STUDY OF THE LABILITY OF COPPER(II)-FULVIC ACID COMPLEXES BY ION-SELECTIVE ELECTRODES AND POTENTIOMETRIC STRIPPING ANALYSIS

Two electroanalytical techniques, direct potentiometry with a copper i on selective electrode (ISE) and potentiometric stripping analysis (PS A), were used to evaluate the lability of copper-fulvic acid complexes (Cu-FA) in 0.5 mol/l sodium perchlorate and in synthetic sea water. T he determinations were performed at pH 6.5, and for sea water also at pH 7.5. By ISE potentiometry, in 0.5 mol/l sodium perchlorate, the FA total complexation capacity (CC) in the 1-150 mg/l FA concentration ra nge, as well as the 1:1 Cu-FA conditional stability constants for two different types of binding sites (K-1 and K-2) and the respective meta l-binding capacities (C-1 and C-2; C-1 + C-2 = CC) were obtained. It w as found that CC was approximately constant over the FA concentration range studied: 1.81 mmol/g of FA or 5.2 mmol/g of total organic carbon (TOC). For 0.2-18 \FA\(t)/\Cu\(t) molar ratio range, the values of th e complexation parameters were as follows: log K-1 = 5.77 +/- 0.09, C- 1 = (4.7 +/- 1.2) x 10(-5) mol/l and log K-2 = 4.22 +/- 0.10, C-2 = (1 4.8 + 1.7)x 10(-5) mol/l (95% confidence limits, 20 degrees C). These parameters were also obtained at 25 degrees C. K-1 and K-2 decreased w ith the decreasing of \FA\(t)/\Cu\(t). The labile copper fraction was determined by PSA, in a large \FA\(t)/\Cu\(t) It molar ratio range (89 -1.6) in 0.5 mol/l sodium perchlorate medium, at -0.5 V vs. SCE and 20 degrees C. The results were compared with the inorganic copper(II) fr action obtained by a computational simulation based on equilibrium con siderations. The lowest lability (dissociation of ca. 8% of the Cu-FA complexes) was found for the highest \FA\(t)/\Cu\(t) ratio, which corr esponded to \Cu-FA\/TOC = 0.06 mmol/g. The lability increased up to ca . 61% when \FA\(t)/\Cu\(t) decreased down to 6 (or \Cu-FA\/TOC = 0.8 m mol/g) and slightly decreased down to ca. 50% for lower \FA\(t)/\Cu\(t ) values (the lowest was 2 mmol/g). In synthetic sea water, the inert copper(II) fraction (i.e., \Cu\(t)/\Cu\(labile) measured by PSA) was n ot very different from that found for 0.5 mol/l sodium perchlorate for identical \FA\(t)/\Cu\(t) at pH 6.5. Similar results were obtained at pH 7.5. These results suggest that the PSA kinetic behaviour of FA in sea water may be not very different from that observed for the simple r medium of similar ionic strength.