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How to choose analytical instruments and precautions for water pH

by:BOQU     2021-05-05


What is the pH of water?

The pH value is the negative logarithm of the hydrogen ion activity in the measured aqueous solution, that is, pHu003d-lgαH+, which is one of the most commonly used indicators in the sewage treatment process. Under the condition of 25oC and pHu003d7, the activity of hydrogen ion and hydroxide ion in water is equal, and the corresponding concentration is 10-7mol/L. At this moment, the water is neutral, and the pH value ﹥of 7 indicates that the water is alkaline. The pH value<7 indicates that the water is acidic.

The value of pH reflects the acidity and alkalinity of water, but it cannot directly indicate the acidity and alkalinity of water. For example, a 0.1mol/L hydrochloric acid solution and a 0.1mol/L acetic acid solution have the same acidity as 100mmol/L, but the pH values u200bu200bof the two are quite different. The pH value of the 0.1mol/L hydrochloric acid solution is 1. The pH value of the 0.1 mol/L acetic acid solution is 2.9.

What are the commonly used methods for pH measurement?

In actual production, in order to quickly and conveniently grasp the pH change 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, colorimetry can also be used. At present, the standard method for determining the pH value of water quality in my country is the potentiometric method (GB 6920-86 glass electrode method), which is generally not affected by color, turbidity, colloidal substances, oxidants and reducing agents, and can measure the pH of clean water. It can also determine the pH value of industrial wastewater polluted by different degrees. This is also a widely used method for determining the pH value of a vast wastewater treatment plant.

The principle of potentiometric determination of pH value is to measure the potential difference between the glass electrode and the reference electrode of known potential and then obtain the potential of the indicator electrode, that is, the pH value. The reference electrode generally uses a calomel electrode or Ag-AgCl electrode, and the calomel electrode is the most commonly used. The center of the pH potentiometer is a DC amplifier, which makes the potential generated by the electrode displayed on the meter head in the form of numbers or pointers after being expanded on the instrument. The potentiometer is generally equipped with a temperature compensation device to calibrate the effect of temperature on the electrode.

The operating principle of the online pH meter used in the wastewater treatment plant is the potentiometric method, and the precautions for use are basically the same as the pH meter in the laboratory. However, since the electrodes used are continuously immersed in wastewater or aeration tanks for a long time in places that contain a lot of oil or microorganisms, in addition to requiring the pH meter to be equipped with an automatic cleaning device for the electrode, manual cleaning is also required according to the water quality and operating experience. . Generally, the pH meter used in the inlet water or aeration tank is manually cleaned once a week, and the pH meter used in the outlet water can be manually cleaned once a month. Regarding the pH meter that can measure temperature and ORP together, it should be protected and maintained in accordance with the operation precautions required by the measurement function.

What are the precautions for pH measurement?

The potentiometer should be kept dry, dust-proof, and punctually energized to ensure that the lead connection part of the electrode input is kept clean to prevent water droplets, dust, oil, etc. from entering. When using AC power, make sure that the ground is good, and the portable potentiometer using dry batteries should be punctual to replace the battery. At the same time, the potentiometer should be punctually verified and zero-adjusted. Once it is properly adjusted, the zero points of the potentiometer and the adjustment and positioning regulators cannot be arbitrarily rotated during the test.

The water used to make standard buffer solutions and rinse electrodes must not contain CO2, the pH value should be between 6.7-7.3, and the conductivity should be less than 2μs/cm. The water treated with anion and cation exchange resin can reach this requirement after being boiled and allowed to cool. The manufactured standard buffer solution should be sealed and stored in a rigid glass bottle or polyethylene bottle, and then stored in a refrigerator at 4oC, which can extend the service life. If it is stored in open-air or stored at room temperature, the service life should generally not exceed 1 Months, the used buffer can’t be poured back into the storage bottle for reuse.

Before the formal measurement, first check whether the instrument, electrode, and standard buffer are normal. And punctually verify the pH meter, the general inspection cycle is one quarter or half a year, and the two-point calibration method is used for the verification. That is, according to the scale of the pH value of the sample to be tested, two standard buffer solutions that are close to it are selected. Generally, the pH difference between the two buffer solutions is at least greater than 2. After positions with the first solution, test the second solution again. The difference between the displayed result of the potentiometer and the standard pH value of the second standard buffer solution should be no more than 0.1 pH unit.

If the error is greater than 0.1 pH unit, use the third standard buffer solution to check. If the error is less than 0.1 pH unit at the moment, it is likely that there is a problem with the second buffer solution. If the error is still greater than 0.1 pH unit, it indicates that there is a problem with the electrode, and the electrode needs to be processed or replaced with a new electrode.

When replacing the standard buffer solution or sample, rinse the electrode with distilled water, absorb the water attached to the electrode with filter paper, and rinse with the solution to be tested to eliminate the mutual influence, which is weak for application It is particularly important when the buffer solution is stable. When measuring the pH value, the aqueous solution should be properly mixed to make the solution uniform and reach electrochemical equilibrium, and when reading, stop stirring and then stand for a while to make the reading stable.

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

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