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 (= tot al alkalinity)] were made on the same sample of Gulf Stream seawater ( S = 35) as a function of temperature (5-35 degrees C) and the ratio of TA/TCO2 (X) (1.0-1.2). Overall the measurements were consistent to +/ -8 mu atm in f(CO2), +/-0.004 in pH, +/-3 mu mol kg(-1) in TCO2, and /-3 mu mol kg(-1) in TA with the thermodynamic constants of Goyet and Poisson (1989), Roy et al. (1993), and Millero (1995). Deviations betw een the measured pH, TCO2, TA and those calculated from various input combinations increase with increasing X when the same constants are us ed. This trend in the deviations indicates that the uncertainties in p K(2) become important with increasing X (surface waters), but are negl igible for samples with the lower X (deep waters). This trend is < 5 m u mol kg(-1) when the pK(2) values of Lee and Millero (1995) are used. The overall probable error of the calculated f(CO2) due to uncertaint ies in the accuracy of the parameters (pH, TCO2, TA, pK(0), pK(1), and pK(2)) is +/-1.2%, which is similar to the differences between the me asured values and those calculated using the thermodynamic constants o f Millero (1995). The calculated values of pK(1) (from f(CO2)-TCO2-TA) agree to within +/-0.004 compared to the results of Dickson and Mille ro (1987), Goyet and Poisson (1989), Roy et al. (1993), and Millero (1 995) over the same experimental conditions. The calculated values of p K(2) (from pH-TCO2-TA) are in good agreement (+/-0.004) with the resul ts of Lee and Millero (1995) and also in reasonable agreement (+/-0.00 8) with the results of Goyet and Poisson (1989), Roy et al. (1993), an d Millero (1995). The salinity dependence of our derived values of pK( 1) and pK(2) (S = 35) can be estimated using the equations determined by Millero (1995).