Complexing Capacity of Natural Water--Its Significance and Measurement

Abstract

Most of the trace metals in natural waters are believed to exist in complexed or chelated forms with the miscellaneous organic ligands of both natural and pollutants origin which regulate the availability of these metals in the system (1–5). Such a mechanism maintains a reservoir of metals ready for biological uses. The abundance of complexing ligands in a body of water thus determines the complexing capacity of the water and hence its regulating capability. The measurement of complexing capacity will add a new parameter to the chemistry of trace metals in the aquatic environment, especially in areas of biological uptake and toxicological studies (6,7). Such information is also pertinent in bioassay work where the knowledge of the possible forms of the metals after being added to a system is important. The measurement of complexing capacity of a water sample is based on the amount of copper ion being complexed by the organic ligands through direct complexation and/or displacement reaction. It is done by spiking several aliquots of a sample with increasing amounts of a copper solution. Then the uncomplexed copper is measured by a differential pulse anodic stripping voltammetric technique (8,9). After the complexing ligands in the sample have been saturated with copper, the peak current of free copper will increase linearly with the amount of the copper spikes. By extrapolating this linear curve back to zero peak current, the intercept on the X-axis represents the complexing capacity of the water expressed as equivalents of μ mole Cu/l. As the differential pulse anodic stripping voltammetry measurement of “free” copper is carried out in a system containing excess of copper ions, the peak current thus obtained represents the concentration of “free” copper. The amount of copper plated out from the labile complexes, if any, becomes insignificant (8). Table I illustrates some complexing capacity values of natural waters. Preliminary work on the chemical characterization of the complexing substances in lake water, using ultra-filtration techniques, indicated that these complexing compounds fall in an arbitrary molecular range of 1,000–10,000. Further investigations are being carried out to isolate and to characterize this fraction (9). The significance of complexing capacity of water on the effect and toxicity of copper ion on algal photosynthesis has also been investigated (10) using the 14C uptake technique with lake waters of different complexing capacity and with the original plankton species. It was observed that complexing capacity of a water body does not imply that the equivalent amount of copper can be taken up (masked) by the water without affecting the plankton growth, although it has been a recognized fact that chelating agents such as NTA, EDTA can mask up the toxicity of copper. Further studies are being carried out to find out the quantitative biological meaning of complexing capacity. The chemical and biological aspects of the problem must go hand in hand.