Symmetrical pH Electrochemical Cell Coupled to Constant Potential Coulometry for Improved Sensitivity and Precision: Part 1. Fundamental Considerations.

pH is a major variable in complex aquatic ecosystems, influencing biological activity, metal speciation and more. The method to routinely measure pH is potentiometric measurements with a glass electrode connected to a reference Ag/AgCl element in contact with the sample through a liquid junction. However, it has been largely replaced by optical pH assays in the field of seawater pH measurements because much better precisions could be achieved. Glass electrodes also suffer from bias such as liquid junction potential changes and temperature influence on the inner solution pH that generate inaccuracies. Moreover, the Nernstian relationship between observed potential and pH results in limited sensitivity. To overcome these limitations, an alternative readout called constant potential coulometry is implemented in the measurement system for increased sensitivity. A symmetrical pH cell is proposed in which two identical glass electrodes, separated by an open 3 M KCl channel, are measured against each other. One of them is kept in a NIST buffer while the other is used to measure the pH of the sample or the calibrant solution. Glass electrodes are evaluated at different temperatures in NIST buffers versus a classical reference electrode and versus each other in a symmetrical flow cell. The latter features open junctions that should improve the repeatability of the liquid junction potential. The determined signal repeatability in a single sample is as low as 0.3 mpH with a precision for consecutive samples of 0.001 pH, which is drastically improved over routinely available pH probes.