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Talk about the working principle of pH and dissolved oxygen analyzer

by:BOQU     2023-03-26
What optimization does the water quality analyzer bring to the living environment? With the rapid development of my country's economy in recent years, the industrial and domestic garbage in cities has increased a lot. At present, the main method of garbage disposal is sanitary landfill, and the landfill is open-air operation. In the event of rain and snow, rainwater and snowwater seep into the landfill area and produce landfill leachate. The leachate is a high-concentration organic wastewater with a wide range of concentration values. It contains hydrocarbons, nitrates, sulfates, and traces of heavy metal ions such as copper, cadmium, and lead. The bacterial index is very high. If it is not treated, it will be directly discharged into the water body. It will seriously pollute the local water environment. In order to protect the water environment, the monitoring of sewage discharge must be strengthened. The design of detection points and the quality of detection instruments (mainly water quality analyzers) play a vital role in water environment monitoring. This article discusses this aspect in conjunction with the design of municipal waste sewage treatment plants. Working principle of water quality analyzer 1. Working principle of pH meter The pH value of water depends on the amount of dissolved substances, so the pH value can sensitively indicate the change of water quality. The change of pH value has a great impact on the reproduction and survival of organisms, and also seriously affects the biochemical function of activated sludge, that is, the effect of treatment. The pH value of sewage is generally controlled between 6.5 and 7. Water is chemically neutral, and some water molecules spontaneously decompose according to the following formula: H2O=H++OH-, that is, decompose into hydrogen ions and hydroxide ions. In a neutral solution, the concentrations of hydrogen ions H+ and hydroxide ions OH- are both 10-7 mol/l, and the pH value is the negative number of the logarithm of the hydrogen ion concentration with base 10: pH=-log, so neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH value is less than 7, and the solution is acidic; otherwise, the solution is alkaline if there is an excess of hydroxide ions. The pH value is usually measured by the potentiometric method, usually using a constant potential reference electrode and a measuring electrode to form a galvanic cell. The electromotive force of the galvanic cell depends on the concentration of hydrogen ions and also on the pH of the solution. The sewage treatment plant uses the company's pH sensors and pH transmitters. As shown in the specific structure, there is a special glass probe sensitive to pH on the measuring electrode, which is made of special glass that can conduct electricity and penetrate hydrogen ions, and has the characteristics of high measurement accuracy and good anti-interference. When the glass probe comes into contact with hydrogen ions, an electric potential is generated. The potential is measured against a reference electrode with a silver wire suspended in a silver chloride solution. If the pH value is different, the corresponding potential is also different, which is converted into a standard 4-20mA output through the transmitter. 2. The working principle of the dissolved oxygen analyzer The oxygen content in the water can fully show the degree of self-purification of the water. For biological treatment plants using activated sludge, it is very important to know the oxygen content of aeration tanks and oxidation ditch. The increase of dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, so it can remove volatile substances and facilitate The naturally oxidized ions purify the sewage. There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic method measurement, and electrochemical method measurement. The amount of dissolved oxygen in water is generally measured by electrochemical method. The sewage treatment plant uses the company's dissolved oxygen sensor and dissolved oxygen transmitter. Oxygen can dissolve in water, and the solubility depends on the temperature, the total pressure of the water surface, the partial pressure and the salts dissolved in the water. The higher the atmospheric pressure, the greater the ability of water to dissolve oxygen. The relationship is determined by Henry's law and Dalton's law. Henry's law states that the solubility of a gas is directly proportional to its partial pressure. Take the company's oxygen measurement sensor as an example, the structure is shown. The electrode consists of a cathode (commonly made of gold and platinum), a counter electrode (silver) with current, and a reference electrode (silver) without current. The electrode is immersed in an electrolyte such as KCl, KOH, and the sensor is covered by a diaphragm. The electrodes and electrolyte are separated from the liquid being measured, thus protecting the sensor from the escape of the electrolyte and the intrusion of foreign substances that would cause contamination and poisoning. A polarizing voltage is applied between the counter electrode and the cathode. If the measuring cell is immersed in water with dissolved oxygen, the oxygen diffuses through the diaphragm and the oxygen molecules present at the cathode (excess of electrons) are reduced to hydroxide ions: O2+2H2O+4e-? 4OH-. An electrochemical equivalent of silver chloride is precipitated on the counter electrode (electron deficient): 4Ag+4Cl-? 4AgCl+4e-. For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons to form a current. The magnitude of the current is proportional to the oxygen partial pressure of the measured sewage. The signal is sent to the transformer together with the temperature signal measured by the thermal resistance on the sensor. The transmitter calculates the oxygen content in the water by using the relationship curve between the oxygen content stored in the sensor and the oxygen partial pressure and temperature, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathodic potential. The response time of the dissolved oxygen sensor is: 90% of the final measured value after 3 minutes, and 99% of the final measured value after 9 minutes; the low flow rate requirement is 0.5cm/s.
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