Abstract
Spectrophotometric pH measurements allow for an accurate quantification of acid-base equilibria in natural waters, provided that the physico-chemical properties of the indicator dye are well known. Here we present the first characterization of purified m-Cresol Purple (mCP) directly linked to a primary pH standard in the salinity range 5-20. Results were obtained from mCP absorption measurements in TRIS buffer solutions. The pHT of identical buffer solutions was previously determined by Harned cell measurements in a coordinated series of experiments. The contribution of the TRIS/HCl component to the ionic strength of the buffer solutions increases towards lower salinity: This was taken into account by extrapolating the determined pK2e2 to zero buffer concentration, thereby establishing access to a true hydrogen ion concentration scale for the first time. The results of this study were extended with previous determinations of pK2e2 at higher and lower salinity and a pK2e2 model was fitted to the combined data set. For future investigations that include measurements in the salinity range 5-20, pHT should be calculated according to this pK2e2 model, which can also be used without shortcomings for salinities 0-40 and temperatures from 278.15 – 308.15 K. It should be noted that conceptual limitations and methodical uncertainties are not yet adequately addressed for pHT determinations at very low ionic strength.
Highlights
PH is a master variable of seawater analysis
Spectrophotometric pH measurements rely on the addition of pH-sensitive indicator dyes, like m-Cresol Purple, to a water sample
The pK2e2 value determined at S = 20 and a TRIS/TRISH+ molality of 0.025 mol·kg-H2O−1 deviated from the value interpolated between the results at TRIS/TRISH+ molalities of 0.01 and 0.04 mol·kg-H2O−1 by more than three times the standard deviation of all measurements, and was removed for further analysis without knowing the source of error
Summary
PH is a master variable of seawater analysis It allows the tracking of numerous biogeochemical processes, including organic matter production and mineralization, and is the most direct measure for ocean acidification (Byrne et al, 2010; Byrne, 2014). Spectrophotometric pH measurements have proven to be the most precise and accurate method and are often considered to be a reference method (Liu et al, 2011; Byrne, 2014). Spectrophotometric pH Measurements in Brackish Waters that the method works reliably in the presence of high concentrations of dissolved organic matter and hydrogen sulfide and supports full CO2 system characterizations even under challenging conditions typical for brackish waters. In order to ensure comparability of pH measurement results, the determination of the dissociation constant should be traceable to a fully characterized primary pH standard, e.g., by Harned cell measurements (Buck et al, 2002; Dickson et al, 2016)
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