Marine Science Faculty Publications

The Role of pH Measurements in Modern Oceanic CO2-system Characterizations: Precision and Thermodynamic Consistency

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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 pHT) total dissolved inorganic carbon (coulometric CT), and total alkalinity (potentiometric AT and spectrophotometric AT). 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 AT data and the first spectrophotometric AT data collected at sea using a double-wavelength, one-step acid addition method. Our data set indicates that spectrophotometric AT measurements (ATspec) are both precise (±1.8 μmol/kg) and accurate; the mean observed difference between ATspec and AT obtained potentiometrically in this study (ATpot) 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 (CT, AT, 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 (CT-AT, pH-CT, and pH-AT) 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 AT and CT 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 AT and CT.

Extensive replication of the shipboard spectrophotometric measurements made it possible to examine directly the sensitivity of derived parameters to variations or errors in input AT and pH. In accord with earlier theoretical treatments of this question, the results presented demonstrate that pH is imprecisely predicted from AT and CT, whereas imprecision in measured pH—at the level typical of spectrophotometric measurements (±0.0004)—contributes negligibly (±0.3 μmol/kg) to imprecision in derived AT and CT.

In view of the high precision of pHTspec measurements and both the precision and reliability of CT measurements (supported by the use of certified SIO reference materials) pHTspec and CTcoul constitute a particularly useful pair of parameters for shipboard study of the oceanic carbon dioxide system. The results indicate, as well, that the AT-pHTspec pairing is of particular interest in ocean regions where simple AT 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 C02.

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Deep Sea Research Part II: Topical Studies in Oceanography, v. 42, issue 2-3, p. 411-429