Dominating redox systems in lakes and rivers; thermodynamic interpretation of in-situ measurements with a platinum electrode

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Abstract
Thorough studies of 28 water bodies, predominantly lakes from the younger moraine area of Schleswig-Holstein, Northern Germany but also some peat bog waters in Niedersachsen, Northern Germany and lake Balaton in Hungary, showed the presence of at least six redox systems: NO 3 − /NO 2 − ; NO 3 − /NH 4 + ; NO 2 − /NH 4 + ; SO/HS−; SO/H2S; and probably Fe(OH)3/Fe2+. Depending on the physicochemical conditions one of these becomes the dominant system determining the electric potential measured with the platinum electrode. The platinum electrode does not respond to any oxygen dominated redox system. The corresponding theoretical redox equations seem to apply to almost all water bodies such as freshwater lakes, rivers, brackish water bodies, sea water and sometimes even humus rich water bodies from peat bogs and black water streams, hydromorphic soils, sediments and sewage sludge. The equations were found not to be valid for some heavily polluted effluents from lakes. The following points are discussed on the basis of these findings: the redox systems to which the normal platinum electrode reacts under natural conditions; equations of redox systems relevant in natural waters; methods of determination of the dominating redox system. the redox systems to which the normal platinum electrode reacts under natural conditions; equations of redox systems relevant in natural waters; methods of determination of the dominating redox system.

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