Potentiometry: Membrane Electrodes

Membrane electrodes, or p-ion electrodes, use membranes with charge-carrying or other components that selectively interact with free analyte ions, generating a potential difference across the membrane.

Glass electrodes have glass membranes with incorporated metal ions that hydrate to form negatively charged sites surrounded by metal counterions.

Hydrogen ions displace singly charged metal ions from these areas with high selectivity at low to moderate pH, which decreases at high pH. 

Crystalline solid-state electrodes have inorganic salt crystal membranes that selectively respond to cations and anions of that salt.

Liquid-membrane electrodes feature hydrophobic membranes containing immiscible liquid organic complexing agents or ion exchangers selective to specific ions.

Gas-sensing electrodes have membranes permeable to the target gases, which diffuse into an internal electrolyte solution.

Potentiometric biosensors are membrane electrodes with immobilized biologically active species on the membrane that react with the analyte and measure product concentrations.