The reliability of the thermodynamic constants for the dissociation of carbonic acid in seawater

Laboratory measurements of all four CO₂ parameters f_CO2 ( = fugacity of CO₂), pH, TCO₂ ( = total dissolved inorganic carbon), and TA ( = total alkalinity)] were made on the same sample of Gulf Stream seawater (S = 35) as a function of temperature (5–35 °C) and the ratio of TA/TCO₂ (X) (1.0–1.2). Overall the measurements were consistent to ±8 μ atm in f_CO2, ± 0.004 in pH, ± 3 μ mol kg⁻¹ in TCO₂, and ± 3 μ mol kg⁻¹ in TA with the thermodynamic constants of Goyet and Poisson (1989), Roy et al. (1993), and Millero (1995). Deviations between the measured pH, TCO₂, TA and those calculated from various input combinations increase with increasing X when the same constants are used. This trend in the deviations indicates that the uncertainties in pK₂ become important with increasing X (surface waters), but are negligible for samples with the lower X (deep waters). This trend is < 5 μ mol kg⁻¹ when the pK₂ values of Lee and Millero (1995) are used.The overall probable error of the calculated f_CO2 due to uncertainties in the accuracy of the parameters (pH, TCO₂, TA, pK₀, pk₁, and pK₂) is ± 1.2%, which is similar to the differences between the measured values and those calculated using the thermodynamic constants of Millero (1995).The calculated values of pK₁, (from f_CO2-TCO₂-TA) agree to within ± 0.004 compared to the results of Dickson and Millero (1987), Goyet and Poisson (1989), Roy et al. (1993), and Millero (1995) over the same experimental conditions. The calculated values of pK₂ (from pH-TCO₂-TA) are in good agreement (± 0.004) with the results of Lee and Millero (1995) and also in reasonable agreement (± 0.008) with the results of Goyet and Poisson (1989), Roy et al. (1993), and Millero (1995). The salinity dependence of our derived values of pK₁ and pK₂, (S = 35) can be estimated using the equations determined by Millero (1995).