The role of pH measurements in modern oceanic CO 2-system characterizations: Precision and thermodynamic consistency

Abstract

In May 1992, surface seawater samples were collected along an equatorial transit (130 to 100°W) and analyzed for total hydrogen ion concentration (expressed as spectrophotometric pH T) total dissolved inorganic carbon (coulometric C T), and total alkalinity (potentiometric A T and spectrophotometric A T). This data set, which presents a striking view of the chemical signature of tropical instability waves, is also unusual in that its “overdetermination” of the CO 2-system in seawater includes both potentiometric A T data and the first spectrophotometric A T data collected at sea using a double-wavelength, one-step acid addition method. Our data set indicates that spectrophotometric A T measurements ( A Tspec) are both precise (±1.8 μmol/kg) and accurate; the mean observed difference between A Tspec and A T obtained potentiometrically in this study ( A Tpot) is 1 μmol/kg. As only two of the three measured parameters are required to characterize the CO 2-system in seawater, this analytical redundancy of our analyses ( C T, A T, pH) afforded an opportunity to evaluate, in terms of thermodynamic consistency, a recent calibration of m-cresol purple, a pH indicator. Using various carbonic acid dissociation constants, measured parameters were combined in pairs ( C T- A T, pH- C T, and pH- A T) to calculate a third parameter for comparison with the shipboard measurements. Depending on the selected set of carbonic acid dissociation constants, the average offset between directly measured and predicted values of A T and C T was as small as ±1 μmol/kg. The results of this study indicate that the present 25°C mCP calibration, paired with the 25°C combined dissociation constants of Hansson and Mehrbach, produces accurate predictions of A T and C T. Extensive replication of the shipboard spectrophotometric measurements made it possible to examine directly the sensitivity of derived parameters to variations or errors in input A T and pH. In accord with earlier theoretical treatments of this question, the results presented demonstrate that pH is imprecisely predicted from A T and C T, whereas imprecision in measured pH—at the level typical of spectrophotometric measurements (±0.0004)—contributes negligibly (±0.3 μmol/kg) to imprecision in derived A T and C T. In view of the high precision of pH Tspec measurements and both the precision and reliability of C T measurements (supported by the use of certified SIO reference materials) pH Tspec and C Tcoul constitute a particularly useful pair of parameters for shipboard study of the oceanic carbon dioxide system. The results indicate, as well, that the A T-pH Tspec pairing is of particular interest in ocean regions where simple A T vs salinity relationships are observed. In light of recent advances, the role of pH measurements in CO 2-system characterizations should be re-evaluated. Spectrophotometric measurements of pH have much to contribute in documenting the oceans' evolving response to anthropogenic C0 2.