Electrochemical sensors, both polarographic and galvanic, consist of an anode and a cathode that are confined in electrolyte solution by an oxygen permeable membrane. Oxygen molecules that are dissolved in the sample diffuse through the membrane to the sensor at a rate proportional to the pressure difference across it. The oxygen molecules are then reduced at the cathode producing an electrical signal that travels from the cathode to the anode and then to the instrument. Since oxygen is rapidly reduced or consumed at the cathode, it can be assumed that the oxygen pressure under the membrane is zero. Therefore, the amount of oxygen diffusing through the membrane is proportional to the partial pressure of oxygen outside the membrane.

In a polarographic sensor, the cathode is gold and the anode is silver. The system is completed by a circuit in the instrument that applies a constant voltage of 0.8 volts to the probe, which polarizes the two electrodes, and a meter to read the dissolved oxygen response from the sensor.
The electrolyte held under the membrane allows the electrical signal to travel from the cathode to the anode. The polarographic sensor operates by detecting a change in this current caused by the variable pressure of oxygen while the potential is held constant at 0.8 V. The more oxygen passing through the membrane and being reduced at the cathode, the greater the electrical signal (current) read by the probe. As oxygen increases, the signal increases and, conversely, as oxygen decreases, the signal decreases. Chemically, this is described as the oxidation of the silver and reduction of oxygen at the gold cathode as follows:

Silver Anode Reaction: 4Ag + 4Cl-→4AgCl + 4e-
Gold Cathode Reaction: O2 + 2H2O +4e-→4OHO

Overall reaction: O2 + 2H2O + 4Ag + 4KCl→4AgCl + 4KOH