This section contains information on the relationships of CO2 in solution, including the solubility of the gas, constants describing the association of carbonate species with protons, and the degree of isotopic fractionation between the various species.

Acid–base buffering

A complete evaluation of the acid–base buffer in seawater requires knowledge of the appropriate physical constants for all significant substances that react with protons. The dominant buffer in seawater is the carbon dioxide system, and the necessary constants for that system are included in this section.

Several different pH scales have been used in marine work, and the definitions and numerical values of most of the important constants are correspondingly affected. For many decades the common approach has been to use measurements with a glass electrode standardized with NBS buffers. Because of small uncertainties associated with the liquid junction potential of glass electrodes, and the greater ionic strength of seawater, this approach leaves some slight uncertainty in the final values. (It seems possible that the spectrophotometric method of measurement will come to be preferred for precise work.) Of the several scales suggested, the pHt scale is currently favored for the most precise work in the open ocean, calibrated with buffers made up to the ionic strength of (usually) seawater with S = 35%. There is no commercial source for the buffers needed to calibrate electrodes on the pHf, pHt, and pHsws scales, and these must be made over a range of salinities in the laboratory, a process requiring considerable care and chemical skill (Dickson et al. 2007). See Appendix F for further comments on pH issues. Extensive discussions of pH measurements in seawater are presented in Dickson et al. (2007) and in Riebesell et al. (2010).