An automated system for spectrophotometric seawater pH measurements

Spectrophotometric pH measurements stand to benefit greatly from the consistency and speed made possible through automation. Here we describe a simple, fast, and precise automated spectrophotometric pH measurement system for seawater samples. The system requires 4 min per analysis, consumes 60 mL seawater from a sample bottle, and requires little operator interaction to obtain repeatability comparable with the best results published with other techniques (+/- 0.0004). The system and the suggested sample handling methods are assessed using over 5000 at-sea measurements obtained during a hydrographic cruise in the Indian Ocean. We estimate the overall measurement uncertainty of the existing, pre-2011, body of at-sea spectrophotometric pH measurements-made using these methods or otherwise-to currently be in the range of 0.01 to 0.02 pH units. However, a new approach for using purified dyes at a range of temperatures and salinities (Liu et al. 2011) stands to greatly reduce this uncertainty for future spectrophotometric pH measurements: our assessment suggests that the overall uncertainty should improve to similar to 0.005 pH units if dye impurities and the indicator's temperature and salinity sensitivity are adequately addressed. Any such improvement in measurement accuracy may provide a basis from which to determine adjustments appropriate for the existing body of spectrophotometric pH measurements made using commercially available (and impure) dyes.