Determination of methane and other small hydrocarbons with a platinum-Nafion electrode by stripping voltammetry

Methane in the gas phase is determined via pre-adsorption on a dispersed platinum electrode backed by a solid polymer electrolyte (SPE) membrane (Nafion) which is in contact with 10M sulfuric acid. The adsorption process is strongly temperature dependent with an activation energy of 8.7kcalmol-1. Prior to the anodic stripping step the platinum electrode is flushed with nitrogen resulting in an acceptable selectivity against electroactive species which could otherwise interfere. During the anodic sweep the adsorbed methane is oxidized at the potential of the onset of the formation of the platinum oxide monolayer. Using a sensing electrode thermostatted at 348K, a linear calibration curve was obtained from 0.1 up to 5% CH4 in nitrogen, with a detection limit of 130ppm (S/N=3). The determination of ethane, propane and butane was also found possible by this scheme and the cross-sensitivity to carbon monoxide and hydrogen could be significantly reduced by means of suitable chemical adsorption filters.