To measure dissolved oxygen in the water, electrochemical sensors with membranes for determination of oxygen can be applied, both polarographic and galvanic sensors can be used.
A most popular type of sensor is the galvanic Mackereth electrode.
Construction & working of Mackereth cell:
The cathode is a perforated silver cylinder that surrounds a lead anode with a potassium bicarbonate aqueous electrolyte. The electrolyte is confined to a silicone rubber membrane that is permeable to oxygen but not water and ions that interfere.
At the cathode, the oxygen diffusing through the membrane is reduced to give a current proportional to the partial pressure of oxygen.
The lead anode is sacrificial and therefore electrodes must be restored in accordance with the actual design and the total amount of oxygen that has spread into the cell.
There are also several variations in the basic design of electrodes in water plants, rivers, lakes, sewage tanks to cope with oxygen determination