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Dissolved oxygen probes provide a convenient approach to an essential direct measurement of molecular oxygen. The membrane isolates the electrode from the sample and the 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 the oxygen concentration or solubility at a specific condition is known.

Solubilisation 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).

Effect of temperature changes

At a constant pressure, the volume of a specific weight of air changes in direct ratio to the absolute temperature (K). The concentration of oxygen at the air-water interface increases by a decrease in the air temperature. The temperature of the water is also important because of two other factors:

•The amount of water vapour 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.

•After oxygen is dissolved in water, it has the same temperature as the water. Both the kinetic energies of the water molecules and the oxygen 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 sulphate, reduce the solubility of oxygen in water.

do_theory.1427798217.txt.gz · Last modified: 31/03/2015 10:36 by pieter