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The numerical modeling of Cd II  speciation amongst the environmental inorganic ligands Cl – , OH – , CO 3  2– , SO 4  2– , and PO 4  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 log 10  β   p,q,r  ° valid at  I   m   = 0 mol kg –1  and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ &amp;epsilon;  , required to calculate log 10  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, ∆ r  H , are reported where available. Unfortunately, with the exception of the Cd II -chlorido system and (at low ionic strengths) the Cd II -sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log 10  {[H + ]/ c °} &amp;lt; 6), in the absence of organic ligands (e.g., humic substances), Cd II  speciation is dominated by Cd 2+ (aq), with CdSO 4 (aq) as a minor species. In this respect, Cd II  is similar to Cu II  [2007PBa] and Pb II  [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 &amp;lt; –log 10  {[H + ]/ c °} &amp;lt; 8.6, the speciation of Cd II  is still dominated by Cd 2+ (aq), whereas for Cu II  [2007PBa] and Pb II  [2009PBa] the carbonato- species MCO 3 (aq) dominates. In weakly acidic saline systems (–log 10  {[H + ]/ c ϒ} &amp;lt; 6; –log 10  {[Cl – ]/ c °} &amp;lt; 2.0) the speciation is dominated by CdCl  n   (2–   n   )+  complexes, ( n  = 1–3), with Cd 2+ (aq) as a minor species. This is qualitatively similar to the situation for Cu II  and Pb II . However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of Cd II  because of the relatively low stability of CdCO 3 (aq). In contrast, the speciation of Cu II  [2007PBa] and Pb II  [2009PBa] in seawater is dominated by the respective species MCO 3 (aq). There is scope for additional high-quality measurements in the Cd 2+  + H +  + CO 3  2–  system as the large uncertainties in the stability constants for the Cd 2+ -carbonato complexes significantly affect the speciation calculations.

pure and applied chemistry

Chemical speciation of environmentally significant metals with inorganic ligands. Part 4: The Cd2+ + OH–, Cl–, CO32–, SO42–, and PO43– systems (IUPAC Technical Report)