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 Ki - p Ki0 = Ai + Bi / T + Ci 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 K10 = - 126.34048 + 6320.813 / T + 19.568224 ln T p K20 = - 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) A1 = 13.4191 S0.5 + 0.0331 S - 5.33 E - 05 S2 B1 = - 530.123 S0.5 - 6.103 S C1 = - 2.06950 S0.5 . For p K 2 ⁎ the parameters are given by (σ = 0.011 and N = 458) A2 = 21.0894 S0.5 + 0.1248 S - 3.687 E - 04 S2 B2 = - 772.483 S0.5 - 20.051 S C2 = - 3.3336 S0.5 . 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).- Publication:
-
Marine Chemistry
- Pub Date:
- 2006
- DOI:
- 10.1016/j.marchem.2005.12.001
- Bibcode:
- 2006MarCh.100...80M
- Keywords:
-
- Seawater;
- Carbonic acid;
- Dissociation constants;
- p K <SUB>1</SUB>;
- p K <SUB>2</SUB>;
- Titration;
- Modeling