The Role of PHT Measurements in Marine COsub2/sub-system Characterizations

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 ⁺ ]+[HSO ₄ ⁻ ]). At this level of precision, pH _T measurements provide an important tool in quality control assessments of other CO ₂ -system parameters (CO ₂ fugacity, total inorganic carbon, total alkalinity). CO ₂ 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 CO ₂ -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 CO ₂ -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, CO ₂ 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 .