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

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.

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.

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).

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.