Membrane electrodes, also known as p-ion electrodes, use membranes that selectively interact with free analyte ions, generating a potential difference across the membrane. The resulting membrane potential, known as the asymmetry potential, is not zero even when analyte concentrations on both sides of the membrane are equal. The membrane's response is typically not selective to a single analyte but proportional to the concentration of all ions in the sample solution capable of interacting at the membrane's active sites. The selectivity coefficient measures the relative response for the interferent over the analyte. A value less than 1.00 indicates good selectivity for the analyte. These electrodes are classified into glass, crystalline solid-state, liquid-based, and gas-sensing electrodes based on the membrane material used in the electrodes.

The glass electrodes have glass membranes with incorporated metal ions. When immersed in a solution, the membrane hydrates, forming negatively charged sites with sodium ions that serve as the counterions. Hydrogen ions from the solution diffuse into the membrane, displacing sodium ions and creating H⁺ selectivity at low pH levels. This selectivity is extended to other cations at high pH levels.

Solid-state electrodes use selectively permeable membranes composed of inorganic salt crystals to measure the cations or anions of the salt used selectively.

Liquid-membrane electrodes provide rapid and accurate assessments of ion concentrations. These electrodes comprise a hydrophobic membrane with selective organic complexing agents or ion exchangers that are selective to certain polyvalent cations or anions in the solution.

Gas-sensing electrodes determine dissolved gas concentrations using gas-permeable membranes that allow the target gas to diffuse into an internal electrolyte solution. The subsequent chemical reaction between the gas and the electrolyte generates a potential difference proportional to the gas concentration in the external solution.

Potentiometric biosensors, including enzyme electrodes, are formed by immobilizing biologically active species on the membrane electrode. The reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode.