TITRATION ALKALINITY OF SEAWATER

Abstract

The titration system is described that was used to measure the total alkalinity of seawater (TA) during the Joint Global Ocean Flux Study (JGOFS) sponsored by the National Oceanic and Atmospheric Administration (NOAA) in the equatorial Pacific. It consists of a piston titrator, a pH meter, and a glass thermostated cell. Since the new pH meters and titrators have RS232 interfaces the system can be easily connected to a personal computer. The computer programs used to carry out the titration and to determine TA, pH(sw) (pH on the seawater scale), and TCO2 from the full titration curve are described. A typical titration takes 20 min and consists of 25 points. Six separate titration cells were calibrated to be used on three systems at sea. The reliability of the electrodes was examined by titrations of 0.7 m NaCl with HCl at a pH near 3 and using seawater buffers at a pH near 8. Although most electrodes did not have Nernstian behavior over the entire pH range, all gave precise values of TA for a given solution. The individual cells were calibrated using standard Na2CO3 and seawater standards prepared in our laboratory and Certified Reference Material (CRM) provided by Dickson. The cells gave reliable values of TA, but the values of pH(sw) were low (0.02) and values of TCO2 were high (20 mu mol kg(-1)) due to the non-Nernstian behavior of the electrodes at a pH near 8.0. If the slope determined from the buffers is used, the titrations yield reliable values of TA, TCO2 and pH(sw). Measurements on Dickson standards with the three cells at sea indicate that the systems have a reproducibility of +/-2-4 mu mol kg(-1) in TA. The titration values of TCO2 determined on the CRMs and the samples collected at sea were about 17 +/- 6 mu mol kg(-1) (fall) and 20 +/- 6 mu mol kg(-1) (spring) too high. This offset in TCO2 is independent of depth and is due to the non-Nernstian behavior of the electrodes. The offset is not due to unknown protolytes.