalent manganese produced on acidifying the brown suspension is directly reacted with ethylenediaminetetraacetic acid to give a pink color.
The result of these two mechanisms determines the concentration of dissolved oxygen, which in turn indicates the production of biomass.
In the first step, manganese(II) sulphate (at 48% of the total volume) is added to an environmental water sample.
In the alkaline solution, dissolved oxygen will oxidize manganese(II) ions to the tetravalent state.
The acid facilitates the conversion by the brown, Manganese-containing precipitate of the Iodide ion into elemental Iodine.
The resulting number (usually reported in parts per million or milligrams per liter) is useful in determining the relative organic strength of sewage or other polluted waters.
This is important because oxygen in trapped air would be included in the measurement and would affect the accuracy of the test.