Thermodynamic studies of the carbonate system in seawater

Abstract

Laboratory measurements of the components of the carbonate system (pH, fCO 2, TCO 2 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/ TCO 2 (1.02 to 1.25). The pH (±0.002) was determined by spectrophotometry; the fCO 2 (±2 μatm) with an infrared detector; the TCO 2 (±2μol kg −1) by coulometry; and the TA (±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- TC0 2). The measurements on CRMs at 25°C were found to be consistent with the thermodynamic constants of Goyet and Poisson (1989) and Roy et al. (1993) to °0.005 in pH, °3 μmol kg −1 in TCO 2 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, in TCO 2 to ±4μmol kg −1 and in TA to ±4μmol kg −1 when these constants were used. The pH and fCO 2 were measured on seawater with a know TA and TCO 2 from 5 to 35°C. The results give slopes of (d In fCO 2 dt) × 10 2 = 4.26 ± 0.03 and (dpH/d t) × 10 2 = 1.65 ± 0.01, in agreement with the values calculated from the thermodynamic constants of Roy et 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- TCO 2 as input parameters. All of these laboratory measurements indicate that the carbonic acid dissociation constants of Roy et al. (1993) and Goyet and Poisson (1989) give the best description of the thermodynamics of the CO 2 system in seawater over temperature from 0 to 40°C, salinity from 33 to 37 and the ratio TA/ TCO 2 from 1.02 to 1.25. By using the p K 1 determined from the equations of Roy et al. (1993), we have used our measurements to determine p K 2 and fitted the results by the equation (σ = 0.0048): p K 2 = 70.8632 − 1386.5890/ T− 10.0561ln( T) + 0.0573 ( S − 35) − [15.4502 T]( S − 35) which is valid for S = 33–37 and T = 273.15–331.15 K.