Comparison of spectrophotometric and electrochemical pH measurements for calculating freshwater pCO₂

Abstract

Inland waters have an important role in the global carbon cycle, contributing significantly to terrestrial carbon fluxes through downstream export and exchange of CO2 with the atmosphere. However, large uncertainties in freshwater inorganic carbon fluxes remain. One contributing factor is uncertainty in carbonate system calculations for estimating the partial pressure of CO2 (pCO2) from pH and alkalinity in freshwater systems. The uncertainty stems largely from inaccurate pH values caused by glass pH electrode measurements in low ionic strength systems. This study compares indicator‐based spectrophotometric and electrochemical pH measurements and their application for calculating freshwater pCO2. Our study found that, compared to a pCO2 reference method, pH electrode‐based estimates of pCO2 were overestimated by 230 ± 200 μatm (n = 54) where indicator‐based spectrophotometric pH estimates of pCO2 were 58 ± 33 μatm (n = 34) over the range of 100–1600 μatm. Furthermore, we found that when ionic strength was assumed to be zero, calculated pCO2 error was ~ 20% of the reference pCO2. A 19‐d field study using autonomous spectrophotometric pH and pCO2 sensors found an average error in calculated pCO2 of −70 ± 57 μatm (n = 1685). Although, our focus is on riverine CO2, these findings and subsequent conclusions apply to all freshwater systems. Spectrophotometric pH measurements will improve future freshwater pCO2 calculations and better quantify inland waters' role in the global carbon budget.

Total alkalinity measurement using an open‐source platform

“…Estimations of p CO 2 as a function of measured A T and pH have been used to constrain CO 2 flux models of lakes and rivers thanks to the great quantity of measurements of these two parameters produced over the preceding century (Cole et al 1994); however, insufficient attention has been given to the quality of these measurements until recently. Unpredictable biases in glass‐electrode pH measurements caused by liquid junction potentials have been demonstrated to bias carbon cycling observations especially in low‐ionic strength waters (Golub et al 2017), leading to the nascent yet growing use of spectrophotometric pH determination in inland waters (Young et al 2022). A similar advancement in technology is needed for lake and river alkalinity measurements, which are produced via a variety of methods with little published accounting for comparability, uncertainty, and bias.…”

An assessment of HgII to preserve carbonate system parameters in organic‐rich estuarine waters

“…Sulfonephthalein indicator dyes for spectrophotometric pH measurements are molecularly characterized using calibration buffers and eliminate bias caused by salinity‐induced variations in the liquid junction potentials of potentiometric pH systems. Spectrophotometric pH T measurements have since been directly compared to potentiometric methods by repeated analyses at low ionic strengths and are shown to be more accurate (French et al, 2002; Young et al, 2022). Notably, spectrophotometric pH T observations are used to calibrate glass pH electrodes and ion sensitive field effect transistor (ISFET) pH electrodes in seawater (Easley and Byrne, 2012) and estuarine waters (Martell‐Bonet and Byrne, 2020).…”

Underway pCO2 Surveys Unravel CO2 Invasion of Lake Superior From Seasonal Variability

“…Direct measurements of CO 2 flux across the air‐water interface are also made via floating chamber or eddy covariance methods (Podgrajsek et al., 2014). Calculation of one inorganic C parameter from two others remains fraught with uncertainty due to ongoing challenges associated with measurement and equilibrium calculations in low‐ionic strength waters (Liu et al., 2020; Minor & Brinkley, 2022; Young et al., 2022). Furthermore, constructing time series of discrete water chemistry measurements is time‐ and labor‐intensive and may not resolve inorganic C cycling in water bodies with high spatial or temporal variability (Schilder et al., 2013).…”