National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2000

The role of pHT 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 pHT measurements can routinely be obtained with an imprecision on the order of ±0.0005 pHT units. This level of imprecision is equivalent to an imprecision in total hydrogen ion concentration of ±0.1% (where pHT = -log[H+]T and [H+]T [H+] + [HSO4]). At this level of precision, pHT 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 pHT 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 pHT 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 pHT 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 pHT values assigned to tris seawater buffers and consequential changes in the calibration of sulfonephthalein pHT 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 pHT.

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