Trace metal ion activities from liquid-liquid partitioning measurements

Abstract

Elucidation of the chemical speciation of trace metals ~n the natural aquatic environment will lead to a better understanding of thei~ distribution and ecological effects. One approach which can provide useful information about the chemical reactivity of metal ions is the measurement of their activity. Phase equilibrium methods are required and liquid-liquid partition equilib"da are applicable. This study 2 utilized model systems to demonstrate this applicability.The partitioning of copper(II) ions as a chelate of acetylacetone was used to determine the trace activity coefficients of the copper(II) electrolyte in the CU(N03)2-HN03-KN03, Cu(N03)2-HCl04-NaCl04 and CuC1 2 -HCl-KCl systems over a wide range of ionic strengths (~).By careful control of pH and acetylacetone concentration only 1-3% of the metal ion was extracted. Under these conditions the amount extracted is proportional to the activity. The concentration of the bis(Acetylacetonato) Copper(II) was determined in the organic phase by spectrophotometric and atomic absorption methods but any convenient concentration technique could be used to measure the amount extracted.A comparison of activity measurements by liquid-liquid partitioning was made with electrochemical measurements by utilizing a copper ion selective electrode. The significantly lower activity coefficient values obtained by the electrochemical method were explained in terms of the liquid junction potential and the necessity for extrathermodynamic approaches to single ion activities.Potential application of the liquid-liquid partitioning method to the determination of trace activity coefficients in natural aquatic systems was demonstrated by extension of the method to measurements in copper(II) amino acid solutions at ~ = 0.001 and ~ = 0.723. The ionic strength adjustments in this case were made with NaCl. A significant difference in the free copper (II) ion activity was observed between solutions of copper(II) glycinate and copper(II) alaninate under identical conditions of metal and ligand concentrations, pH and ionic strength. Lum-Shue-Chan and Chau (4) to conclude that the free copper ion and its inorganic complexes are more toxic than are its organic ccmplexes.They estimated that ionic copper becomes toxic to planktonic algae at concentrations of approxtmate1y 10-10 M. Stiff (5) and Pagenkopf, Russo and Thurston (6) indicated that reduced copper toxicity results from bicarbonate alkalinity through formation of carbonate complexes or precipitates. Sylva (7) showed that the influence of organic complexing agents such as humic and fulvic acid cannot be overlooked. 2More recently, detailed studies utilizing equilibrium calculations have provided increased evidence that copper toxicity is indeed related to the activity of the free copper ion. Sunda and Guillard (8), utilizing cultures of Thalassiosira pseudonana and Nannochlous atomus, found that both growth rate inhibition and the copper content of the cells were related to the activity of the cupric ion. The growth rates of Thalassios...

Copper toxicity and extracellular release in Selenastrum capricornutum

“…f.olvent extraction has been used to measure ionic copper concentrations (Kennish, 1978). The methocl extracts only a fe'"J· percent of the bound copper thus perturbing the sys"tem very little.…”