Metrology for pH Measurements in Brackish Waters—Part 1: Extending Electrochemical pHT Measurements of TRIS Buffers to Salinities 5–20

Jens D. Müller,Andrew G. Dickson,Beatrice Sander,Frank Bastkowski,David R. Turner,Steffen Seitz,Gregor Rehder

doi:10.3389/fmars.2018.00176

Abstract

Harned cell pHT measurements were performed on 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) buffered artificial seawater solutions in the salinity range 5 20, at three equimolal buffer concentrations (0.01, 0.025, 0.04 mol·kg H2O 1), and in the temperature range 278.15 – 318.15 K. Measurement uncertainties were assigned to the pHT values of the buffer solutions and ranged from 0.002 to 0.004 over the investigated salinity and temperature ranges. The pHT values were combined with previous results from literature covering salinities from 20 to 40. A model function expressing pHT as a function of salinity, temperature and TRIS/TRIS·H+ molality was fitted to the combined data set. The results can be used to reliably calibrate pH instruments traceable to primary standards and over the salinity range 5 to 40, in particular, covering the low salinity range of brackish water for the first time. At salinities 5-20 and 35, the measured dependence of pHT on the TRIS/TRIS·H+ molality enables to extrapolate quantities calibrated against the pHT values, e.g. the dissociation constants of pH indicator dyes, to be extrapolated to zero TRIS molality. Extrapolated quantities then refer to pure synthetic seawater conditions and define a true hydrogen ion concentration scale in seawater media.

Highlights

Changes in seawater pH are linked to changes in the concentration of dissolved inorganic carbon and alkalinity
This study extends the characterization of TRIS buffer solutions by Harned cell measurements to brackish waters and provides a consistent pHT model for the salinity range 5–40
It is emphasized that minor assumptions and uncertainties remain in the pHT assignment and restrict the accuracy of all currently available TRIS buffer characterizations

Summary

Introduction

Changes in seawater pH are linked to changes in the concentration of dissolved inorganic carbon and alkalinity. Acidification trends cannot reliably be predicted from changes in pCO2 as is the case in open ocean waters with stable alkalinity levels (Doney et al., 2009), but require direct pH measurements. Brackish waters are typically characterized by high loads of dissolved organic matter that contribute significantly to alkalinity, which results in erroneous results if alkalinity is used as an input parameter for CO2 system calculations (Kulinski et al, 2014). Under such conditions, accurate and precise pH measurements are extremely valuable for a complete determination of the CO2 system. It was shown that this can in principle be achieved by spectrophotometric pH measurements, because the methods work reliably even in the presence of high amounts of dissolved organic matter (Müller et al, 2018)

Methods

Results

Discussion

Conclusion