Trace elements play an important role in the environment, in particular in the biosphere. Some trace elements are very toxic, e. g. As, Cd, Hg, Pb; others, e. g. Fe, Zn, Mo are essential for life and found in organisms in the form of bioinorganic complexes. For many elements the difference between essential and toxic concentrations is not great. Within the last few decades the concentrations of trace elements in the environment have changed appreciably as a result of especially traffic, households and industry. The determination of trace elements in natural waters, identification of their chemical form and their separation are therefore of great importance. Chemical Forms of Trace Elements in Natural WatersTrace elements may be present in natural waters (ground water, river water, lakes and sea-water) in the form of cationic and anionic species or neutral complexes, in colloidal form or in suspended matter. The suspended solids can either be filtered off or separated by centrifuging. For filtration, membrane filters of 0.45-#m pore size are mostly used. Particles passing such filters are considered to *

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