This lecture covers the principles of ionic balances, oxidation-reduction reactions, and the functioning of electrochemical potentiometric sensors. It explains the Nernst law, electron exchange, and the role of liquid electrolytes and electrodes in sensor technology. The lecture also delves into the construction and operation of glass electrodes, the selection of membrane materials for specific ion detection, and the use of ISFETs in chemical sensing applications. Additionally, it discusses the measurement of oxygen concentration in combustion gases using solid electrolyte sensors and the constitution of lambda probes for air/fuel ratio optimization.