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

Abstract

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.

Reconciliation of pH25 and pHinsitu acidification rates of the surface oceans: A simple conversion using only in situ temperature

“…The average standard error is just ± 0.00036 pH unit (or just ± 0.0001 pH unit when the KNOT data is excluded) (Table ). The slopes are consistent with DeGrandpre et al ()'s calculations of −0.015 pH unit °C −1 at the HOT and BATS stations and −0.016 pH unit °C −1 for the coastal water in the northeast Pacific Ocean, suggesting the ranges of the slopes (the a 1 values) of the surface world oceans are fairly narrow as will be shown and discussed later.…”

Measuring pH in the Arctic Ocean: Colorimetric method or SeaFET?

“…However, the mCP has only been characterised above 5°C by Liu et al (2011) and care should be taken when using equations outside their definition range. In this case, the extinction coefficients and dissociation constants (pKa') have been shown to be close to linear with temperature (DeGrandpre et al, 2014;Liu et al, 2011) and extending equations to lower temperatures may lead to negligible errors. This assumption is supported by comparison with equations from DeGrandpre et al (2014) defined for a temperature down to 0°C showing a discrepancy lower than 0.010 in pKa' at 0°C.…”

“…In this case, the extinction coefficients and dissociation constants (pKa') have been shown to be close to linear with temperature (DeGrandpre et al, 2014;Liu et al, 2011) and extending equations to lower temperatures may lead to negligible errors. This assumption is supported by comparison with equations from DeGrandpre et al (2014) defined for a temperature down to 0°C showing a discrepancy lower than 0.010 in pKa' at 0°C. During this investigation, the SSS remained within the range of the (Liu et al, 2011) mCP characterisation.…”

A simple colorimetric method for determining seawater alkalinity using bromophenol blue

“…Spectrophometric indicator-based methods, in theory, are simple, fast, and precise. Spectrophotometric pH typically has a precision of 0.004 2 0.001 pH units (Easley and Byrne 2012;DeGrandpre et al 2014); although the accuracy of spectrophotometric pH measurements can be adversely affected by impurities in the indicator dyes (Yao et al 2007;Lai et al 2016).…”