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

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 (ΔVTris⁎) 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 ΔVTris⁎ were fitted to the equation ΔVTris⁎=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 0.9cm−3mol−1. Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200bar, and can be expressed as:∆pHTris=−4.528+0.04912tPln10RT.where T is temperature in Kelvin, R is the universal gas constant (83.145cm3barK−1mol−1), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately −0.02 at 200bar.