Calcium and inorganic carbon speciation in Spanish Mediterranean brackish waters

The concentrations of free ions and major inorganic complexes of calcium and inorganic carbon (i.e. free Ca²⁺, free HCO^-₃, free CO^2-₃ , H₂CO₃ , CaSO⁰₄ , CaCO⁰₃ , CaHC0⁺₃ , NaHCO⁰₃ , MgHCO⁺₃ , NaCO^-₃ and MgCO⁰₃ ) have been calculated according to an ion-pair model, on the basis of their relative abundance in total concentrations, in Spanish Mediterranean brackish waters. Calcium complexes amount to 25% of total calcium, whereas bicarbonate and carbonate complexes reach 10 and 47% of total alkalinity respectively. NaHCO⁰₃ and MgHCO⁺₃ expressed as percentages of total alkalinity, and NaCO^-₃ , MgCO⁰₃ and free CO^2-₃ expressed as percentages of total carbonate are dependent on ionic strength. NaCO^-₃ , MgCO⁰₃ referred to total carbonate and free HCO₃^- , and free CO^2-₃ as percentages of total alkalinity are strongly dependent on pH. Inorganic carbon complexes amount to an important fraction of total alkalinity when I (ionic strength)> 0.05 and pH> 8.5, when I> 0.10 and pH> 8.3, and when I> 0.20 and pH> 8.1. Calcium complexes cannot be expressed as single function of pH or ionic strength. Their maximum percentages appear at high pH values and moderate or high ionic strength, when CaSO⁰₄ and CaCO⁰₃ coexist at significant levels.