Oxygen consumed means the oxygen that the oxidizable compounds of sewage
and water consume when treated in an acid solution with potassium
permanganate. The expression is synonymous with oxygen required, oxygen
absorbed, and oxygen-consuming capacity. It should not be confused with
biochemical oxygen demand.
As the carbon, not the nitrogen, in organic matter is oxidized by
potassium permanganate, oxygen consumed is considered by some an
indication of the amount of carbonaceous organic matter present. The
determination indicates, however, only part of the carbon, the
proportion varying in different samples because the carbon in
nitrogenous matter is not so readily oxidized as that in carbonaceous
organic matter. Furthermore, it does not directly differentiate the
carbon present in unstable organic matter from that in fairly stable
organic matter, such as is sometimes referred to as residual humus
matter. As nitrite nitrogen, ferrous iron, sulfide, and other oxidizable
mineral substances reduce potassium permanganate, corrections for them
should be made in the determination.