Dissociation constants of carbonic acid in seawater as a function of salinity and temperature

Abstract

Potentiometric measurements of the stoichiometric constants on the seawater pH scale for the dissociation of carbonic acid in seawater ( K 1 ⁎ = [H +][HCO 3 −]/[CO 2] and K 2 ⁎ = [H +][CO 3 2−]/[HCO 3 −]) have been made as a function of salinity (1 to 50) and temperature (0 to 50 °C). The results have been fitted to the equations ( T/ K) p K i − p K i 0 = A i + B i / T + C i ln ⁢ T . The values of p K i 0 in pure water are taken from the early work of Harned and Davis (1943) and Harned and Scholes (1941) p K 1 0 = − 126.34048 + 6320.813 / T + 19.568224 ⁢ ln ⁢ T p K 2 0 = − 90.18333 + 5143.692 / T + 14.613358 ⁢ ln ⁢ T . The value of the adjustable parameters A i , B i and C i for p K 1 ⁎ are given by ( σ = 0.0054 and N = 466) A 1 = 13.4191 S 0.5 + 0.0331 ⁢ S − 5.33 ⁢ E − 05 S 2 B 1 = − 530.123 S 0.5 − 6.103 ⁢ S C 1 = − 2.06950 S 0.5 . For p K 2 ⁎ the parameters are given by ( σ = 0.011 and N = 458) A 2 = 21.0894 S 0.5 + 0.1248 ⁢ S − 3.687 ⁢ E − 04 S 2 B 2 = − 772.483 S 0.5 − 20.051 ⁢ S C 2 = − 3.3336 S 0.5 . The values of p K 1 ⁎ and p K 2 ⁎ determined in this study are in good agreement with the seawater (SW) measurements of Mehrbach et al. (1973) and Mojica-Prieto and Millero (2002) from S = 15 to 45 and 0 to 40 °C. The values of p K 1 ⁎ near S = 35 are also in reasonable agreement with the measurements in artificial seawater (ASW) of Goyet and Poisson (1989) and Roy et al. (1993) from 0 to 35 °C. The values of p K 2 ⁎ in real seawater, however, do not agree with the measurement made in artificial seawater at temperatures above 5 °C. Calculations of p K 1 ⁎ and p K 2 ⁎ near 25 °C using an ionic interaction model (Millero and Roy, 1997) suggest that the p K 2 ⁎ results in SW are more reliable than in ASW. The equations from this study should be valid from S = 0 to 50 and t = 0 to 50 °C for most estuarine and marine waters (check values at S = 35 and t = 25 °C are p K 1 ⁎ = 5.8401 and p K 2 ⁎ = 8.9636).