Thermodynamic studies of the carbonate system in seawater

Abstract

Laboratory measurements of the components of the carbonate system (pH, fCO(2), TCO2 and TA) were made on Gulf Stream seawater and Certified Reference Material (CRM) as a function of salinity (33-37), temperature (0-40 degrees C) and the ratio TA/TCO2 (1.02 to 1.25). The pH (+/-0.002) was determined by spectrophotometry; the fCO(2) (+/-2 mu atm) with an infrared detector; the TCO2(+/- 2 mu mol kg(-1)) by coulometry; and the TA (+/-2 mu mol 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-TCO2). The measurements on CRMs at 25 degrees C were found to be consistent with the thermodynamic constants of Goyet and Poisson (1989) and Roy el al. (1993) to +/-0.005 in pH, +/-3 mu mol kg(-1) in TCO2 and +/-3 mu mol kg(-1) in TA. The measurements on Gulf Stream seawater from 0 to 40 degrees C were also found to be consistent in pH to +/-0.006, in TCO2 to +/-4 mu mol kg(-1) and in TA to +/-4 mu mol kg(-1) when these constants were used. The pH and fCO(2) were measured on seawater with a known TA and TCO2 from 5 to 35 degrees C. The results give slopes of (d In fCO(2) dr) x 10(2) = 4.26 +/- 0.03 and (dpH/dt) x 10(2) = 1.65 +/- 0.01, in agreement with the values calculated from the thermodynamic constants of Roy el al. (1993) and Goyet and Poisson (1989) over the same temperature range. The measured fugacities were internally consistent to +/-7 mu atm with the values calculated with these same constants and with pH-TA or pH-TCO2 as input parameters. All of these laboratory measurements indicate that the carbonic acid dissociation constants of Rov et 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 degrees C, salinity from 33 to 37 and the ratio TA/TCO2 from 1.02 to 1.25. By using the pK(1) determined from the equations of Roy et al. (1993), we have used our measurements to determine pK(2) and fitted the results by the equation (sigma = 0.0048): pK(2) = 70.8632 - 1386.5890/T - 10.0561 In(T) + 0.0573 (S - 35) - [15.4502/T](S - 35) which is valid for S = 33-37 and T = 273.15-331.15 K.