The effects of pressure on pH of Tris buffer in synthetic seawater

Highlights

• Pressure dependence of equimolar Tris buffer pH in synthetic seawater was quantified to 200 bar.

• ΔV_Tris^⁎ was quantified between 10 and 30 °C in synthetic seawater.

• Results are in excellent agreement with reported values measured in 0.725 mol kg^− 1 NaCl solution.

Abstract

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (ΔV) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The ΔV of Tris in seawater media of salinity 35 (ΔV_Tris^⁎) was determined between 10 and 30 °C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined ΔV_Tris^⁎ were fitted to the equation ΔV_Tris^⁎ = 4.528 + 0.04912t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm^− 3 mol^− 1. Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as:

$$∆{\mathrm{pH}}_{\text{Tris}}=\frac{-\left(4.528+0.04912t\right)P}{ln\left(10\right)\mathit{RT}.}$$

where T is temperature in Kelvin, R is the universal gas constant (83.145 cm³ bar K^− 1 mol^− 1), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately − 0.02 at 200 bar.

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:

$$\mathrm{pH}=\mathrm{p}{K}_a+{\mathit{log}}_{10}\left(\frac{\left[A^{-}\right]}{\left[\mathit{HA}\right]}\right)$$

In equimolar buffer solutions, where the deprotonated and protonated forms of the buffer are at equal concentrations, pH is equivalent to the pK_a. Therefore the change in pK_a is equivalent to the change in pH of the solution. The effect of pressure on K_a can be expressed as

$$\mathit{RT}\left(\frac{\partial \mathit{ln}{K}_a}{\partial P}\right)=-∆V$$

where 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 (K_a^P) and at atmospheric pressure (K_a⁰):

$$log\left(\frac{K_a^{\mathrm{P}}}{K_a^0}\right)=\frac{-∆\mathit{VP}}{ln\left(10\right)\mathit{RT}}$$

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 ΔV_Tris 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 ΔV_Tris 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], ΔV_tris should be quantified in seawater media to validate the extension of ΔV_Tris measured in NaCl solution to seawater.

Here, we report ΔV_Tris^⁎ 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 K_Tris up to 200 bar. Excellent agreement with ΔV_Tris in 0.725 mol kg^− 1 NaCl solution was observed [Rodriguez et al., 2015], validating the extension of the reported ΔV_Tris in NaCl solution to seawater media. Sources of uncertainty for the reported values are explored.