Oxygenation (environmental)

The oxygen saturation in water is a relative measure of the amount of dissolved oxygen, relative to the equilibrium concentration with respect to air at atmospheric pressure ( 1013 hPa).

Dissolved oxygen may be measured in standardized units for solution concentrations, for example, millimoles O2 per liter ( mmol / l), milligrams O2 per liter ( mg / L), milliliters O2 per liter ( ml / l) or parts per million (ppm). As in the medical context, but also the percentage can be specified relative to the equilibrium concentration of O2, which would be set at a given temperature and salinity of the water and the current partial pressure of oxygen of the air. Well aerated water in free exchange with the ambient air by definition therefore has an oxygen saturation of 100%. The colder the water, the more O2 can be solved, the saline or the lower the atmospheric pressure, the less. This results from the gas laws of physics.

Examples:

  • 0 ° C, atmospheric pressure, fresh water: 14.6 mg / l = 100 % saturation
  • 10 ° C, atmospheric pressure, fresh water: 11.3 mg / l = 100 % saturation
  • 20 ° C, atmospheric pressure, fresh water: 9.1 mg / l = 100 % saturation

Solubility tables (based on the water temperature) and corrections for different salinities and pressures found, inter alia on the USGS website. Such tables in which the O2 concentration of the solution are given in mg / l, based on elaborate laboratory tests equations. Tables covered with relative specification, the O2 concentration of the solution to the variables temperature and salinity ( as they are used by oceanographers ) based on the equation of Weiss ( 1970) for normal pressure:

States with low saturations between 0 and 30 % are often referred to as hypoxic. An O2 saturation of 0% means anoxia. Most fish can in water with an O2 saturation not survive <30 %. Intact sea water is saturated to 80-110 %, the supersaturation ( values ​​over 100%) is caused by the photosynthesis of phytoplankton. Even at high oxygen saturations can be harmful to organisms.

The oxygen content of a solution can be measured by an oxygen or Clark electrode. Clark et al. described in 1953 for the first time an amperometric method for the in vitro and in vivo determination of oxygen in blood. They used a cellophane -covered electrode assembly, which is still used today in various modified forms for the determination of oxygen in solution.

As the working electrode used in the original Pt - cathode as the reference electrode is an Ag anode, which is covered with a AgCl layer. Both electrodes are immersed in a potassium chloride-containing electrolyte solution. The electrolyte chamber with the electrodes being covered by a gas permeable membrane. Today are used as gas-permeable membranes of polyethylene, tetrafluoroethylene, polyvinyl chloride, among others. Membrane-covered electrodes have the advantage that the electrode processes take place in an optimized electrolyte and thus be defined electrochemical conditions. A constant DC voltage between 0.6 and 0.9 V is applied between the Pt electrode and the reference electrode. In this voltage range, the current is virtually independent of the applied voltage. The current-voltage curve shows a plateau region here. The flow is only dependent on the oxygen concentration in the solution is in the range known as the working point. To maintain the oxygen-dependent concentration gradients fresh solution should be brought to the membrane by stirring or continuous flow onto forever.

Electrode processes in alkaline electrolyte:

Anode: Ag 4 4 Cl ⇄ 4 AgCl 4 e -

Cathode: O2 2 H2O 4 e - ⇄ 4 OH

Commercial Clark electrode also use other combinations of metals as electrodes, for example, gold to silver, or more recently, " self-polarizing " gold against lead.

The oxygen saturation of the water is often used for the preliminary estimate of the water quality class.

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