How water quality analyzers work

The working principle of the water quality analyzer mainly has the following two aspects:

The working principle of a pH meter The pH 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 great influence on the reproduction and survival of organisms, and it also seriously affects the biochemical action of activated sludge, that is, it affects the treatment effect. 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-, which decomposes into hydrogen ions and hydroxide ions. In the neutral solution, the concentrations of hydrogen ion H and hydroxide ion OH- are both 10-7 mol/l, and the pH is the negative number of the logarithm of the hydrogen ion concentration at the base of 10: pH = -log, so neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH is less than 7 and the solution is acidic; conversely, if the hydroxide ion is excessive, the solution is alkaline. 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 primary battery. The electromotive force of the primary battery depends on the concentration of hydrogen ions and also depends on the pH of the solution. The plant uses a CPS11 pH sensor and a CPM151 pH transmitter. The specific structure is shown in Fig. 1. There is a special glass probe with sensitive pH response 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. When the glass probe is in contact with hydrogen ions, a potential is generated. The potential is measured by a silver control reference electrode suspended in a silver chloride solution. With different pH values, the corresponding potentials are also different. It is converted by the transmitter to a standard 4-20mA output.

Second, the principle of dissolved oxygen analyzer The oxygen content of water can fully show the degree of water self-purification. For a biological treatment plant using activated sludge, it is very important to know the oxygen content of the aeration tank and the oxidation ditch. The increase of dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, thus enabling the removal of volatile substances and ease of use. Naturally oxidized ions purify sewage. There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, and electrochemical measurement.

Dissolved oxygen in water is generally measured electrochemically. The plant uses a COS4 dissolved oxygen sensor and a COM252 dissolved oxygen transmitter. Oxygen is soluble in water and the solubility depends on the temperature, the total pressure on the surface of the water, the partial pressure and the 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 states that the solubility of gas is proportional to its partial pressure. Take the COS4 oxygen measurement sensor as an example. The electrode consists of a cathode (usually made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in electrolytes such as KCl and KOH. The sensor is covered with a membrane and the membrane The electrodes and the electrolyte are separated from the liquid being measured, thus protecting the sensor, preventing both the escape of the electrolyte and the ingress of foreign substances, resulting in 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 diffuses through the membrane and oxygen molecules that are present at the cathode (excess electrons) are reduced to hydroxide ions: O2 2H2O 4e-®4OH-. Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4Ag 4Cl-®4AgCl 4e-. For each oxygen molecule, the cathode emits 4 electrons, the counter electrode accepts electrons, forms a current, and the size of the current is measured with the structure of the pH measuring electrode (left) and the reference electrode (right) in FIG. 1. FIG. 2 Three-electrode COS Dissolving Oxygen Sensor The oxygen partial pressure of sewage is proportional to the structure diagram. This signal is sent to the transmitter together with the temperature signal measured by the thermal resistance of the sensor. The relationship between oxygen content, oxygen partial pressure and temperature stored in the sensor is calculated. The oxygen content in the water is then converted to a standard signal output. The function of the reference electrode is to determine the cathode potential. The response time of the COS4 DO sensor was 90% of the final measurement after 3 minutes, and 99% of the final measurement after 9 minutes; the minimum flow rate requirement was 0.5 cm/s.