Thermodynamic studies of the carbonate system in seawater

Laboratory measurements of the components of the carbonate system (pH, fCO2, TCO2 and TA) were made on Gulf Stream seawater and Certified Reference Material (CRM) as a function of salinity (33–37), temperature (0–40°C) and the ratio TA/TCO2 (1.02 to 1.25). The pH (±0.002) was determined by spectrophotometry; the fCO2 (±2 μatm) with an infrared detector; the TCO2 (±2μol kg−1) by coulometry; and theTA (±2μmol kg−1) by potentiometric titrations. The results were used to examine the internal consistency of the thermodynamics of the carbonate system by calculation of various parameters (e.g.,TA) from two input parameters (e.g., pH-TC02). The measurements on CRMs at 25°C were found to be consistent with the thermodynamic constants of Goyet and Poisson (1989) and Royet al. (1993) to °0.005 in pH, °3 μmol kg−1 inTCO2 and ±3 μmol kg−1 in TA. The measurements on Gulf Stream seawater from 0 to 40°C were also found to be consistent in pH to ±0.006, inTCO2 to ±4μmol kg−1 and in TA to ±4μmol kg−1 when these constants were used. The pH andfCO2 were measured on seawater with a knowTA andTCO2 from 5 to 35°C. The results give slopes of (d InfCO2 dt) × 102 = 4.26 ± 0.03 and (dpH/dt) × 102 = 1.65 ± 0.01, in agreement with the values calculated from the thermodynamic constants of Royet al. (1993) and Goyet and Poisson (1989) over the same temperature range. The measured fugacities were internally consistent to ±7 μatm with the values calculated with these same constants and with pH-TA or pH-TCO2 as input parameters. these laboratory measurements indicate that the carbonic acid dissociation constants of Royet al. (1993) and Goyet and Poisson (1989) give the best description of the thermodynamics of the CO2 system in seawater over temperature from 0 to 40°C, salinity from 33 to 37 and the ratio TA/TCO2 from 1.02 to 1.25. By using the pK1 determined from the equations of Royet al. (1993), we have used our measurements to determine pK2 and fitted the results by the equation (σ = 0.0048): 70.8632 − 1386.5890/T− 10.0561ln(T) + 0.0573 (S − 35) − [15.4502T](S − 35) which is valid for S = 33–37 and T = 273.15–331.15 K.