How the Electrode Method Reads Dissolved Oxygen

The electrode method determines dissolved oxygen by turning a chemical reduction into an electrical current. At its heart is a membrane-covered electrochemical cell: a gas-permeable membrane separates the water sample from an internal electrolyte, and two electrodes—a cathode and an anode—sit within that electrolyte. 

Oxygen molecules diffuse across the membrane in proportion to their partial pressure in the water, dissolve into the thin electrolyte film, and reach the cathode, where they are reduced. The resulting current is the analytical signal.

Two variants share this architecture but differ in how the reduction is driven. In the polarographic, or Clark-type, sensor, an external voltage is applied between the electrodes to hold the cathode at a potential where oxygen is readily reduced. In the galvanic sensor, the electrode materials themselves generate the driving potential, so no external polarising voltage is needed. In both cases, the current flowing through the cell is proportional to the rate at which oxygen arrives at the cathode, and therefore to the oxygen partial pressure in the sample.

That proportionality is the key to quantification. Because the membrane controls diffusion, the sensor behaves as a diffusion-limited device: oxygen consumption at the cathode is small relative to the flux through the membrane, so the current tracks the external oxygen level rather than depleting it. 

The instrument converts the current into a dissolved oxygen concentration using the temperature-dependent solubility of oxygen and, where relevant, compensation for salinity and atmospheric pressure. A built-in temperature sensor is essential, because both membrane permeability and oxygen solubility change with temperature; without compensation, the reading would drift with the water’s thermal state rather than its oxygen content.

Practical operation follows from the same mechanism. The sensor must be calibrated against a known oxygen condition—typically water-saturated air or a zero-oxygen solution—so that the current-to-concentration relationship is anchored. Because oxygen transport through the membrane is sensitive to flow, the electrode needs a defined stirring or flow regime; in still water, a stagnant boundary layer can depress the reading。

while in a flowing stream the response stabilises. Membrane fouling, electrolyte depletion and cathode surface contamination all alter the diffusion path or the reduction kinetics, so periodic cleaning, membrane replacement and electrolyte renewal are part of reliable long-term use.