Development Of Underway Measurement System For Seawater Total Alkalinity

Total alkalinity (TA) is the measure of buffering capacity and one of the four parameters which characterize the carbonate system of seawater. The TA data of high temporal and spatial resolution are greatly needed for the researches of ocean carbon cycle, ocean acidification and other marine biogeochemical processes. Underway instrument for TA measurement is the best way to obtained TA data of high spatial resolution. However, the overall performance of this kind of instruments reported in literatures is poor and thus have been scarcely applied. In this study, we developed an underway measurement system for seawater TA based on our previously developed automated TA analyzer, and tested its performance in lab and at sea. The obtained results are as follows:(1) By integrating our previously developed automated TA analyzer with a rapid thermostatic device and an in-line debubbler, optimizing the running conditions and modifying some parts, an underway measurement system for seawater TA was developed. The precision of the system during a continuous test of6days in lab was±1.5μmol/kg (n=7), and no significant drift was found. The thermostatic device could bring the sample temperature in the range of3.29～37.10℃to25℃in6.5min, which means the developed system can be applied in the monitoring of seawater with wide temperature range. The tests at sea showed that the system could run smoothly for many days, indicating its adaptability on board. The throughput time of the system was6.5min, which implying a high measuring frequency. However, the precipitation of BCG in the titrant and the loosing of the interface between pump tube and related tubing would lead to the increase of TA, showing significant drift of the system. Furthermore, a systematic error of15～19μmol/kg was found between the data obtained by our system at sea and that obtained in lab by analyzing the samples, which were collected at sea, with conventional method.(2) To solve the problem found in the previous tests, the system was modified by using new case of the automated analyzer, new thermostatic device for water sample and titrant, and the connection of pump tube. The modified system consists of only two parts, the automated analyzer and thermostatic devices for water sample and titrant. It is compact, light, portable and good in appearance. The precision of the system during a continuous test of4days in lab was±1.89μmol/kg (n=3), and no significant drift was found. The agreement of the system with the conventional method was found to be1.10±1.22μmol/kg (n=8), indicating a significant accuracy improvement after modification.In brief, the system developed in this study can be applied in the underway measurement of seawater TA with high spatial resolution. It is portable with high precision and accuracy and less calibration. More work is needed to evaluate its performance in the future.