An automated system for spectrophotometric seawater pH measurements

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.

Phytoplankton Blooms at Increasing Levels of Atmospheric Carbon Dioxide: Experimental Evidence for Negative Effects on Prymnesiophytes and Positive on Small Picoeukaryotes

“…For seawater pH determination, a similar spectrophotometric approach as described in Carter et al (2013) was adopted (see Supplement S-3.1 for details). Prior to pH measurements seawater samples were gently pressure-filtered (see Supplement S-3.2 for details), in a similar fashion as described in Bockmon and Dickson (2014).…”

Traceability of pH to the Mole

“…UV-Vis spectrophotometry, with m-cresol purple indicator, has gained support as an alternative to potentiometry with pH electrodes [17].…”

Technical Note: Maximising accuracy and minimising cost of a potentiometrically regulated ocean acidification simulation system

“…McGraw et al, 2010) provides a high degree of precision (±0.0004, Carter et al, 2013;Clayton and Byrne, 1993) compared to potentiometric techniques (±0.002-0.001, Dickson et al, 2007) and has been used to regulate OA simulation systems with minimal variation around target pH values (±0.02, McGraw et al, 2010). However, spectrophotometric pH regulation can prove extremely expensive, as these systems must be custom-designed (Wilcox-Freeburg et al, 2013).…”

“…Sources of error in our measurement of pH include the following: spectrophotometric measurement of buffer pH (±0.004, Carter et al, 2013); differences between buffer salinity and seawater salinity (< 0.005, Dickson et al, 2007); and the potentiometric measurement of seawater pH (±0.01-0.002, pH meter specifications).…”