An automated analyzer was developed to achieve fast, precise and accurate measurements of seawater total alkalinity (AT) based on single-point titration and spectrophotometric pH detection. The single-point titration was carried out in a circulating loop which allowed the titrant (hydrochloric acid and bromocresol green solution) and a seawater sample to mix at a constant volume ratio. The dissolved CO2 in the sample-titrant mixture was efficiently removed by an inline CO2-remover, which consists of a gas-permeable tubing (Teflon AF2400) submerged in a sodium hydroxide (NaOH) solution. The pH of the mixture was then measured with a custom-made spectrophotometric detection system. The analyzer was calibrated against multiple certified reference materials (CRMs) with different AT values. The analyzer features a sample throughput time of 6.5 minutes with high precision (±0.33-0.36 µmol kg-1, n=48) and accuracy (-0.33±0.99 µmol kg-1, n=10). Intercomparison with a traditional open-cell AT titrator showed overall good agreement of 0.88±2.03 µmol kg-1 (n=22). The analyzer achieved excellent stability without recalibration over 11 days, during which time 320 measurements were made with a total running time of over 40 hours. Due to its small size, low power consumption requirements, and its ability to be automated, the new analyzer can be adapted for underway and in situ measurements.
Li Q., Wang F., Wang Z. A., Yuan D., Dai M., Chen J., Dai J. & Hoering K., in press. An automated spectrophotometric analyzer for rapid single-point titration of seawater total alkalinity. Environmental Science & Technology. Article (subscription required).
