Toward Unified pH of Saline Solutions

Abstract

Fluctuations of pH in coastal systems are generally surveyed through potentiometric pH measurements. A new concept of a unified pH scale was introduced with the great advantage of enabling comparability of absolute values, pHabs, pertaining to any medium. Using water as an anchor solvent, yielding pHabsH2O, enables referencing the pHabs values to the conventional aqueous pH scale. The current work aims at contributing to implement pHabsH2O to saline solutions. To this purpose, differential potentiometric measurements, with a salt bridge of ionic liquid [N2225][NTf2], were carried out aiming at overcoming problems related to residual liquid junction potentials that affect the quality of such measurements. The ability to measure pHabsH2O with acceptable uncertainty was evaluated using Tris-Tris·HCl standard buffer solutions prepared in a background matrix close to the characteristics of estuarine systems (salinity of 20) as well as with NaCl solutions with ionic strength between 0.005 and 0.8 mol kg−1. The present study shows that for high ionic strength solutions, such as seawater, challenges remain when addressing the assessment and quantification of ocean acidification in relation to climate change. Improvements are envisaged from the eventual selection of a more adequate ionic liquid.