DO Theory

Introduction

Dissolved oxygen probes provide a convenient approach to essentially direct measurement of molecular oxygen. The membrane isolates the electrode from the sample, and oxygen is detected as it diffuses across the membrane. The probe can be used to measure any system where oxygen is present. Calibration of the sensor is readily achieved if knowledge of oxygen concentration or solubility at a specific condition is known.

Solubilization of Oxygen from the atmosphere into water

The solubility of oxygen in water is achieved by physical solution which does not involve chemical interaction between the compounds. The solubility equilibrium is a function of the following factors:

* the concentration of oxygen in the gas phase at the water-atmosphere interface 
* the attractive molecular forces between water molecules and oxygen molecules 
* the kinetic energy of the water and oxygen molecules

Physical changes in temperature and pressure have a pronounced effect on the solubility factors.

Effect of Pressure Changes

The percent concentration of oxygen molecules in air is essentially constant in the atmosphere close to earth (approximately 21% by volume and 23% by weight). The actual number of oxygen molecules per unit volume of air depends upon the temperature and pressure of the air. Air is compressible, and, at a constant temperature, a specific weight of gas will change in volume in a reverse ratio to pressure. The practical effect is that the amount of oxygen at an interface of air and water decreases as the pressure of air is decreased, or since the percentage concentration of oxygen in the air remains constant, the actual concentration of oxygen at the air-water interface is directly proportional to atmospheric pressure (Henry's Law). At Denver, Colorado, (elevation approximately 5000') the concentration of oxygen at the interface is about 84% of that which would exist at sea level.

Effect of Temperature Changes

At a constant pressure, the volume of a specific weight of air changes in direct ratio to the absolute temperature (Kelvin Scale = Degrees Centigrade + 273; or Rankin Scale = Degrees Fahrenheit + 460). As air cools from 100^oF to 0^oF the volume decreases by 17.9%, and at 0^oF the concentration of oxygen per unit volume is 21.8% greater than it is at 100^oF. Thus, the concentration of oxygen at the air-water interface increased by a decrease in the air temperature.

The temperature of the water is also important because of two other factors:

* The amount of water vapor in the air at the air-water interface increases as the temperature of water increases, which results in a decrease in oxygen concentration at the interface. The decrease is about 6% as the temperature of water changes from 32^oF to 100^oF. 
* After oxygen is dissolved in water, it is the same temperature as the water. Both the kinetic energies of the water molecules and the O₂ molecules are directly proportional to absolute temperature. Higher kinetic energies tend to overcome the attractive molecular forces between the water and oxygen molecules and contribute to decreased solubility of oxygen at higher water temperatures.

Effect of Dissolved Materials

The presence of dissolved materials in water potentially can reduce the solubility of oxygen if the dissolved materials interact with water to decrease the attractive molecular forces between water and oxygen. For example, dissolved inorganic salts, such as sodium chloride, potassium chloride, or sodium sulfate, reduce the solubility of oxygen in water.

Theory

A polarographic molecular oxygen probe consists of two metals of different nobility which serve as electrodes. The more noble metal is the cathode. If a potential of about 0.5 volts is applied to the two electrodes (negatively to the cathode) and the electrodes are immersed in an electrolyte, molecules of oxygen dissolved in the electrolyte will diffuse to the surface of the cathode and pick up electrons which, in combination with water, will produce hydroxyl ions. At essentially the same time, hydroxyl ions will give up electrons at the anode and form an oxide. The resulting transfer of electrons establishes a current flow through an external circuit and may be displayed on a millivolt or microampere meter.

The membraned Oxygen electrode offers the following advantages:

1. The membrane encloses the two electrodes in a captured volume of electrolyte, ensuring constant electrolyte strength and purity so that ions which might otherwise "poison" the probe are not present.

2. The membrane excludes materials that do not diffuse through it. As a result, most materials in the sample that might "poison" the cathode, or might cause an erroneous output from the electrode system are excluded. Potential interferences are limited to reactive gases which diffuse through the membrane, such as chlorine.