Oxygen is constantly present in dissolved form in surface waters. The content of dissolved oxygen in water characterizes the oxygen regime of the reservoir and is of great importance for assessing its ecological and sanitary status. Oxygen should be contained in water in sufficient quantities, providing conditions for the breathing of hydrobionts. It is also necessary for the self-cleaning of reservoirs, because Participates in the processes of oxidation of organic and other impurities, decomposition of dead organisms. The decrease in the concentration of the RK indicates a change in the biological processes in the reservoir, the contamination of the reservoir by biochemically intensely oxidizing substances (primarily organic ones). Consumption of oxygen is also due to chemical processes of oxidation of impurities contained in water, as well as the respiration of aquatic organisms.
The intake of oxygen into the body of water takes place by dissolving it in contact with air (absorption), and also as a result of photosynthesis by aquatic plants, i.e. As a result of physicochemical and biochemical processes. Oxygen also enters water bodies with rain and snow water. Therefore, there are many reasons that cause an increase or decrease in the concentration of dissolved oxygen in water.
Dissolved oxygen in water is in the form of hydrated O2 molecules. The content of the PK depends on the temperature, atmospheric pressure, the degree of water turbulence, the amount of precipitation, the mineralization of water, etc. At each temperature value, there is an equilibrium oxygen concentration, which can be determined from special reference tables compiled for normal atmospheric pressure. The degree of saturation of water with oxygen corresponding to the equilibrium concentration is assumed to be 100%. The solubility of oxygen increases with decreasing temperature and mineralization and with increasing atmospheric pressure.
In surface waters, the dissolved oxygen content can range from 0 to 14 mg / l and is subject to significant seasonal and diurnal fluctuations. In eutrophic and heavily polluted water bodies, there may be a significant oxygen deficit. Reducing the concentration of PK to 2 mg / l causes a mass death of fish and other hydrobionts.
In the water of reservoirs in any period of the year up to 12 noon, the concentration of PK should be at least 4 mg / l. The maximum permissible concentration of oxygen dissolved in water for fishery reservoirs is 6 mg / l (for valuable fish species) or 4 mg / l (for other breeds).
Dissolved oxygen is a very unstable component of the chemical composition of waters. When it is determined, sampling should be carried out carefully: avoid contact of water with air before fixing oxygen (binding it to an insoluble compound).
Controlling the oxygen content in water is an extremely important problem, in the solution of which practically all sectors of the national economy are interested, including ferrous and non-ferrous metallurgy, chemical industry, agriculture, medicine, biology, fish and food industry, environmental protection services. The content of PK is determined both in uncontaminated natural waters and in sewage after treatment. Waste water treatment processes are always accompanied by monitoring of oxygen content. The determination of PK is part of the analysis when determining another important water quality indicator - biochemical oxygen demand (BOD).