Experimental Design Considerations in Measuring pH for the Wendy Schmidt Ocean Health XPRIZE Competition to Improve pH-sensing Technologies

Ocean acidification is having widespread and varying effects on marine ecosystems. However, understanding these effects requires scientists to distinguish anthropogenic pH changes from natural variability, which in turn requires high-quality measurements over increased spatiotemporal scales. Glass pH electrodes are currently the most widespread pH-sensing technology used for seawater pH measurements; however, they tend to measure inconsistently as a consequence of a variety of random and systematic effects that are inherent in such measurements. Such effects introduce errors of unknown magnitude. Despite the recognized need for improved pH sensors for seawater pH measurement, spectrophotometric and ion-selective field-effect transistor approaches have become more prevalent only in the last 25 years but are cost-prohibitive for many potential users. The Wendy Schmidt Ocean Health XPRIZE competition addressed this technology gap by incentivizing research into improving the performance and affordability of pH sensors. The accuracy, precision, and stability of competitors’ sensors were evaluated in a series of laboratory and field experiments. We describe the experimental design of these evaluations as well as measures taken to understand spatiotemporal variability in the test tank pH values and to reduce uncertainties in the measurement of pH. The chief concerns were controlling temperature, ensuring homogenous pH across the testing area, and establishing appropriate quality control procedures. This competition provides a template for future sensor evaluations by demonstrating how multiple sensors can be evaluated simultaneously in laboratory, coastal, and deep sea applications.