How Electrochemical Sensors Measure Dissolved Oxygen in Water

Electrochemical dissolved oxygen measurement relies on a simple principle: oxygen that diffuses into a sensor is reduced at a cathode, and the resulting current is proportional to the oxygen flux. The challenge is to make that current depend on the oxygen concentration in the water rather than on stirring, temperature, or electrode history.

In a typical Clark-type polarographic sensor, a gas-permeable membrane separates the internal electrolyte from the sample. Oxygen from the water dissolves in the membrane, diffuses through it, and enters a thin electrolyte film covering the cathode. A fixed polarizing voltage is applied between the cathode and the anode. At the cathode, oxygen is reduced, in alkaline electrolyte, according to O₂ + 2H₂O + 4e⁻ → 4OH⁻. At a silver anode, silver is oxidized and precipitates as silver chloride in a chloride-containing electrolyte. The electron flow between the electrodes constitutes the sensor current.

The key to quantitative measurement is the diffusion-limited regime. When the applied voltage is sufficiently negative, essentially every oxygen molecule reaching the cathode is reduced immediately, so the current is controlled not by reaction kinetics but by the rate at which oxygen arrives. 

Because the membrane imposes a fixed diffusion resistance, this arrival rate is proportional to the oxygen partial pressure across the membrane. Henry's law then links that partial pressure to the dissolved oxygen concentration in the water at a given temperature. In this regime the current is linear with oxygen concentration, which is why a two-point calibration—zero oxygen and air-saturated water—is usually sufficient.

Galvanic sensors work on the same electrochemical principle but dispense with the external polarizing voltage. The cell itself generates a potential difference between a noble cathode and a base-metal anode, and the diffusion-limited current is again proportional to oxygen flux. Galvanic cells are convenient for field use because they need no polarizing circuit, but their output can drift as the anode is consumed.

Several practical factors shape the measurement. Temperature affects membrane permeability, electrolyte conductivity, and reaction kinetics, so all electrochemical DO sensors require temperature compensation, usually from an integrated thermistor. Flow dependence arises because a stagnant water layer outside the membrane adds an uncontrolled diffusion resistance; stirring or a flowing sample minimizes this boundary layer, and modern membrane sensors are designed to reduce—though not eliminate—this sensitivity. 

Fouling of the membrane by biofilm, oil, or suspended solids lowers the apparent reading and is a common cause of drift. Membrane rupture, electrolyte depletion, and anode passivation are other failure modes. Hydrogen sulphide, sulphur dioxide, and chlorine can permeate the membrane and interfere electrochemically, so their presence must be considered when interpreting readings. Salinity also reduces oxygen solubility and is often corrected using a conductivity-derived factor.