Heterogeneous membrane electrodes

A dry heterogeneous membrane cast from an oil in water emulsion for an electrochemical sensing electrode for direct exposure to a sample, the membrane comprising

• a hydrophobic compartment, the hydrophobic compartment including a gas permeable polymer and having a water vapor diffusion coefficient;
• a hydrophilic compartment, the hydrophilic compartment including a hydrophilic polymer and having an aqueous electrolyte diffusion coefficient;
• and
• a dry internal reagent reservoir, wherein the hydrophilic compartment includes the internal reagent reservoir;
• wherein the hydrophobic compartment is in excess by volume over the hydrophilic compartment for the water vapor diffusion coefficient of the hydrophobic compartment to be higher than the aqueous electrolyte diffusion coefficient of the hydrophilic compartment and
• wherein at least one of the gas permeable polymer and the hydrophilic polymer is at least partially cross-linked, and
• wherein the internal reagent reservoir contains at least a bicarbonate salt and a pH sensitive redox couple.