Seawater pH measurements in the field: A DIY photometer with 0.01 unit pH accuracy

Abstract A portable light-emitting-diode (LED) photometer has been developed to provide low-cost seawater pH measurements. The benefits of the new system include a simple “do-it-yourself” construction design, a hundredfold reduction in cost relative to benchtop spectrophotometric systems, routine calibration-free operation in the field, and precision and accuracy well suited to applications such as education, coastal zone monitoring (including citizen science programs), and aquaculture and aquarium management. The photometer uses a high-sensitivity light-to-voltage integrated circuit as a detector, two LED light sources, and an open-source Arduino microcontroller for system control and data processing. Measurements are based on observations of absorbances of a pH-sensitive indicator. With meta-cresol purple, a sulfonephthalein indicator appropriate to natural seawater, the photometer system produces pH T measurements within 0.01 units of state-of-the-art spectrophotometric measurements (7.6 ≤ pH ≤ 8.2, 30 ≤  S  ≤ 36.2, and 15 °C ≤  t  ≤ 30 °C) and has a pH precision of ± 0.002. Measurement accuracy is achieved with a one-time calibration that relates absorbance ratios measured by the broadband photometer ( R B ) to absorbance ratios measured by a high-quality (narrowband) spectrophotometer ( R N ). Calculation of R N from R B allows the use of published algorithms that yield seawater pH as a function of R N , temperature, and salinity.