CrossRef enabled

PAC Archives

Archive →

Pure Appl. Chem., 2013, Vol. 85, No. 12, pp. 2249-2311

Published online 2013-11-23


Chemical speciation of environmentally significant metals with inorganic ligands. Part 5: The Zn2+ + OH, Cl, CO32–, SO42–, and PO43– systems (IUPAC Technical Report)

Kipton J. Powell1*, Paul L. Brown2, Robert H. Byrne3, Tamás Gajda4, Glenn Hefter5, Ann-Kathrin Leuz6, Staffan Sjöberg7 and Hans Wanner6

1 Department of Chemistry, University of Canterbury, Christchurch, New Zealand
2 Rio Tinto Technology and Innovation, 1 Research Avenue, Bundoora VIC 3083, Australia
3 College of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016, USA
4 Department of Inorganic and Analytical Chemistry, A. József University, P.O. Box 440, Szeged 6701, Hungary
5 Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia
6 Swiss Federal Nuclear Safety Inspectorate, CH-5200 Brugg, Switzerland
7 Department of Chemistry, Umeå University, S-901 87 Umeå, Sweden

Abstract: The numerical modeling of ZnII speciation amongst the environmental inorganic ligands Cl, OH, CO32–, SO42–, and PO43– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10 βp,q,r° valid at Im = 0 mol·kg–1 and 25 °C (298.15 K), and reports the empirical reaction ion interaction coefficients, ∆ε, required to calculate log10 βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆rH, are reported where available. There is scope for additional high-quality measurements for the Zn2+ + H+ + CO32– system and for the Zn2+ + OH and Zn2+ + SO42– systems at I > 0. In acidic and weakly alkaline fresh water systems (pH < 8), in the absence of organic ligands (e.g., humic substances), ZnII speciation is dominated by Zn2+(aq). In this respect, ZnII contrasts with CuII and PbII (the subjects of earlier reviews in this series) for which carbonato- and hydroxido- complex formation become important at pH > 7. The speciation of ZnII is dominated by ZnCO3(aq) only at pH > 8.4. In seawater systems, the speciation at pH = 8.2 is dominated by Zn2+(aq) with ZnCl+, Zn(Cl)2(aq), ZnCO3(aq), and ZnSO4(aq) as minor species. This behaviour contrasts with that for CuII and PbII for which at the pH of seawater in equilibrium with the atmosphere at 25 °C (log10 {[H+]/c°} ≈ 8.2) the MCO3(aq) complex dominates over the MCln(2–n)+ species. The lower stability of the different complexes of ZnII compared with those of CuII, PbII, and CdII is also illustrated by the percentage of uncomplexed M2+ in seawater, which is ca. 55, 3, 2, and 3.3 % of [MII]T, respectively.