Proton-selective electrode for pH sensing

Cellulose-based carbon electrodes with pores of different widths (~ 2.6 and ~ 4 nm) have been fabricated by the pyrolysis of cellulose filter paper, followed by mild activation and surface treatment in nitric acid. Such electrodes were introduced into solutions with different pH values and their potential was measured vs. a reference electrode. Nernstian behavior between measured potentials and solution pH with a slope value of 57 mV per decade was only demonstrated for the carbon with the smaller pore size. This carbon also has proton-selective properties, unlike the carbons with wide pores. Based on the modified Donnan model of the electrical double-layer structure, it is hypothesized that the micropores of proton-selective carbons can hold a fixed amount of protons, regardless of the composition of the solution outside the micropores, when a Donnan potential between the carbon-electrode micropores and the external solution is established.