The effects of pressure on pH of Tris buffer in synthetic seawater
Introduction
Open ocean pH is declining at rates between − 0.001 to − 0.002 pH yr− 1 as atmospheric CO2 increases and the surface ocean equilibrates with larger partial pressures of CO2 [Dore et al., 2009, Byrne et al., 2010, Bates et al., 2014]. Decreases in pH may have large impacts on ocean ecosystems, particularly on organisms that make calcium carbonate skeletons and shells [Doney et al., 2009, Barton et al., 2012, Bednaršek et al., 2012]. It is expected that rates of pH change will not be uniform [Feely et al., 2008, Wootton et al., 2008, Takeshita et al., 2015], and different regions will experience varying degrees of impact. The rate of decrease has been observed directly at a few long-term time series stations [Bates et al., 2014] using highly reproducible spectrophotometric pH measurements [Clayton and Byrne, 1993, Liu et al., 2011]. The expense and expertise required to sustain such observations prevent them from being scaled to large areas of the ocean. Alternatively, a monitoring system that utilizes autonomous chemical sensors may alleviate such problems and greatly improve our understanding of the spatial and temporal variability of the rate of pH decline in the ocean [Johnson et al., 2007]. However, in order to establish such a chemical sensor network, the development and implementation of stringent calibration protocols are necessary.
The assignment of proper pH values to a suitable buffer solution is essential in obtaining accurate pH measurements [Bates, 1973]. Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater (referred to as just Tris buffer hereafter) has been widely accepted as the primary pH standard for oceanographic pH measurements [Dickson, 1993, DelValls and Dickson, 1998], and has been used to calibrate potentiometric pH measurements [Millero et al., 1993, Martz et al., 2010], and to characterize indicator dyes used in spectrophotometric pH measurements [Clayton and Byrne, 1993, Liu et al., 2011]. Although the temperature and salinity dependence of the dissociation constant of Tris has been quantified at atmospheric pressure [DelValls and Dickson, 1998], its pressure dependence has yet to be determined in seawater media. As the number of in situ pH measurements at high pressures is expected to increase rapidly in the next decade due to improvements in robust pH sensor technology [Johnson et al., 2016] and development of the deep-sea Free Ocean Carbon Enrichment system [Barry et al., 2014], characterizing Tris buffer in seawater under high pressures will meet a crucial need in sensor calibration and traceability.
The pH of a buffer solution can be quantified by the Henderson-Hasselbach equation:
In equimolar buffer solutions, where the deprotonated and protonated forms of the buffer are at equal concentrations, pH is equivalent to the pKa. Therefore the change in pKa is equivalent to the change in pH of the solution. The effect of pressure on Ka can be expressed aswhere ΔV [cm3 mol− 1] is the difference in partial molal volume of the acid dissociation reaction, R is the universal gas constant (83.145 cm3 bar K− 1 mol− 1), T is temperature in Kelvin, and P is pressure in bar [Byrne and Laurie, 1999, Millero, 2001]. This can be rearranged to give the relationship between the dissociation constant at in situ gauge pressure (KaP) and at atmospheric pressure (Ka0):
Note that this equation does not include the partial molal compressibility term, as this only becomes significant at higher pressures than we investigated in this study [Millero, 2001]. P refers to gauge pressure hereafter. The ΔVTris has been measured at infinite dilution and in low ionic strength solutions (≤ 0.1 mol dm− 3 NaCl) using dilatometry [Katz and Miller, 1971, Kitamura and Itoh, 1987] and spectrophotometry [Neuman et al., 1973]. More recently, the ΔVTris in 0.725 mol kg− 1 NaCl was reported between 5 and 25 °C by measuring density and sound speed, and using Pitzer equations to interpret the results [Rodriguez et al., 2015]. These results can be extended to seawater media, as agreement of ΔV in artificial seawater media (salinity = 35) and 0.725 mol kg− 1 NaCl solutions have been shown for various weak acid-base species [Millero, 2001]. However, since ΔV is dependent on ionic strength and solution composition [Byrne and Laurie, 1999], ΔVtris should be quantified in seawater media to validate the extension of ΔVTris measured in NaCl solution to seawater.
Here, we report ΔVTris⁎ at 10 to 30 °C, where the * symbol refers to seawater media of salinity 35. A potentiometric cell consisting of an Ion Sensitive Field Effect Transistor (ISFET) pH sensor and a Chloride-Ion Selective Electrode (Cl-ISE) was utilized to quantify the effects of pressure on KTris up to 200 bar. Excellent agreement with ΔVTris in 0.725 mol kg− 1 NaCl solution was observed [Rodriguez et al., 2015], validating the extension of the reported ΔVTris in NaCl solution to seawater media. Sources of uncertainty for the reported values are explored.
Section snippets
Background and theory
The pressure dependence of KTris can be calculated by monitoring the pH of equimolar Tris buffer solutions over a range of pressures. The left hand side of Eq. (1) is equivalent to the difference in solution pH at 0 and experimental P (pH0 − pHP) for an equimolar solution, given that the concentration of H+ and OH− is negligible compared to the buffer concentration:
Therefore ΔVTris⁎ can be quantified from a linear regression between RTln(pH0 − pHP) versus gauge pressure. We
Materials and methods
Temperature and pressure cycles were carried out in a custom system capable of reproducing a T-P range of 0 to 40 °C, and 0 to 200 bar. The ISFET and Cl-ISE was placed into a pressure vessel consisting of a titanium housing with an inert PEEK insert as the wetted material [Johnson et al., 2016]. The T and P of the housing is controlled by a temperature bath (Thermo Scientific, RTE-7) and an ISCO 260D Syringe pump, respectively. The pressure chamber was placed in an air bath (controlled by the
Results and discussion
A repeatable f(P) was observed at all temperatures, and decreased by ~ 6 mV over 200 bar (Fig. 1). There was a small, but noticeable effect of temperature on f(P), and generally was lower at lower temperatures. However, this effect was very repeatable at all temperatures. Agreement between replicate assessments of f(P) at all temperatures performed roughly a month apart was better than 180 μV (maximum difference), and typically was better than 100 μV. This demonstrates the high reproducibility and
Conclusion
The ΔVTris⁎ was quantified between 10 and 30 °C by measuring the change in pH of the buffer solution between 1.01 and 200 bar. pH of the solution was measured using a modified high pressure tolerant ISFET pH sensor and a Cl-ISE as a reference electrode. A custom system was utilized to control experimental temperature and pressure. The pressure coefficient f(P) of the cell ISFET | Tris Buffer | Cl-ISE was quantified in aqueous HCl solution prior to measurements in a certified Tris buffer solution. Our
Acknowledgements
This work was funded by the National Oceanographic Partnership Program award N00014-10-1-0206 and by the David and Lucile Packard Foundation.
References (36)
- et al.
Best practices for autonomous measurement of seawater pH with the Honeywell Durafet
Methods Oceanogr.
(2014) - et al.
Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results
Deep Sea Res. Part I Oceanogr. Res. Pap.
(1993) - et al.
The pH of buffers based on 2-amino-2-hydroxymethyl-1,3-propanediol (“tris”) in synthetic sea water
Deep Sea Res. Part I Oceanogr. Res. Pap.
(1998) pH buffers for sea water media based on the total hydrogen ion concentration scale
Deep Sea Res. Part I Oceanogr. Res. Pap.
(1993)The effect of pressure on the solubility of minerals in water and seawater
Geochim. Cosmochim. Acta
(1982)- et al.
The use of buffers to measure the pH of seawater
Mar. Chem.
(1993) Measurement of pHT values of Tris buffers in artificial seawater at varying mole ratios of Tris: Tris·HCl
Mar. Chem.
(2014)- et al.
The partial molal volume and compressibility of Tris and Tris–HCl in water and 0.725 m NaCl as a function of temperature
Deep Sea Res. Part I Oceanogr. Res. Pap.
(2015) - et al.
Use of a free ocean CO2 enrichment (FOCE) system to evaluate the effects of ocean acidification on the foraging behavior of a deep-sea urchin
Environ. Sci. Technol.
(2014) - et al.
The Pacific oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: implications for near-term ocean acidification effects
Limnol. Oceanogr.
(2012)
Determination of pH Theory and Practice
A time-series view of changing surface ocean chemistry due to ocean uptake of anthropogenic CO2 and ocean acidification
Oceanography
Extensive dissolution of live pteropods in the Southern Ocean
Nat. Geosci.
ISFET, theory and practice
Influence of pressure on chemical equilbria in aqueous systems - with particular reference to seawater
Pure Appl. Chem.
Direct observations of basin-wide acidification of the North Pacific Ocean
Geophys. Res. Lett.
Ocean acidification: the other CO2 problem
Annu. Rev. Mar. Sci.
Physical and biogeochemical modulation of ocean acidification in the central North Pacific
Proc. Natl. Acad. Sci. U. S. A.
Cited by (9)
Assessment of pH dependent errors in spectrophotometric pH measurements of seawater
2020, Marine ChemistryCitation Excerpt :The Honeywell Ion Sensitive Field Effect Transistor (ISFET) pH sensor, originally developed for industrial applications (Sandifer and Voycheck, 1999), exhibits exceptional performance in seawater (Martz et al., 2010). Its precision and stability is sufficient to quantify thermodynamic constants, such as the pressure effects on the pKa of Tris buffer in artificial seawater (Takeshita et al., 2017). In particular, 100% Nernstian response of the ISFET to hydrogen ion activity was demonstrated over a pH range of 2–12 by directly comparing its response to the platinum Standard Hydrogen Electrode in a universal buffer solution with ionic strength similar to seawater (Takeshita et al., 2014).
Spectrophotometric determination of pH and carbonate ion concentrations in seawater: Choices, constraints and consequences
2019, Analytica Chimica ActaCitation Excerpt :Tris buffer or calculated pH values from the TA-DIC couple are commonly used for quality control of pH measurements. On the contrary, measurements with purified indicator can actually provide an indication of the quality of buffers [22,96,107–114]. Different pH indicators have been used for seawater analysis, but direct comparisons of analytical results for the same sample using different indicators are rare.
Assessment of the suitability of Durafet-based sensors for pH measurement in dynamic estuarine environments
2018, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Spurred on by the findings of Yao et al. (2007), the seawater pH community is collectively seeking to develop a high-precision spectrophotometric methodology using purified indicator dyes for pH measurement over the full temperature and salinity range of natural waters (DeGrandpre et al., 2014; Lai et al., 2016; Liu et al., 2011; Patsavas et al., 2013a, 2013b; Soli et al., 2013). Parallel work for pH measurements under near-zero temperatures (DeGrandpre et al., 2014; Loucaides et al., 2017; Papadimitriou et al., 2016; Rérolle et al., 2016), at high pressures (Hopkins et al., 2000; Rodriguez et al., 2015; Soli et al., 2013; Takeshita et al., 2016a), and below open-ocean salinities (French et al., 2002; Gabriel et al., 2005; Gallego-Urrea and Turner, 2017; Hammer et al., 2014; Lai et al., 2016; Mosley et al., 2004; Yao and Byrne, 2001) has also been completed. The availability of a new series of variable mole-ratio TRIS:TRIS-HCl buffers calibrated using a Harned Cell have produced buffers of similar composition to natural waters of S < 20 that possess the required buffering capacity needed for method development (Pratt, 2014).
A Novel Lab-on-Chip Spectrophotometric pH Sensor for AutonomousIn SituSeawater Measurements to 6000 m Depth on Stationary and Moving Observing Platforms
2021, Environmental Science and TechnologyAutonomous in situ calibration of ion-sensitive field effect transistor pH sensors
2021, Limnology and Oceanography: Methods