Water Quality: ph Determination - Glass Electrode Method (Continued from Table Above)

5. instruments

5.1 Acidometer or ion concentration meter. Instruments used for routine testing should be accurate to at least 0.1 pH unit, with a pH range from 0 to 14. If there are special needs, instruments with higher precision should be used.

5.2 Glass electrode and mercury electrode.

6. Sample storage

Best to measure on site. Otherwise, the sample should be kept at 0 to 4°C after sampling and measured within 6 hours after sampling.

7. steps

7.1 Instrument calibration: The operating procedures are carried out according to the instrument instruction manual. First, bring the water sample and the standard solution to the same temperature, record the measurement temperature, and adjust the temperature compensation knob of the instrument to this temperature.

Calibrate the instrument with a standard solution whose pH differs from that of the water sample by no more than 2 pH units. Remove the electrode from the standard solution, rinse thoroughly and blot dry with filter paper. Then immerse the electrode in the second standard solution. Its pH is about 3 pH units different from the first standard solution. If the difference between the instrument response and the pH (S) value of the second standard solution is greater than 0.1 pH unit, check whether there is a problem with the instrument, electrode or standard solution. When all three are normal, the sample can be measured.

7.2 Sample determination

When measuring the sample, first rinse the electrode carefully with distilled water, then rinse with water sample, then immerse the electrode in the sample, shake or stir carefully to make it uniform, let it stand, and record the pH value when the reading is stable.

8. Precision

*Based on the results of repeated measurements in a laboratory of domestic drinking water, lightly, moderately, and heavily polluted surface water and some types of industrial wastewater samples with a pH value in the range of 2.21 to 13.23.

**Based on the measurement results of 7 artificial water samples and 1 ground water sample with pH values in the range of 1.41 to 11.66 measured by this method using ten different types of acidimeters and four different types of electrodes used by 10 laboratories in Beijing.

9. Comments

9.1 Soak the glass electrode in distilled water for more than 24 hours before use.

9.2 When measuring pH, the bulb of the glass electrode should be completely immersed in the solution and slightly higher than the ceramic core end of the calomel electrode to avoid clinking the glass during stirring.

9.3 It must be noted that there should be no bubbles between the inner electrode of the glass electrode and the bulb and between the inner electrode of the calomel electrode and the ceramic core to prevent circuit breakage.

9.4 The liquid level of the saturated potassium chloride solution in the calomel electrode must be higher than the mercury body. There should be a few potassium chloride crystals at room temperature to ensure the saturation of the potassium chloride solution. However, care must be taken not to have too many potassium chloride crystals to prevent blocking the path with the molten liquid to be measured.

9.5 When measuring pH, in order to reduce the dissolution or volatilization of carbon dioxide in the air and water samples, the water sample bottle should not be opened in advance before measuring the water sample.

9.6 When the surface of the glass electrode is contaminated, it needs to be treated. If there is inorganic salt scaling, it can be dissolved with warm dilute hydrochloric acid; for insoluble scaling such as calcium and magnesium, it can be dissolved with EDTA disodium solution. If it is stained with oil, it can be cleaned with acetone. After the electrode is treated according to the above method, it should be soaked in distilled water for one day and night before use. Be careful not to use absolute ethanol or dehydrating detergent to treat the electrode.

10. Test report

The test report should include the following content:

a. Date, time and location of sampling;

b. Sample preservation method;

c. The date and time when the sample was measured;

d. The temperature of the sample during measurement;

e. Measurement results (the pH value should be taken to the nearest 0.1 pH unit. If there are special requirements, the number of calibration digits can be calculated according to the needs and the accurate result of the instrument);

f. Other circumstances that need to be explained.

Additional notes:

This standard is proposed by the National Environmental Protection Administration.

This standard is formulated by the Beijing Environmental Protection Monitoring Center.

The main drafter of this standard is Dong Shuying.

The National Environmental Protection Administration is responsible for interpreting this standard.