Cobalt interference in the “non-steady state clean water test” for the evaluation of aerator equipment—II. Occurrence and magnitude for various natural water systems

Water soluble and insoluble oxidizing compounds are formed during the non-steady state clean water test in certain natural water systems causing interference with Winkler dissolved oxygen determination. This results in pseudo high oxygen transfer efficiency of aerator devices. The interfering precipitate consists mainly of hydrated cobaltic oxide. Evaluation of the electrochemical-oxygen equivalent measurements made the presence of hydrated cobalt dioxide in the precipitate plausible. The presence of hydrogen peroxide in the filtrate suggested the formation of peroxodi- and peroxomono-sulfuric acid during the reaeration step. The pH along with the buffering capacity of a test water are the most critical water quality parameters directly affecting the magnitude of the cobalt interference. At pH values of 6.9 and below the cobalt and peroxide interference in the non-steady state clean water tests can be eliminated. Maximum cobalt interference was found to occur at pH values of 10–11.