The structural principle of dissolved oxygen electrode and its use

Dissolved oxygen (DO) is an abbreviation for Dissolved Oxygen, which is a parameter that characterizes the concentration of oxygen in an aqueous solution. A dissolved oxygen electrode is a current-type electrode for determining oxygen dissolved in a liquid based on the polarographic principle.

1. Classification of dissolved oxygen electrodes
There are several methods for determining DO: such as chemical Winkler method, electrode method, mass spectrometer, etc. Here we mainly introduce the electrode method. The dissolved oxygen electrode was first invented by Clark (1956). It is a current-type electrode covered by a gas permeable film. DO electrodes can be divided into two categories: primary cells (Galvanic) and polargrafic.

2. Principle of DO electrode measurement
Primary battery type: generally consists of a noble metal such as platinum, gold or silver; the anode is composed of lead. PbCl2 or Pb(AcO)2 is formed in the presence of an electrolyte such as KCl or lead acetate, and the galvanic electrode does not require an applied voltage. The Polargrafic type electrode requires an additional polarization voltage of 0.6-0.8V. The cathode is generally composed of a noble metal such as platinum or gold; the anode is composed of silver. The polarographic electrode needs to be applied with a constant voltage, and the electrolyte participates in the reaction, so the electrolyte must be replenished at certain time intervals.

Polarographic and galvanic cell types Polarographic electrodes generally have a longer life but are more expensive. The output currents differ by an order of magnitude. The electrode response time is typically 90S. It seems to be difficult to determine Kla or transition phenomena. Some electrodes can respond to less than 30S. The surface of the polarographic electrode has a small cathode surface, generally having a diameter in the range of 1 to 50 μm, and a reduction current is formed at the nA level. Therefore, a special electronic amplification device is required. Generally, the requirements of the cathode material are very high, such as the purity of platinum or silver is above 99.999%.

The surface of the galvanic electrode requires a smooth surface, and its area is proportional to the reduction current. The general diameter is 5-10 mm. The reduction current is 5-25μA at 28°C, so it can be directly connected to a 5 or 10 mV automatic potentiometer via a potentiometer without a dedicated electronic amplifier. The anode material of the primary battery type is also required to be high, and the purity is above 99.999%. Generally, the anode is formed into a cylindrical shape, and the surface area thereof is required to be several times larger than the cathode area. This is easy to do with the polarographic type electrode, so that it can be made smaller. The anode of the primary battery type is much larger to meet this ratio requirement. DO electrode structure: generally composed of a cathode, an anode, an electrolyte, and a plastic film.
Electrolyte: Generally, the formulation of the electrolyte is considered confidential, and the business is not easy to disclose. The preparation of the electrolyte is very particular, and it needs to use ion-free water. Some contaminated ions will seriously affect the performance of the electrode. The pharmaceutical reagents used are required to be at least grade AR. The electrolyte is useful, KOH; KCl, Pb(AcO)2, and the like. Film: Generally, a copolymer of polytetrafluoroethylene (F4) or polytetrafluoroethylene-polyhexafluoropropylene is used, and polychloropropene, polyethylene, polypropylene, and the like have also been used. Its main performance is in line with the high temperature resistance of the DO electrode (>200 °C) and good ventilation performance. Its thickness is also very particular, the thinner the film, the higher the sensitivity, generally in the range of 0.01-0.05 mm. Membrane performance is very important for a good electrode response. The membrane is required to have a high permeability to oxygen and a low permeability to CO2. Electrode response Our simple analysis of electrode performance shows that the electrode response is related to the electrode constant, k: k = π2D/d2. D is the diffusion coefficient of the film, and d is the film thickness. The larger K, the faster the response. Of course, the structure of the electrode will greatly affect the performance of the electrode. Pressure compensation membranes The electrodes used in the tanks are generally equipped with a pressure compensation membrane, and the electrodes for small glass fermenters are usually vented. Pressure compensated membrane weight is required to cope with thermal expansion of the electrolyte during autoclaving. Generally, it is made of silica gel.

Working principle Oxygen in water must pass through the membrane to reach the surface of the cathode in order to be reduced by the electrode. Therefore, oxygen has to overcome some resistance in diffusing to the surface of the cathode, the most important of which is the resistance of the liquid film close to the film and the resistance of the film itself. For the galvanic electrode type, it is very important that the main resistance should fall on the film, that is, the resistance of the film is much larger than the resistance of the liquid film, so that the influence of the resistance change caused by the flow of the measured liquid on the oxygen diffusion can be minimized. . Therefore, it can be seen from the formula (1) that the oxygen measurement is substantially the measurement of the diffusion rate of oxygen. IS = N FA (Pm/dm)P0 (1) where IS is the output current, N is the number of electrons obtained by reduction of oxygen, F is the Faraday constant, A is the cathode surface area, the diffusion coefficient of the Pm plastic film, and dm is the film Thickness, P0 is the partial pressure of oxygen in the liquid to be tested. Based on this principle, when measuring the DO in the viscous fermentation broth, the galvanic electrode should use a thicker film as much as possible, so that the resistance of the liquid film can be changed, and the fluctuation of the output current is smaller. For polarographic electrodes, fluid motion has no effect on the output of the electrode.

Note In fact, the DO electrode measures not the dissolved oxygen concentration but the oxygen activity or the oxygen partial pressure. Air and oxygen-free nitrogen are typically used to calibrate 100% and zero. The true dissolved oxygen concentration in the liquid can be determined chemically.

3. Technical specifications of DO electrode (1) Stability: This means that when the measured DO is unchanged, the current output should remain unchanged for a long time, otherwise the electrode cannot be used. However, the drift of the electrode output is inevitable. In general, the standard deviation with time is allowed at SD = 0.1%/d. Of course, the smaller the SD, the better.

(2) Sterilization resistance: It is required to be able to withstand high pressure steam sterilization at 131 ° C for 1 h.

(3) Response time: refers to the speed at which the electrode output tracks the change of dissolved oxygen concentration, which is a measure of the sensitivity of the electrode, in response to the required time of 95% or 90%. Usually in 30 s ~ 2 min. For on-line measurement, it is required to have higher sensitivity. For the galvanic electrode, it is also feasible that the 90% response time is within 3 min. The measurement can repeatedly place the electrode in the oxygen-free water and air, and repeatedly measure the pure nitrogen and air in the water in the tank.

(4) Working life of the electrode: This means that the electrolyte can maintain the normal measurement time. Of course, the longer the better, generally at least 1 month, and the good electrode can reach more than half a year. As for the life of the electrode should not be less than 3-5 years.

(5) Residual current: refers to the output of the electrode in the absence or zero oxygen state in the liquid. Of course, the smaller the better, generally less than 1%. This can be placed in oxygen free water or measured by nitrogen.

(6) Linear range: This refers to the range of dissolved oxygen concentration proportional to the output of the electrode. Of course, the wider the better, the range of 0-50% pure oxygen is generally allowed.

4. There are several problems in the application of current-type electrodes.

(1) Unit of DO concentration: There are currently 3 units representing DO concentration.
The first type is Dissolved Oxygen Tension (DOT), expressed in atmospheric pressure or mmHg. The DOT in 100% air-saturated water is 0.2095×760 = 159 (mm Hg column). This representation is often used in medical units. The second method is the absolute concentration, expressed in mg O2 / L pure water or ppm. This method is mainly used in environmental protection units. The absolute concentration of dissolved oxygen in water can be measured by Winkler's chemical method, but the electrode method is not acceptable unless it is pure water. To this end, the fermentation industry uses only the third method, air saturation (%). This is because in an aqueous solution containing a solute, particularly a salt, the absolute oxygen concentration is lower than that of pure water, but it is substantially the same when measured by an oxygen electrode. It is also unrealistic to measure the DO in the fermentation broth by chemical method, because the redox substance in the fermentation liquid interferes with the measurement. Therefore, it is expressed by % air saturation. This can only be compared under similar conditions at the same temperature, tank pressure, and aeration. This method can reflect the physiological metabolic changes of bacteria and the effect on product synthesis. Therefore, in application, it is necessary to calibrate the electrodes before inoculation. The method is based on a certain temperature, tank pressure and aeration agitation to reduce the medium to 100% saturation of the air.
(2) DO positioning is generally performed after the medium is sterilized, and the DO electrode needs to be calibrated before fermentation. The method is to adjust the current output to 100 under stirring, aeration and culture temperature, and then inoculate after it is stabilized, and can not be moved after inoculation until the end of fermentation. It is generally not possible to perform calibration during fermentation. To investigate whether the DO electrode is working properly, it can be judged from the following phenomena. Suspend stirring or add sugar, replenish, refuel, and replenish water.
(3) Drift and membrane clogging are the main problems faced by DO electrodes in use. After sterilization, the electrode output is difficult to reproduce. Therefore, the electrodes need to be corrected frequently.
(4) Evaporation of organic solvents in electrolytes is a common problem, leading to an early decline in electrode performance. It also occurs during electrode storage.

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