THE ROLE OF PH MEASUREMENTS IN MODERN OCEANIC CO2-SYSTEM CHARACTERIZATIONS - PRECISION AND THERMODYNAMIC CONSISTENCY

In May 1992, surface seawater samples were collected along an equatorial transit (130 to 100 degrees 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 (AT(spec)) are both precise (+/- 1.8 mu mol/kg) and accurate; the mean observed difference between A(Tspec) and A(T) obtained potentiometrically in this study (A(Tpot)) is 1 mu mol/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 mu mol/kg. The results of this study indicate that the present 25 degrees C mCP calibration, paired with the 25 degrees 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 mu mol/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) PHTspec 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)-PHTspec 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.