Pure and Applied Chemistry

Abstract: The numerical modeling of Zn^II speciation amongst the environmental inorganic ligands Cl^–, OH^–, CO₃^2–, SO₄^2–, and PO₄^3– 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 β_p,q,r° valid at I_m = 0 mol·kg^–1 and 25 °C (298.15 K), and reports the empirical reaction ion interaction coefficients, ∆ε, required to calculate log₁₀ β_p,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⁺ + CO₃^2– system and for the Zn²⁺ + OH^– and Zn²⁺ + SO₄^2– systems at I > 0. In acidic and weakly alkaline fresh water systems (pH < 8), in the absence of organic ligands (e.g., humic substances), Zn^II speciation is dominated by Zn²⁺(aq). In this respect, Zn^II contrasts with Cu^II and Pb^II (the subjects of earlier reviews in this series) for which carbonato- and hydroxido- complex formation become important at pH > 7. The speciation of Zn^II 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 Cu^II and Pb^II 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 MCl_n^(2–n)+ species. The lower stability of the different complexes of Zn^II compared with those of Cu^II, Pb^II, and Cd^II is also illustrated by the percentage of uncomplexed M²⁺ in seawater, which is ca. 55, 3, 2, and 3.3 % of [M^II]_T, respectively.