FY 2000

The role of pH_T measurements in marine CO₂-system characterizations

Byrne, R.H., S. McElligott, R.A. Feely, and F.J. Millero

Deep-Sea Res. Pt. I, 46, 1985–1997, doi: 10.1016/S0967-0637(99)00031-X (1999)

Spectrophotometric pH_T measurements can routinely be obtained with an imprecision on the order of ±0.0005 pH_T units. This level of imprecision is equivalent to an imprecision in total hydrogen ion concentration of ±0.1% (where pH_T = -log[H⁺]_T and [H⁺]_T [H⁺] + [HSO₄⁻]). At this level of precision, pH_T measurements provide an important tool in quality control assessments of other CO₂-system parameters (CO₂ fugacity, total inorganic carbon, total alkalinity). CO₂ fugacities and total alkalinities calculated using measured pH_T and total inorganic carbon, for the large data set considered in this work, have relative precisions on the order of 0.15 and 0.1%, respectively. The precision of total inorganic carbon calculated via pH_T and salinity-normalized-alkalinity is on the order of 0.02% or better. In this work, using the NOAA 1992 boreal autumn Equatorial Pacific (EqPac) dataset, it is shown that CO₂-system variables calculated via pH_T can be used to enhance both the precision and accuracy of directly measured parameters. Through the procedures described in this work significant problems were revealed in the initial version of the 1992 NOAA dataset, and the dataset was greatly improved. Additionally, in this work, we revisit CO₂-system thermodynamic consistency issues in view of changes in the pH_T values assigned to tris seawater buffers and consequential changes in the calibration of sulfonephthalein pH_T indicators. As the principal result of a +0.0047 increase in the pK of meta cresol purple, CO₂ fugacity calculations and measurements are in very good agreement for the NOAA 1992 boreal autumn EqPac dataset. We note, as well, that due to a reassignment of the titrant acid concentration used in the NOAA 1992 boreal autumn dataset, measured total alkalinities are in good agreement with total alkalinities calculated from total carbon and pH_T.