Nano-litre proton/hydrogen titration in a dual-plate platinum-platinum generator-collector electrode micro-trench

A generator-collector "nano-litre cavity" based on two opposed platinum-coated gold film electrodes with 19 μm gap and either 38 μm or 83 μm depth is employed for the titration of protons and/or hydrogen. Diffusional feedback currents between generator and collector electrode in a platinum-platinum dual-plate sensor configuration provide a sensitive analytical tool for cycleable redox systems, here, the proton/hydrogen redox system in the presence of oxygen. Experimental data are compared with the theoretical expression proposed by Hubbard and Peters (A.T. Hubbard, D.G. Peters, Critical Reviews in Analytical Chemistry, 1973, 3, 201-242) suggesting a dominating transport effect of "slow diffusers". Good agreement is observed even at relatively high proton concentrations where hydrogen bubble nucleation within the micro-trench starts to occur. Low concentration measurements are strongly affected by oxygen reduction locally consuming/depleting protons. Catalytic activity of the platinum deposit is crucial for maximising the current output.