Operation of pH meters and dissolved oxygen analyzers
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 changes in water quality. Changes in pH value have a great impact on the reproduction and survival of organisms. At the same time, they also seriously affect the biochemistry of activated sludge, which affects the treatment effect. The pH value of sewage is generally controlled between 6.5 and 7. Water is chemically neutral, and certain water molecules spontaneously decompose according to the following formula: H2O=H++OH-, that is, into hydrogen ions and hydroxide ions. In a neutral solution, the concentrations of hydrogen ions H+ and hydroxide ions OH- are both 10-7mol/l, and the pH value is the negative of the base 10 logarithm of the hydrogen ion concentration: pH=-log, so neutral The pH value of the solution is equal to 7. If there are excess hydrogen ions, the pH value is less than 7 and the solution is acidic; conversely, if there are excess hydroxide ions, the solution is alkaline.
The pH value is usually measured by the potentiometric method. A reference electrode with a constant potential and a measuring electrode are usually used to form a primary cell. The electromotive force of the primary cell depends on the concentration of hydrogen ions and the pH of the solution. The plant uses pH sensors and pH transmitters. There is a special glass probe that is sensitive to pH on the measuring electrode. It is made of special glass that can conduct electricity and permeate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference performance. When the glass probe comes into contact with hydrogen ions, an electric potential is generated. The potential is measured using a silver wire reference electrode suspended in a silver chloride solution. Different pH values produce different potentials, which are converted into standard 4-20mA output through a transmitter.
2. Working principle of dissolved oxygen analyzer
The oxygen content in water can fully indicate the degree of self-purification of water. For biological treatment plants that use activated sludge, it is very important to understand the oxygen content of aeration tanks and oxidation ditches. Increased dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, thereby removing volatile substances and easily Naturally oxidized ions purify sewage. There are three main methods for measuring 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 using electrochemical methods.
Oxygen is soluble in water, and its solubility depends on temperature, total pressure at the water surface, partial pressure, and dissolved salts 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 holds 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 with current (silver), and a reference electrode without current (silver). The electrode is immersed in an electrolyte such as KCl, KOH , the sensor is covered by a diaphragm, which separates the electrode and electrolyte from the liquid being measured, thus protecting the sensor, preventing the electrolyte from escaping, and preventing the intrusion of foreign substances that may cause contamination and poisoning. A polarizing voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, oxygen will diffuse through the separator, and the oxygen molecules present on the cathode (excess electrons) will be reduced to hydroxide ions: O2+2H2O+4e-® 4OH-. An electrochemical equivalent of silver chloride is precipitated on the counter electrode (electron deficiency): 4Ag+4Cl-® 4AgCl+4e-. For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts the electrons to form a current. The size of the current is proportional to the oxygen partial pressure of the measured sewage. This signal, together with the temperature signal measured by the thermal resistor on the sensor, is sent to the transformer. The transmitter uses the relationship curve between the oxygen content, oxygen partial pressure, and temperature stored in the sensor to calculate the oxygen content in the water, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathode potential. The response time of the dissolved oxygen sensor is: 90% of the final measurement value is reached after 3 minutes, and 99% of the final measurement value is reached after 9 minutes; the low flow rate requirement is 0.5cm/s.
