75. A low-cost spectrophotometric system for automated and high-frequency measurements of seawater pH

Hugh L. Doyle (1)* and Christina M. McGraw (2)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, 7005, Australia
2 School of Science and Technology, University of New England, Armidale, New South Wales, 2351, Australia

Background
Automated spectrophotometric procedures allow rapid and precise seawater pH measurements. However, such systems are time-consuming to build and typically quite expensive (ca. $20,000). Here, we present an inexpensive (ca $4,500) and fully automated spectrophotometric pH system.

Methods
This system integrates an Ocean Optics spectrometer with a custom flow cell. Automated fluid handling (including sampling and dye mixing) is controlled through a series of diaphragm pumps. By using small-volume flow cells and miniature pumps, analysis time and volume is kept to a minimum (<2 minutes and ~3 mL, respectively). An intuitive user interface was designed to simplify the measurement and minimise operator error.

Findings
To help ensure accurate measurements, instrument-specific calibrations are performed on each device using purified meta-cresol purple dye and following the procedures of Liu et al. (2011, Environ. Sci. Technol., doi: 10.1021/es200665d) over a range of temperatures and salinities. This calibration was further tested using seawater reference materials from the Dickson Laboratory at Scripps Institution of Oceanography. Finally, the system was assessed using at-sea measurements obtained during a hydrographic cruise in the Southern Ocean (Earth-Ocean-Biosphere Interactions, RV Investigator 2016).

Conclusions
The low cost, rugged construction, and reliability of this device makes it ideally suited for ocean acidification studies where accurate, high-frequency measurements are needed.