Increasing the Accuracy of pH Measurements in Estuarine and Brackish Water: the Need for an Improved Pitzer Model for TRIS in Seawater

pH measurements in seawater on the total scale are calibrated against TRIS-seawater buffers. The current state of the art for definition of pH in these buffers uses Harned cell measurements. However, while these measurements have only been made in the salinity range 20 – 40, pH measurements in estuarine and brackish waters at lower salinities require the assignment of pH values to low salinity TRIS buffers. The assigned pH values of TRIS-seawater buffers at salinities less than 20 are currently based on (i) interpolation with measurements made in pure TRIS solutions at low ionic strength, and/or (ii) on measurements made in sodium chloride solutions. An additional problem is that as the salinity decreases, the composition of the buffer departs ever more from standard seawater, in order to maintain a minimum concentration of TRIS in the buffer. An alternative approach is to develop a Pitzer model for TRIS in seawater that is sufficiently accurate to reproduce both the Harned cell data in the salinity range 20 – 40, and the low ionic strength TRIS data. This approach would also allow the increasing proportion of TRIS at lower salinities to be treated effectively. In order to test this approach, the MIAMI model for the seawater electrolyte, together with published Pitzer parameters for TRIS, has been used to calculate the Harned cell potentials for TRIS-seawater buffers at 25°C in the salinity range 20 - 40. Comparison with experimental measurements shows significant and consistent offsets. We examine the extent to which this offset can be reduced by re-evaluation of TRIS parameters using updated data on the sodium chloride / water system, and identify priorities for new measurements that would yield increased accuracy of the Pitzer model for TRIS-seawater buffers.