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Abstract

An ion-exchange method was used to determine complexation constants for the Nioxalate and Ni-carbonate systems in a NaClO 4 background electrolyte. The Ni-oxalate data were interpreted in terms of a single Niox(aq) complex having log K 1 values for Ni 2+ + ox 2− ⇔ Niox(aq) of 3.9 ± 0.1 (I.S. = 0.5 mol-L −1 p[H] = 7.1) and 4.4 ± 0.1 (I.S. = 0.1 mol-L −1 p[H] = 8.6) at 22 ± 1 • C. Specific ion-interaction theory (SIT) was used to obtain log K • 1 = 5.17 ± 0.05 (95% confidence level and ε = −0.23 ± 0.15) at I.S. = 0. The Ni-carbonate studies were carried out at p[H] values of 7.5, 8.5, and 9.6 in 0.5 mol-L −1 NaClO 4 /NaHCO 3 solutions. The NiCO 3 (aq) species was the dominant complex in the [CO 2− 3 ] concentration ranges studied at all three p[H] values. A log K 1 value for Ni 2+ + CO 2− 3 ⇔ NiCO 3 (aq) of 2.9 ± 0.3 was deduced at I.S. = 0.5 mol-L −1 . Extrapolating this value to zero ionic strength using the SIT approach yielded log K • 1 = 4.2 ± 0.3 (95% confidence level and ε = −0.26 ± 0.04). The data allowed upper bound values for the complexation constants for NiHCO + 3 and Ni(CO 3 ) 2− 2 to be estimated, i.e., log K • < 1.4 for Ni 2+ + HCO − 3 ⇔ NiHCO + 3 , and log K • 2 < 2 for NiCO 3 (aq) + CO 2− 3 ⇔ Ni(CO 3 ) 2− 2 , respectively.

Development of an In Situ Ion-Exchange Technique for the Determination of Free Cd, Co, Ni, and Zn Concentrations in Freshwaters

“…The reliability of these Ni-carbonate constants, which were derived by estimation procedures, has been questioned (Hummel and Curti 2003). With an ion-exchange method, Baeyens et al (2003) measured a log K NiCO3 of 4.2 ± 0.3 and estimated a log K NiHCO3? \ 11.7 (at zero ionic strength).…”