Key points for water quality testing operations in sewage treatment plants part seven

39.What are water acidity and alkalinity?

The acidity of water refers to the amount of substances contained in the water that can neutralize strong bases. There are three types of substances that form acidity: strong acids that can completely dissociate H+ (such as HCl, H2SO4), weak acids that partially dissociate H+ (H2CO3, organic acids), and salts composed of strong acids and weak bases (such as NH4Cl, FeSO4). Acidity is measured by titration with a strong base solution. The acidity measured with methyl orange as the indicator during titration is called methyl orange acidity, including the acidity formed by the first type of strong acid and the third type of strong acid salt; the acidity measured with phenolphthalein as the indicator is called phenolphthalein acidity, It is the sum of the above three types of acidity, so it is also called total acidity. Natural water generally does not contain strong acidity, but contains carbonates and bicarbonates that make the water alkaline. When there is acidity in the water, it often means that the water has been contaminated by acid.

In contrast to acidity, water alkalinity refers to the amount of substances in the water that can neutralize strong acids. Substances that form alkalinity include strong bases (such as NaOH, KOH) that can completely dissociate OH-, weak bases that partially dissociate OH- (such as NH3, C6H5NH2), and salts composed of strong bases and weak acids (such as Na2CO3, K3PO4, Na2S) and other three categories. Alkalinity is measured by titration with a strong acid solution. The alkalinity measured using methyl orange as the indicator during titration is the sum of the above three types of alkalinity, which is called total alkalinity or methyl orange alkalinity; the alkalinity measured using phenolphthalein as the indicator is called phenolphthalein base. Degree, including alkalinity formed by the first type of strong base and part of the alkalinity formed by the third type of strong alkali salt.

The measurement methods of acidity and alkalinity include acid-base indicator titration and potentiometric titration, which are generally converted into CaCO3 and measured in mg/L.

40.What is the pH value of water?

The pH value is the negative logarithm of the hydrogen ion activity in the measured aqueous solution, that is, pH=-lgαH+. It is one of the most commonly used indicators in the sewage treatment process. Under 25oC conditions, when the pH value is 7, the activities of hydrogen ions and hydroxide ions in the water are equal, and the corresponding concentration is 10-7mol/L. At this time, the water is neutral, and the pH value > 7 means that the water is alkaline. , and pH value <7 means the water is acidic.

The pH value reflects the acidity and alkalinity of water, but it cannot directly indicate the acidity and alkalinity of water. For example, the acidity of 0.1mol/L hydrochloric acid solution and 0.1mol/L acetic acid solution is also 100mmol/L, but their pH values are quite different. The pH value of 0.1mol/L hydrochloric acid solution is 1, while The pH value of 0.1 mol/L acetic acid solution is 2.9.

41. What are the commonly used pH value measurement methods?

In actual production, in order to quickly and easily grasp the changes in pH value of wastewater entering the wastewater treatment plant, the simplest method is to roughly measure it with pH test paper. For colorless wastewater without suspended impurities, colorimetric methods can also be used. At present, my country's standard method for measuring the pH value of Water Quality is the potentiometric method (GB 6920--86 glass electrode method). It is usually not affected by color, turbidity, colloidal substances, oxidants, and reducing agents. It can also measure the pH of clean water. It can also measure the pH value of industrial wastewater polluted to different degrees. This is also a widely used method of measuring pH value in most wastewater treatment plants.

The principle of potentiometric measurement of pH value is to obtain the potential of the indicating electrode, that is, the pH value, by measuring the potential difference between a glass electrode and a reference electrode with a known potential. The reference electrode generally uses a calomel electrode or an Ag-AgCl electrode, with the calomel electrode being the most commonly used. The core of the pH potentiometer is a DC amplifier, which amplifies the potential generated by the electrode and displays it on the meter head in the form of numbers or pointers. Potentiometers are usually equipped with a temperature compensation device to correct the effect of temperature on the electrodes.

The working principle of the online pH meter used in wastewater treatment plants is potentiometric method, and the precautions for use are basically the same as those of laboratory pH meters. However, because the electrodes used are continuously soaked in wastewater or aeration tanks and other places containing a large amount of oil or microorganisms for a long time, in addition to requiring the pH meter to be equipped with an automatic cleaning device for the electrodes, manual cleaning is also required based on Water Quality conditions and operating experience. Generally, the pH meter used in the inlet water or aeration tank is manually cleaned once a week, while the pH meter used in the effluent water can be manually cleaned once a month. For pH meters that can simultaneously measure temperature and ORP and other items, they should be maintained and maintained according to the usage precautions required for the measurement function.

42.What are the precautions for measuring pH value?

⑴The potentiometer should be kept dry and dust-proof, powered on regularly for maintenance, and the input lead connection part of the electrode should be kept clean to prevent water droplets, dust, oil, etc. from entering. Ensure good grounding when using AC power. Portable potentiometers that use dry batteries should replace the batteries regularly. At the same time, the potentiometer must be regularly calibrated and zeroed for calibration and maintenance. Once debugged properly, the zero point of the potentiometer and the calibration and positioning regulators cannot be rotated at will during the test.

⑵The water used to prepare the standard buffer solution and rinse the electrode must not contain CO2, have a pH value between 6.7 and 7.3, and a conductivity of less than 2 μs/cm. Water treated with anion and cation exchange resin can meet this requirement after boiling and letting it cool. The prepared standard buffer solution should be sealed and stored in a hard glass bottle or polyethylene bottle, and then stored in a refrigerator at 4oC to extend the service life. If stored in the open air or at room temperature, the service life generally cannot exceed 1 Months, used buffer cannot be returned to the storage bottle for reuse.

⑶ Before formal measurement, first check whether the instrument, electrode, and standard buffer are normal. And the pH meter should be calibrated regularly. Usually the calibration cycle is one quarter or half a year, and the two-point calibration method is used for calibration. That is, according to the pH value range of the sample to be tested, two standard buffer solutions that are close to it are selected. Generally, the pH value difference between the two buffer solutions must be at least greater than 2. After positioning with the first solution, test the second solution again. The difference between the display result of the potentiometer and the standard pH value of the second standard buffer solution should not be greater than 0.1 pH unit. If the error is greater than 0.1 pH unit, a third standard buffer solution should be used for testing. If the error is less than 0.1 pH units at this time, there is most likely a problem with the second buffer solution. If the error is still greater than 0.1 pH unit, there is something wrong with the electrode and the electrode needs to be processed or replaced with a new one.

⑷When replacing the standard buffer or sample, the electrode should be fully rinsed with distilled water, and the water attached to the electrode should be absorbed with filter paper, and then rinsed with the solution to be measured to eliminate mutual influence. This is important for the use of weak buffers. This is especially important when using solutions. When measuring the pH value, the aqueous solution should be stirred appropriately to make the solution uniform and achieve electrochemical equilibrium. When reading, the stirring should be stopped and allowed to stand for a while to allow the reading to be stable.

⑸ When measuring, first rinse the two electrodes carefully with water, then rinse with the water sample, then immerse the electrodes in a small beaker containing the water sample, shake the beaker carefully with your hands to make the water sample uniform, and record the pH value after the reading is stable.