Spectrophotometric pH measurement in the ocean: Requirements, design, and testing of an autonomous charge‐coupled device detector system

Abstract

A newly designed system for high quality discrete spectrophotometric measurements of pHT using a low‐cost charge‐coupled device (CCD) detector is described. Considerations and requirements for the choice of spectrophotometers with a CCD detector instead of scanning spectrophotometers with photomultiplier detector are elucidated. The presented system is evaluated in the laboratory for system accuracy and short‐term precision and at sea for long‐term precision and at‐sea capability. Derived system characteristics are a (1s) short‐term precision of ±0.0012 pH units and a (1s) long‐term precision at sea of ±0.0032 pH units based on Certified Reference Materials (CRM). Such long‐term precision is equivalent to a deviation of ±1.1 to 2.2 µmol kg−1 in total dissolved inorganic carbon (TCO2) and ±1.4 to 2.1 µmol kg−1 in total alkalinity (TA), depending on temperature and the TCO2/TA ratio. Overdetermination of the CO2 system (TCO2, TA, pHT) from surface‐to‐deep water profiles support the accuracy and precision assessment in comparison to earlier data. With careful design and testing, low‐cost CCD spectrophotometers can be used for high accuracy pH measurements.