Determination of sea water alkalinity by direct equilibration with carbon dioxide

Shipboard measurements of total alkalinity are very demanding. Currently used procedures invariably involve painstaking measurements of combined titrant-acid and sea water volumes, as well as careful standardization and storage of acids. Most measurement procedures require frequent calibrations of pH electrodes and well-behaved potentiometric systems in electrically and mechanically noisy environments. We have recently observed that spectrophotometric measurements at sea using double-beam spectrophotometers and multiwavelength measurements provide highly precise and convenient determinations of sea water pH. Spectrophotometric pH measurements involving observations of absorbance ratios are inherently calibrated, thereby obviating periodic buffer standardizations. In this work we have combined spectrophotometric pH measurements with sea water pCO sub(2) equilibrations for the purpose of demonstrating the precision and remarkable convenience of CO sub(2) equilibration methods in determinations of sea water alkalinity. Our reported procedures increase the precision of previous CO sub(2) equilibrium alkalinity measurements by a factor of 10.