Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties: Artificial seawater from 0 to 45 °C

Accurate chemical speciation models of solutions containing the ions seawater have applications in calculation carbonate system equilibria and trace metal natural waters, determination pH. Existing models, based on Pitzer formalism for activity coefficients, do not yet agree with key experimental data (potentiometric determinations H+ Cl− products acidified artificial seawaters) and, critically, include uncertainty estimates. This hampers also their further development (for which contributions individual ion interactions equilibrium constants need to be known). We therefore implemented Waters Millero (Mar. Chem. 149, 8-22, 2013) Clegg Whitfield (Geochim. et Cosmochim. Acta 59, 2403-2421, 1995) seawater, within a generalised treatment uncertainties, as first step towards more complete model standard pH buffers. addition enables both total any model-calculated quantity (e.g., pH, speciation) estimated, all interaction parameters constants. Both been fully documented (and some corrections made). Estimates variances covariances were obtained by Monte Carlo simulation, simplifying assumptions. The tested against measured electromotive forces (EMFs) cells seawaters. mean offsets (measured – calculated) at 25 °C are: 0.046 ± 0.11 mV (artificial without sulphate, 0.280 mol kg−1 0.879 ionic strength); −0.199 0.070 salinities 5 45). Results are similar other temperatures. These differences compare an overall EMFs about 0.04 mV. Total uncertainties calculated dominated just few contributions: mainly H+-Cl−, Na+-Cl−, H+-Na+-Cl− interactions, thermodynamic dissociation constant HSO4−. makes it likely that accuracy can readily improved, recommendations work made. It is shown used calibration marine ‘total’ scale accurately predicted only slight modification original suggesting they contribute extension lower salinities.