Considerations for the measurement of spectrophotometric pH for ocean acidification and other studies

Indicator-based spectrophotometric pH is commonly used for the analysis of seawater because of its high precision and long-term reproducibility. Users come from an increasingly diverse range of disciplines, primarily motivated by studies focused on the causes and effects of ocean acidification. While the analysis is readily implemented and straightforward, there are many variables that must be predetermined or measured, all of which can contribute uncertainty to the measurement. The indicator equilibrium constant and molar absorption coefficient ratios are available in the literature, but for various reasons, the conditions of analysis can be different, creating errors. Most of the parameters are temperature, salinity, and pressure dependent, posing potential additional errors. Indicator impurities and indicator perturbation of the sample pH also create uncertainties. We systematically evaluate all of the sources of error and compute how the errors propagate into CO₂ equilibrium calculations of the partial pressure of CO₂ (pCO₂) and calcium carbonate saturation states (Ω). The primary sources of uncertainty originate from wavelength and absorbance errors in low quality or poorly functioning spectrophotometers (0.007 to 0.020 pH units) and indicator impurities (0.000 to >0.040 pH units). These errors generate pCO₂ and Ω uncertainties of 11-200 μatm and 0.08-0.38, respectively, depending upon the pH value and its uncertainty.