FY 1995

The role of pH measurements in modern oceanic CO₂-system characterizations: Precision and thermodynamic consistency

Clayton, T.D., R.H. Byrne, J.A. Breland, R.A. Feely, F.J. Millero, D.M. Campbell, P.P. Murphy, and M.F. Lamb

Deep-Sea Res. II, 42(2–3), 411–429, doi: 10.1016/0967-0645(95)00028-O (1995)

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 CO₂-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 CO₂-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 dissocation 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 CO₂-system characterizations should be re-evaluated. Spectrophotometric measurements of pH have much to contribute in documenting the oceans' evolving response to anthropogenic CO₂.