How to apply pH meter and dissolved oxygen meter in sewage treatment
1 How a pH meter works
The pH of any solution can be expressed in terms of hydrogen ion concentration. Due to the ionization effect of water itself, every liter of pure water contains 10-7 grams of [H+] at 22 ° C, and the atomic weight and atomic valence of hydrogen are both 1, so each liter of pure water contains 10-7 gram equivalents of [H+] ], because [H+] in pure water is produced due to the dissociation of water molecules themselves, that is:
H2O=H++OH-
Pure water is neutral, that is, the concentrations of the two ions are equal, [H+]=[OH-], the product of which is a constant temperature constant, called ion product K water.
K water=[H+][OH-]=10-7*10-7=10-14
The above formula is applicable to any acid-base solution, that is, any aqueous solution, in which [H+][OH-] product is equal to 10-14. For example, in some aqueous solution [H+]=10-3, then [OH-] must be 10-11, so for any kind of aqueous solution, as long as [H+] is known, then [OH-] is easy to obtain, usually The pH is defined by the negative value of the common logarithm of the hydrogen ion concentration, expressed as: pH=-lg[H+],
So the pH of a neutral 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 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, which affects the treatment effect. The pH value of sewage is generally controlled between 6.5 and 7.
The pH value is usually measured by the potentiometric method. A constant potential reference electrode and a measuring electrode are commonly 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. Type sinker pH meter. The measuring electrode of the instrument adopts a special glass electrode sensitive to pH, which has the characteristics of high measurement accuracy and good anti-interference. When it is immersed in the solution to be tested, the hydrogen ions in the solution to be tested are balanced with the hydrogen ions in the hydration layer on the surface of the electrode bulb, and at the same time, the solution inside and outside the glass bulb and the hydration layer on the inner wall of the electrode bulb generate a potential difference, and the glass electrode The interior is filled with a buffer solution with a fixed pH value, and the lead-out electrode is immersed in the internal solution to form a half-cell, and the half-cell formed by immersing the calomel electrode in a saturated potassium chloride solution is simultaneously introduced into the converter for measurement. Different pH values correspond to different potentials, which are converted into 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 the aeration tank. The increase of dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, thus removing volatile substances and substances that are prone to natural oxidation ions to 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 directly proportional to its partial pressure.
The oxygen measurement sensor consists of a cathode (commonly made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrodes are immersed in electrolytes such as KCl and KOH, and the sensor is covered by a diaphragm. The coating separates the electrodes and electrolyte from the liquid to be measured, and only dissolved gas can penetrate the coating, thus protecting the sensor, preventing the electrolyte from escaping, and preventing the intrusion of foreign substances to cause pollution 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 [OH-]. The electrochemical equivalent of silver chloride is precipitated on the counter electrode (electron deficiency). For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons to form a current: 4Ag+4Cl-=4AgCl+4e-.
The magnitude of the current is proportional to the partial pressure of oxygen in the measured sewage. The signal is sent to the transmitter together with the temperature signal measured by the thermal resistance on the sensor. The relationship curve calculates 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 cathodic potential.
3. Characteristics of pH meter
The glass on the pH electrode will gradually age over time, the gradient (the change in the electrode output potential caused by a unit pH change) will deteriorate, and it will take a long time to reach a stable potential. The service life of general electrodes can reach two years. In addition, temperature also has a great influence on aging. The aging degree of storage at 100°C for several weeks is equivalent to the aging degree of storage at room temperature for one year.
The pH meter has the advantages of accurate measurement, high reliability, and convenient installation and maintenance. It is also sensitive to pollution and requires frequent calibration. Generally, it is calibrated every one to one and a half months, and the electrode is replaced every two years.
4. Features of dissolved oxygen meter
The dissolved oxygen meter has the characteristics of easy installation, long calibration period (3~4 months), insensitivity to other substances, etc., and can monitor the use of the electrolyte in the membrane and probe. Generally, the electrolyte and membrane should be replaced every one to three years. .
