How pH Meters and Dissolved Oxygen Analyzers Work
First, the 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, a constant potential reference electrode and a measuring electrode are used to form a galvanic cell. The electromotive force of the galvanic cell depends on the concentration of hydrogen ions and also depends on the pH of the solution. The plant uses pH sensors and pH transmitters. There is a special glass probe sensitive to pH on the measuring electrode, which is made of special glass that can conduct electricity and permeate 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. Working principle of dissolved oxygen analyzer
The oxygen content in water can fully show the degree of self-purification of 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.
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 proportional to its partial pressure.
Taking the oxygen measurement sensor as an example, 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 Among them, the sensor is covered by a diaphragm, which separates the electrodes and electrolyte from the liquid to be measured, thus protecting the sensor, preventing the electrolyte from escaping, and preventing the intrusion of foreign substances that 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 on 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.
