Titrimetric determination of dissolved oxygen in waste water

Winkler's procedure for the determination of dissolved oxygen has been modified to cope with substantial amounts of oxidizing or reducing substances in the samples. The manganese(III) is treated with an excess of hydroquinone, the unconsumed part of which is then titrated with standard ceric sulphate solution. A blank determination is also carried out, in which the order of addition of reagents is reversed so that the solution is always acidic, to allow for the interfering substances present. Results obtained by using this modified procedure on clean samples are in good agreement with results obtained by the usual Winkler method.