Specific Problems in the Measurement and Intrepretation of Complexation Phenomena in Seawater

Characterization of metal complexation phenomena in seawater is a formidable challenge. Adequate characterization of inorganic complexation schemes is currently limited by a paucity of well-defined hydrolysis constant data for chemical species which are important in the weakly alkaline conditions of normal seawater. Other important problems include the neglect of equilibria involving ligands and metals which form highly insoluble solids, and a poor understanding of the influence of pressure on stability constant behavior. Characterization of metal complexation by organic ligands in seawater is especially problematic. There is presently a large gulf between laboratory investigations of metal complexation by structurally characterized organic ligands, and investigations of complexation by natural organics whose molecular architecture is essentially unknown. In view of the wide variety of techniques used to investigate the complexation behavior of natural marine organics, and the diverse and occasionally conflicting attributes reported for metal complexation by natural organics in seawater, it would be most valuable if some level of effort were devoted to analysis of shared samples. In view of the multi-faceted strengths of the diverse procedures available for characterization of natural organo-metal interactions, conjugate analysis using a variety of techniques on identical samples seems essential for bringing a reasonable level of constraint to interpretation of complexation observations. 1. ComDlexation Phenomena in Seawater The fertility of the oceans and the chemical composition of seawater are intricately interwoven '(85MH, 91BD). Many elements in seawater exhibit widely varying solution concentrations due to their participation in complex, biologically-mediated cycles: uptake in and on biogenic particles in the surface ocean, downward transport via biological debris, regeneration (return to solution) at depth, and return to the surface ocean via oceanic mixing. While the distribution of many elements in the oceans is thus strongly influenced by ocean biology, the growth characteristics of organisms in the surface ocean, in turn, bear the strong imprint of ocean chemistry. The role of ocean chemistry, and, in particular solution chemistry, in the oceans' biological productivity is especially evident in relationships which exist between marine planktonic organisms and a variety of bioactive cations found in seawater at nanomolar and lower concentrations. For a wide variety of metals including Co", Mn", Ni", Cu", Zn", and Fe"' it is generally observed that bioavailability is intimately related to each metal's distribution among various chemical forms (speciation) in solution (90HM, 91S, 93HM, 94S, 94PB). For these nutrient metals, and others, partitioning of total metal among free hydrated cations, M"', and forms, MG, complexed with solution ligands (4) exerts a major influence on the, oceans' biogeochemical cycles. While the acknowledged importance of chemical speciation in marine biogeochemistry (85MH, 91BD, 91s) has made chemical speciation an area of intense interest, the experimental difficulties typically encountered in identifymg and investigating species of suspected importance in seawater has made some aspects of speciation assessments very controversial. In the discussion which follows, specific problems and limitations in the description of complexation phenomena in seawater will be addressed. This account will be divided into inorganic and organic complexation assessments. In keeping with the general practice of saying most about that which is least understood, the greater part of the following discussion will be devoted to the very important phenomenon of metal association with marine organic matter.