Principle of measuring method of dissolved oxygen meter
In the sewage treatment process, the pollutants are decomposed by the activated mud by increasing the oxygen content in the sewage to achieve the purpose of sewage purification. Measuring the oxygen content helps determine the best purification method and the most economical aeration tank configuration. The measurement data of oxygen content in the biological fermentation process can guide the process, such as judging the critical oxygen concentration of the fermentation process, the oxygen supply capacity of the fermenter, the activity of the bacteria and the growth amount of the bacteria, etc., and according to the fermentation time. Oxygen and aerobic changes to guide the feeding operation.
principle
Solubility depends on temperature, pressure and dissolved salts in water
I. Dissolved Oxygen Analyzer Measurement Principle The solubility of oxygen in water depends on temperature, pressure and dissolved salts in water. The sensing part of the dissolved oxygen analyzer is composed of a gold electrode (cathode) and a silver electrode (anode) and a potassium chloride or potassium hydroxide electrolyte. The oxygen diffuses through the membrane into the electrolyte to form a measurement loop with the gold electrode and the silver electrode. 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 receives electrons, generating an electric current. The whole 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 partial pressure of oxygen, and there is a linear relationship between the current and the oxygen concentration at the same temperature.
display method
Second, the representation of dissolved oxygen content There are three different ways to represent dissolved oxygen: oxygen partial pressure (mmHg); percent saturation (%); oxygen concentration (mg / L or 10-6), these three methods Essentially no different.
(1) Partial pressure representation: The 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 oxygen partial pressure (mmHg); P H2O is the water vapor partial pressure; 0.209 is the oxygen content in the air.
(2) Percent saturation representation: Since the aeration fermentation is very complicated, the oxygen partial pressure cannot be calculated. In this case, the expression of percent saturation is most suitable. For example, if the standard dissolved oxygen is set to 100% and 0% at zero oxygen, the dissolved oxygen content during the reaction is the percentage of the calibration.
(3) Oxygen concentration representation: According to Henry's law, the oxygen concentration is proportional to its partial pressure, ie: C=Po2 × a, where C is the oxygen concentration (mg/L); Po2 is the oxygen partial pressure (mmHg); a is the solubility coefficient (mg/mmHg·L). The solubility coefficient a is not only related to temperature but also to the composition of the solution. For a constant temperature aqueous solution, a is a constant, then the oxygen concentration can be measured. The oxygen concentration notation is not commonly used in the fermentation industry, but is expressed in terms of oxygen concentration in processes such as sewage treatment and drinking water.
Factors affecting dissolved oxygen measurement
Factors Affecting Dissolved Oxygen Measurement The solubility of oxygen depends on temperature, pressure, and dissolved salts in water. In addition, oxygen diffuses through the solution faster than through the membrane. If the flow rate is too slow, interference may occur.
1. Influence of temperature Due to temperature changes, the diffusion coefficient of the membrane and the solubility of oxygen will change, directly affecting the current output of the dissolved oxygen electrode, and the thermistor is often used to eliminate the influence of temperature. As the temperature rises, the diffusion coefficient increases and the solubility decreases. The effect of temperature on the solubility coefficient a can be estimated according to Henry's law, and the temperature versus membrane diffusion coefficient β can be estimated by Arrhenius's law.
(1) Solubility coefficient of oxygen: Since the solubility coefficient a is affected not only by temperature but also by the composition of the solution. The actual oxygen concentration of the different components may also be different at the same partial pressure of oxygen. According to Henry's law, the oxygen concentration is proportional to its partial pressure. For dilute solutions, the temperature change solubility coefficient a changes by about 2% / °C.
(2) Diffusion coefficient of membrane: According to Arrhenius's law, the relationship between solubility coefficient β and temperature T is: C=KPo2·exp(-β/T), where it is assumed that K and Po2 are constant, then β can be calculated. At 25 ° C, it was 2.3% / °C. When the solubility coefficient a is calculated, the diffusion coefficient of the membrane can be calculated by comparing the instrument indication with the analytical analysis value (the calculation process is omitted here), and the diffusion coefficient of the membrane is 1.5%/°C at 25 °C.
2. Effect of Atmospheric Pressure According to Henry's law, the solubility of a gas is proportional to its partial pressure. The partial pressure of oxygen is related to the altitude of the area. The difference between the plateau and the plain area can reach 20%. It must be compensated according to the local atmospheric pressure before use. Some instruments are equipped with a barometer inside, which can be automatically calibrated at the calibration time; some instruments are not equipped with a barometer, and the calibration should be set according to the data provided by the local weather station. If the data is incorrect, it will lead to a large measurement error.
3. The dissolved oxygen in the brine in the solution is significantly lower than the dissolved oxygen in the tap water. For accurate measurement, the effect of salt content on dissolved oxygen must be considered. At a constant temperature, the dissolved oxygen is reduced by about 1% for each 100 mg/L increase in salt content. If the salt used in the calibration of the meter is low, and the salt content of the actually measured solution is high, it will cause errors. In actual use, the salt content of the measured medium must be analyzed for accurate measurement and correct compensation.
4. The flow rate of the sample through the membrane is slower than the diffusion through the sample, and the electrode membrane must be in complete contact with the solution. For the flow-through detection method, oxygen in the solution diffuses into the flow cell, causing oxygen loss in the solution close to the membrane, causing diffusion interference and affecting the measurement. For accurate measurement, the flow rate of the solution flowing through the membrane should be increased to compensate for the oxygen lost by the diffusion, and the minimum flow rate of the sample is 0.3 m/s.
Attention problem
Fourth, the problem of attention For the dissolved oxygen analyzer, as long as the selection, setting, and maintenance are appropriate, generally can meet the measurement requirements of the process. The main problems with the poor use of the dissolved oxygen analyzer are: improper use and maintenance; abnormal temperature compensation due to internal leakage of the electrode; reduced electrode input impedance.
1. Daily maintenance The daily maintenance of the instrument mainly includes cleaning, verifying and regenerating the electrode regularly.
(1) The electrode should be cleaned once every 1~2 weeks. If there is contamination on the diaphragm, it will cause measurement error. Care should be taken when cleaning, taking care not to damage the diaphragm. Put the electrode in clean water and wash it. If the dirt cannot be washed away, carefully scrub with a soft cloth or cotton cloth.
(2) Zero and span should be re-calibrated from February to March.
(3) The regeneration of the electrode is performed approximately once a year. When the measurement range is not adjusted, it is necessary to regenerate the dissolved oxygen electrode. Electrode regeneration involves replacing the internal electrolyte, replacing the diaphragm, and cleaning the silver electrode. If the silver electrode is observed to be oxidized, it can be polished with fine sandpaper. (4) If the electrode is found to be leaking during use, the electrolyte must be replaced.
2. The calibration method of the meter calibration meter can generally be calibrated by standard liquid or on-site sampling.
(1) Standard solution calibration method: Standard solution calibration generally adopts two-point calibration, namely zero calibration and span calibration. The zero calibration solution can be used with 2% Na2SO3 solution. The range calibration solution can select 4M KCl solution (2mg/L) according to the meter measurement range; 50% methanol solution (21.9mg/L).
(2) On-site sampling calibration method (Winkler method): In actual use, the Winkler method is used to perform on-site calibration of the dissolved oxygen analyzer. There are two cases when using this method: the meter reading is M1 when sampling, the analysis value is A, and the meter reading is still M1. In this case, only the meter reading is equal to A; the meter reading is M1, the test analysis value is A, and the meter reading of the meter is changed to M2. At this time, the meter reading cannot be equal to A, but the meter reading should be adjusted to 1 MA × M2. 3. Problems in use should be noted Pay attention to the following problems during use: due to the high impedance of the dissolved oxygen electrode (about 20MΩ), the distance between the dissolved oxygen electrode and the converter is up to 50m; when the dissolved oxygen electrode is not in use, it should be in working condition, which can be connected to dissolved oxygen conversion. On the device. Electrodes that are placed or regenerated (replacement of electrolyte or membrane) should be placed in an oxygen-free environment for 1~2h before use. Because of temperature changes, the diffusion and oxygen solubility of the electrode film are greatly affected. Long time (about 10min), so that the temperature compensation resistance is balanced; the oxygen partial pressure is related to the altitude of the area, the meter must be compensated according to the local atmospheric pressure before use; when the salt content of the measurement solution is high, the meter calibration should be A solution of equivalent salt content is used; for flow-through measurement, the minimum flow rate through the electrode is required to be 0.3 m/s.
Detection method
Iodometric method
(GB7489-87) (Iodometric)
Iodometric method (equivalent to international standard ISO
5813-1983) is the standard method for determining dissolved oxygen in water. It is the earliest method for detecting dissolved oxygen by using chemical detection method and high measurement accuracy. The principle is to add manganese sulfate and alkaline potassium iodide to the water sample to form manganese hydroxide precipitate. At this time, the manganese hydroxide is extremely unstable and rapidly oxidizes with water to form manganese manganate:
4MnSO4+8NaOH = 4Mn(OH)2↓+4Na2SO4 (1)
2Mn(OH)2+O2 = 2H2MnO3↓ (2)
2H2MnO3+2Mn(OH)3 = 2MnMnO3↓+4H2O (3)
The concentrated sulphuric acid is added to react the dissolved dissolved oxygen (present in the form of MnMnO3) with the potassium iodide added in the solution to precipitate iodine:
4KI+2H2SO4 = 4HI+2K2SO4 (4)
2MnMnO3+4H2SO4+HI = 4MnSO4+2I2+6H2O (5)
The starch is used as an indicator, and the released iodine is titrated with sodium thiosulfate to calculate the dissolved oxygen content [3]. The chemical equation is:
2Na2S2O3+I2 = Na2S4O6+4NaI (6)
Let V be the amount of Na2S2O3 solution (mL), M be the concentration of Na2S2O3 (mol/L), a is the volume of water sample taken during the titration (mL), DO can be calculated as follows [2]:
DO(mol/L)= (7)
In the absence of interference, this method is applicable to a variety of water samples having a dissolved oxygen concentration greater than 0.2 mg/L and less than twice the saturation of oxygen (about 20 mg/L). When the water may contain nitrite, iron ions, free chlorine, it may interfere with the determination. In this case, the iodometric method should be used. The specific method is to add NaN3 solution when adding manganese sulfate and alkaline potassium iodide solution, or add alkaline potassium iodide-sodium azide solution to the water sample. When Fe3+ is high, add KF complex mask. our. The iodometric method is suitable for clean water such as source water and surface water. Iodometric method is a traditional method for measuring dissolved oxygen. The measurement accuracy is high and the accuracy is good. The measurement uncertainty is 0.19mg/L [4]. However, this method is a purification method, which takes a long time and is cumbersome to meet the requirements of online measurement [5]. At the same time, easily oxidized organic substances such as tannic acid, humic acid and lignin may interfere with the measurement. Oxidized sulfur compounds, such as sulfide thiourea, also interfere with the respiratory system that is prone to oxygen depletion. When such materials are contained, the electrochemical probe method [6] is preferred, including the current measurement method and conductivity measurement method to be described below.
Reagent used
4.1 Manganese sulfate solution Weigh 120g of manganese sulfate tetrahydrate (MnSO4.4H2O), or 96g of manganese sulfate monohydrate, dilute to 250ml with water, if not clear, filter.
4.2 Alkaline potassium iodide-sodium azide solution Dissolve 125 g of sodium hydroxide in 100 ml of water, dissolve 37.5 g of potassium iodide in 50 ml of water, and dissolve 2.5 g of sodium azide in water. Mix the three and dilute to 250ml with water.
4.3 Copper sulfate-sulfamic acid inhibitor dissolves 32 g of sulfamic acid in 475 g of water, dissolves 50 g of copper sulfate in 500 ml of water, mixes, adds 25 ml of glacial acetic acid, and mixes.
4.4 Potassium dichromate 0.025mol/l Pipette 0.25mol/l potassium dichromate 25.00ml into a 250ml volumetric flask and add water to the mark.
4.5 1+5 sulfuric acid
4.6 1% starch solution is consistent with cod starch,
4.7 Sodium thiosulfate solution Weigh 3.2g sodium thiosulfate dissolved in boiling cooling water, add 0.2g sodium carbonate, dilute to 1000ml with water, store in brown bottle, and calibrate with 0.025mol/l potassium dichromate before use. ,Methods as below
In a 250ml iodine flask, add 100ml water and 1g potassium iodide, add 10ml 0.025mol / l potassium dichromate solution, 5ml (1 + 5) sulfuric acid, cover the plug, shake, dark in the dark for 5min, with thiosulfuric acid Sodium titration to pale yellow, add 1ml starch, continue titration until the blue just fades, record the amount of V
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