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

Abstract

Harned cell pH T 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-H 2 O −1 ), and in the temperature range 278.15-318.15 K. Measurement uncertainties were assigned to the pH T values of the buffer solutions and ranged from 0.002 to 0.004 over the investigated salinity and temperature ranges. The pH T values were combined with previous results from literature covering salinities from 20 to 40. A model function expressing pH T 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 5-40, in particular, covering the low salinity range of brackish water for the first time. At salinities 5-20 and 35, the measured dependence of pH T on the TRIS/TRIS·H + molality enables extrapolation of quantities calibrated against the pH T 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.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…These thermodynamic parameters are characterized through careful laboratory experiments and are not as well defined for some of the less commonly sampled waters such as in sea ice regions and coastal waters. In the case of sea ice, where waters are routinely below 0°C, commonly used carbonate system constants do converge, but brackish coastal waters may require more work [142,143]. Coastal waters may also contain an unknown contribution of organic alkalinity that is not parameterized in the inorganic carbonate model [144,145].…”

Metrology of pH Measurements in Brackish Waters—Part 2: Experimental Characterization of Purified meta-Cresol Purple for Spectrophotometric pHT Measurements

“…, it accounts for the concentrations of both, the free hydrogen ions, H + , and hydrogen sulfate ions, expressed as the total sulfate concentration SO − 4 T divided by the dissociation constant of hydrogen sulfate K HSO − 4 (Müller et al, 2018b). Previously, the required seawater buffer solutions with pH values assigned on the total scale were only available at salinities ≥ 20 (DelValls and Dickson, 1998).…”

“…TRIS buffered ASW solutions were prepared at salinities 5, 10, 15, 20, and at 35 for consistency assessments with previous results, according to Müller et al (2018b). At each salinity, three buffer solutions contained equimolal TRIS/TRISH + molalities (b TRIS/TRIS•H + ) of 0.01, 0.025, 0.04 mol•kg-H 2 O −1 .…”

“…This allows the impact of the solution composition on the pH T of the buffers and on the dissociation behavior of the dye to be corrected by extrapolation of the determined pK 2 e 2 values to zero TRIS/TRISH + molality. The preparation and bottling of the buffer solutions was performed by the national metrological institute of Germany, Physikalisch-Technische Bundesanstalt (PTB), in parallel (same lab, day, stock solutions, and person, but separate weighings) to the preparation of identical buffer solutions for Harned cell pH T measurements (Müller et al, 2018b). Solutions prepared at PTB were stored in 500 mL glass bottles (Schott DURAN, GL 45 polypropylene screw caps).…”

“…1. Prior to the experiments presented here, Harned cell measurements of TRIS buffered artificial seawater (ASW) solutions were preformed to determine pH values on the total scale (pH T ) at salinities ranging from 5 to 20, temperatures from 278.15 to 318.15 K and equimolal TRIS/TRIS•H + molalities of 0.01, 0.025, and 0.04 mol•kg-H 2 O −1 (Müller et al, 2018b).…”