What is Dissolved Oxygen (D.O.)?

DEFINITION:

Oxygen meters are used to determine the quantity of dissolved oxygen in liquids. Two measurement scales are commonly utilized: parts per million (ppm); or percentage saturation (%), which is defined as the percentage of oxygen dissolved in 1 liter of water.

It is necessary to determine the quantity of dissolved oxygen present in water because it is an indicator of water quality. It is therefore important to monitor aquarium water where low concentrations indicate pollution.

One application of oxygen meters is in fish keeping inside small bodies of water such as aquariums. Here oxygen levels have to be continuosly monitored both to attain optimal conditions for spawning and to prevent high mortality caused by low levels.

PRINCIPLE OF OPERATION:

The conventional measurement system of D.O. consist of a meter and a Clark-type polarographic probe. The probe is the most important and delicate part of the system. The probe consist of a Silver (Ag) anode(+) wrapped with a platinum (Pt) wire that functions as the cathode(-). These are inserted into a protective cap filled with Potassium Chloride (KCl) electrolyte solution. The cap ends in a Teflon membrane, a gas permeable material that allows passage of oxygen present in the solution but not passage of the solution itself.

By applying a potential of 790mV, the oxygen present in the cell is reduced to Hydroxide ions (OH^-) at the cathode, and Silver Chloride (AgCl) is deposited at the anode. This reaction provokes a current flux with an intensity proportional to the quantity of oxygen present in the sample. The meter converts the current into the corresponding concentration of dissolved oxygen.

It should be noted that the solubility of oxygen is dependent on many factors including temperature, atmospheric pressure and salinity. Oxygen meters are available that automatically compensate for these variables and insure even greater accuracy.