Oxygen saturation (environment)

The oxygen saturation in water is a relative measure of the dissolved amount of oxygen , based on the equilibrium concentration compared to air under standard conditions (1,013.25 hPa).

Dissolved oxygen can be measured in standardized units for solution concentrations , for example millimoles O ₂ per liter ( mmol / l ), milligrams O ₂ per liter (mg / l), milliliters O ₂ (under standard conditions ) per liter (ml / l) or parts per million (ppm mass) . As in the medical context, however, the percentage based on the equilibrium concentration of O ₂ that would arise at a given temperature and salt content of the water and the current oxygen partial pressure of the air can also be specified. Well-aerated water in free exchange with the ambient air therefore has an oxygen saturation of 100% by definition. The colder the water, the more O ₂ can be dissolved, the more salty the water or the lower the atmospheric pressure, the less. This results from the gas laws of physics.

Examples of 100% O ₂ saturation of fresh water under air at normal pressure:

• 0 ° C: 14.6 mg / l
• 10 ° C: 11.3 mg / l
• 20 ° C: 9.1 mg / l

Solubility tables (based on the water temperature) and corrections for different salt contents and pressures can be found on the USGS website, among others. Such tables, in which the O ₂ solution concentration is given in mg / l, are based on equations worked out in laboratory tests. Tables with relative details of the O ₂ solution concentration based on the variables temperature and salinity (as used by oceanographers ) are based on the equation by Weiss (1970) for normal pressure:

${\displaystyle \ln DO=A_{l}+A_{2}{100 \over T}+A_{3}\ln {T \over 100}+A_{4}{T \over 100}+S[B_{1}+B_{2}{T \over 100}+B_{3}({T \over 100})^{2}]}$

wherein , , , , , , , T = temperature in Kelvin , S = salt content in g / kg, DO = Dissolved Oxygen in ml / L. ${\displaystyle A_{l}=-173.4292}$ ${\displaystyle A_{2}=249.6339}$ ${\displaystyle A_{3}=143.3483}$ ${\displaystyle A_{4}=-21.8492}$ ${\displaystyle B_{l}=-0.033096}$ ${\displaystyle B_{2}=0.014259}$ ${\displaystyle B_{3}=-0.001700}$

Conditions with low saturations between 0 and 30% are often referred to as hypoxic. An O ₂ saturation of 0% is called anoxia. Most fish can not survive in water with an O ₂ saturation <30%. Intact seawater is 80–110% saturated, the oversaturation (values ​​over 100%) is caused by photosynthesis of the phytoplankton . Excessive oxygen saturation can also be harmful to organisms.

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

In the original, a Pt cathode serves as the working electrode, and an Ag anode covered with an AgCl layer is used as the reference electrode. Both electrodes are immersed in an electrolyte solution containing potassium chloride. The electrolyte space with the electrodes is covered by a gas-permeable membrane. Today polyethylene, tetrafluoroethylene, polyvinyl chloride, among others, are used as gas-permeable membranes. Membrane-covered electrodes have the advantage that the electrode processes take place in an optimized electrolyte and thus defined electrochemical conditions are present. 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 practically independent of the applied voltage. The current-voltage curve shows a plateau area here. In the range designated as the working point, the current is only dependent on the oxygen concentration in the solution. To maintain the oxygen-dependent concentration gradient, fresh measurement solution must always be brought to the membrane by stirring or continuous flow.

Electrode processes with alkaline electrolytes:

Anode: 4 Ag + 4 Cl ^-    ⇄ 4 AgCl + 4 e ^-

Cathode: O ₂   + 2 H ₂ O + 4 e ^-    ⇄ 4 OH ^-

Commercial Clark electrodes also use other metal combinations as electrodes, for example gold versus silver or, more recently, “self-polarizing” gold versus lead.

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

literature

• RF Weiss: The Solubility of Nitrogen, Oxygen and Argon in Water and Seawater. In: Deep-Sea Research. 17, 1970, ISSN  0146-6313 , pp. 721-735.

Individual evidence

1. ↑ USGS (PDF; 53 kB).
2. ^ LC Clark, R Wolf, D Granger, Z Taylor: Continuous recording of blood oxygen tensions by polarography . In: J Appl Physiol . 6, 1953, pp. 189-193. PMID 13096460
3. ^ JW Severinghaus, PB Astrup: History of blood gas analysis . IV. Leland Clark's oxygen electrode. In: J Clin Monit . 2, 1986, pp. 125-139. PMID 3519875