Combined Voltammetric Measurement of pH and Free Chlorine Speciation Using a Micro-Spot sp2 Bonded Carbon-Boron Doped Diamond Electrode.

This work demonstrates the use of an sp2-bonded carbon microspot boron doped diamond (BDD) electrode for voltammetric measurement of both pH and analyte concentration in a pH-dependent speciation process. In particular, the electrode was employed for the voltammetric detection of pH and hypochlorite (OCl-) in unbuffered, aerated solutions over the pH range 4-10. Knowledge of both pH and [OCl-] is essential for determination of free chlorine concentration. The whole surface of the microspot BDD electrode was found active toward the voltammetric oxidation of OCl-, with OCl- showing a characteristic response at +1.5 V vs SCE. In contrast, it was only the surface integrated quinones (Q) in sp2-bonded carbon regions of the BDD electrode that were responsible for the voltammetric pH signal. A Nernstian response for pH (gradient = 63 ± 1 mV pH-1) was determined from proton coupled electron transfer at the BDD-Q electrode, over the potential range -0.4-0.5 V vs SCE. By measuring both OCl- and pH voltammetrically, over the pH range 4-10, the OCl- oxidative current was found to correlate extremely well with the predicted pH-dependent [OCl-] speciation profile.