Choosing the Correct Internal Reference Redox Species for Overcoming Reference Electrode Drift in Voltammetric pH Measurements

Reference electrode (RE) drift is a common problem when electrodes are used for pH determination, especially over extended periods of time or in complex media. For voltammetric pH measurements, one method to mitigate against RE drift is to add a second pH insensitive redox species (the internal reference, IREF) and measure the difference in peak potential, <i>E</i> _diff, between the signal associated with the pH sensitive species, <i>E</i> _pH, and IREF, <i>E</i> _IREF. This work strategically explores how to choose the correct IREF species. For these studies, a quinone-functionalized boron doped diamond (BDD-Q) electrode is employed as the pH sensing electrode over the pH range of 4-9. To avoid errors in reporting of the real <i>E</i> _pH and <i>E</i> _IREF values, there must be a minimum separation between the two peaks. Moreover, the distance on the potential axis between a peak and that of the current response pertaining to water electrolysis must also be considered. For the BDD-Q pH electrode, an operable potential window for IREF is established and the IREF redox species, hexachloroiridate (IrCl₆ ^2-/3-), is determined to be most appropriate, showing an ∼0.08 pH error over the pH range of 4-9 and reducing to ∼0.02 pH error over the pH range of 6-8. The use of <i>E</i> _diff is further assessed via the voltammetric measurement of dissolved carbon dioxide (CO₂) in a Stow-Severinghaus arrangement over the partial pressure range of 30.4-152.0 mmHg. The <i>R</i> ² linearity of the calibration line (=0.998) is shown to be equivalent and in agreement with theory when plotting either <i>E</i> _diff or <i>E</i> _pH versus CO₂ partial pressure. This data bodes well for the use of <i>E</i> _diff as a measurement signal in Stow-Severinghaus dissolved CO₂ transcutaneous sensors, where continuous measurement of pH over several days is required.