- Solubility depends on temperature, pressure and dissolved salt in water
Dissolved oxygen analyzer measurement principle The solubility of oxygen in water depends on the temperature, pressure and the dissolved salts in the water. The sensing part of the dissolved oxygen analyzer is composed of gold electrode (cathode), silver electrode (anode) and potassium hydroxide electrolyte. Oxygen diffuses through the membrane into the electrolyte and the gold electrode and silver electrode form a measurement loop. When a polarization voltage of 0.6 ~ 0.8V is applied to the electrode of the dissolved oxygen analyzer, oxygen diffuses through the membrane, the cathode releases electrons, and the anode accepts electrons to generate current. The entire reaction process is: anode Ag+Cl→AgCl+2e- cathode O2+2H2O+4e→4OH- According to Faraday’s law: the current flowing through the electrode of the dissolved oxygen analyzer is proportional to the oxygen partial pressure, and there is a linear relationship between the current and the oxygen concentration at the same temperature.
- Representation method
The expression method of dissolved oxygen content There are three different expression methods for dissolved oxygen content: oxygen partial pressure (mmHg); percent saturation (%); oxygen concentration (mg/L or 10-6). These three methods are essentially It’s no different.
(1) Partial pressure representation: Oxygen partial pressure representation is the most basic and essential representation. According to Henry’s law, P=(Po2+P H2O)×0.209, where P is the total pressure; Po2 is the partial pressure of oxygen (mmHg); P H2O is the partial pressure of water vapor; 0.209 is the oxygen content in the air.
(2) Percent saturation representation: Because aeration fermentation is very complicated, the partial pressure of oxygen cannot be calculated. In this case, the percentage saturation representation is the most appropriate. For example, if the dissolved oxygen is set at 100% during calibration and 0% at zero oxygen, the dissolved oxygen content during the reaction is the percentage of the calibration time.
(3) Oxygen concentration representation: According to Henry’s law, oxygen concentration is proportional to its partial pressure, namely: C=Po2×a, where C is oxygen concentration (mg/L); Po2 is oxygen partial pressure (mmHg); a is Solubility coefficient (mg/mmHg·L). The solubility coefficient a is not only related to temperature, but also related to the composition of the solution. For an aqueous solution with a constant temperature, a is a constant, and the oxygen concentration can be measured. The oxygen concentration expression method is not commonly used in the fermentation industry, but it is used to express the oxygen concentration in the processes of sewage treatment and drinking water.
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