The free proton concentration scale for seawater pH

Abstract

Due to the use of multiple pH scales in seawater, studies related to the pH of marine environments are unnecessarily complicated and prone to inaccuracy. It is desirable that a standard scale be adopted, and we suggest the free proton pH scale be used. To ensure that the free proton scale is accurate and consistent with primary pH measurements in seawater, we use the Pitzer ion-interaction model to re-evaluate the free scale pH and stoichiometric dissociation constants of HSO4− and H2CO3. The Pitzer model is validated over a salinity range of 5 to 45 and temperatures of 0 to 45°C using the measured mean activity coefficient of HCl in artificial seawater. The resulting standard potential and stoichiometric dissociation constants are compared to previous estimates. The validated model results are used to evaluate more accurate values for the pH of equi-molal (m=0.04molkg−1) amine buffers on the free proton concentration scale, which can be used to calibrate free scale pH measurements in natural waters.