Improvements in potentiometric determinations of the CO2 oceanic system using seawater sub-standards and CO2 reference materials

Abstract

During the Cither 3 cruise along WOCE section A14, where 107 full-water column stations were surveyed, pH and alkalinity were measured potentiometrically. Corrections carried out on the raw data set have allowed us to improve the data quality. The correction procedure was focused on monitoring: (i) calibrations, (ii) liquid junction potential and (iii) seawater sub-standards; CO2 reference material (CRM) has been used to check this procedure. Accuracy of pH15 measurements (NBS scale) and alkalinity was 0.004 and 1.4 μmol.kg-1, respectively. Certified total inorganic carbon on CRM, batch 24, is 1987.55 μmol.kg.-1. A value of 1987.8 ± 2.2 μmol.kg-1 was obtained following the correction procedure. This improvement was independently verified by means of the covariances between nitrate and pH, and between silicate and normalized alkalinity in North Atlantic Deep Water.