An automated system for spectrophotometric seawater pH measurements

B. R. Carter, J. A. Radich, H. L. Doyle, A. G. Dickson

Limnol. Oceanogr. Methods 11:16-27 (2013) | DOI: 10.4319/lom.2013.11.16

ABSTRACT: 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 ~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.