Procedures for Measurement of Carbonate Ion Concentrations in Seawater by Direct Spectrophotometric Observations of Pb(II) Complexation

The health of coral reefs and calcareous plankton is strongly influenced by the carbonate saturation state of seawater. Calculations of carbonate saturation states currently require measurements of two CO ₂ system parameters, such as pH and total dissolved carbon, plus thermodynamic calculations that relate carbonate ion concentrations to directly measured parameters. In this work we report novel procedures for direct measurements of carbonate ion concentrations and saturation states in seawater. Measurements are obtained via ultraviolet spectroscopic observations of Pb(II) spectra as the relative concentrations of PbCO ₃ ⁰ and an ensemble of lead chloride complexes vary in response to dissolved CO ₃ ²⁻ . Measurement precision is enhanced by parameterization in terms of absorbance ratios. The PbCO ₃ ⁰ stability constant, and Pb(II) molar absorbance ratios in seawater, were determined at 25 °C over a range of salinity between 36 and 20. The procedures described in this work are well suited to measurements throughout the normal range of carbonate ion concentrations in the oceans. Rapid equilibration rates for Pb(II) carbonate complexation make the procedures described in this work well suited to rapid direct analysis in situ.