Glass electrode method

Method summary

Use pH glass electrode as indicator electrode and saturated calomel electrode as reference electrode (pH composite electrode can also be used) , immersed in the solution being tested to form a battery. At 25°C, every unit change in solution pH produces a potential difference of 59.16mV. After calibrating and positioning with a standard buffer, place the electrode into the sample and read the pH value directly on the pH meter. There is a compensation device on the instrument for temperature differences.

This method is suitable for measuring the pH value of groundwater, drinking water, surface water and industrial wastewater, and is one of the important indicators of the environment. The precision of its measurement range depends on the selected pH electrode and pH meter.

The color, turbidity, colloidal substances, oxidants, reducing agents and higher salt content of the water do not interfere with the measurement; the influence of temperature can be corrected through the compensation device on the instrument. In a strong acidic solution with a pH of 1, there will be a so-called acid error, which can be measured according to the acidity; in an alkaline solution with a pH of 10, there will be an error due to the presence of a large amount of sodium ions, making the reading low, usually called sodium Difference. To eliminate the sodium difference, in addition to using a special low-sodium difference electrode, you can also use a standard solution with a pH value similar to the measured solution to calibrate the instrument. Temperature affects the potential of the electrode and the ionization balance of the water. Attention must be paid to adjusting the compensation device of the instrument to be consistent with the temperature of the solution, so that the temperature error between the sample being tested and the standard buffer solution used to calibrate the instrument is within ±1°C.

Instruments and Apparatus

Acidometer.

Glass electrode or (pH composite electrode).

Saturated calomel electrode or (pH composite electrode).

Reagent

Preparation of pH standard buffer solution: Weigh the specified amount of reagent into a beaker according to Table 80.5, dissolve it in deionized water, move it to a 1000mL volumetric flask, and keep the temperature at 25 ℃, dilute to volume with deionized water. The deionized water used must be boiled to remove carbon dioxide before use, and cooled before use. The conductivity of the deionized water should be less than 2 μS/cm, and the pH should be between 5.6 and 6.0.

Table 80.5 Preparation of pH standard buffer solution

Continued table

Note: The reagent needs to be dried at 110°C for 2 hours before use.

Analysis steps

Adjust the instrument according to the instructions of the selected acidimeter. And install the activated pH glass electrode and saturated electrode or (pH composite electrode) on the electrode holder.

Select a standard buffer solution (Table 80.6) with a pH value similar to that of the water sample, wash the electrode 2 to 3 times, pour 30 to 50 mL of this solution into a beaker, and calibrate according to the "positioning" steps in the instrument instructions instrument. Correct the pH value of the standard buffer solution according to the temperature of the solution during measurement according to the data in Table 80.6.

Table 80.6 pH values ??of five standard solutions at different temperatures

Wash the electrode 2 to 3 times with the water sample to be measured, and take 30 to 50mL of water sample in a clean beaker for measurement. Read the pH value directly from the instrument meter.

Notes

1) The pH value is best measured on site. Otherwise, the sample should be kept at 0 ~ 4℃ after sampling, and the measurement should be performed within 6 hours after sampling.

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

3) When measuring the pH value, the bulb of the glass electrode should be completely immersed in the solution and slightly higher than the ceramic core of the calomel electrode to avoid being damaged during stirring.

4) It must be noted that there must be no air 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.

5) The liquid level of the saturated potassium chloride solution in the calomel electrode must be higher than the mercury body. There should be a small amount of potassium chloride crystals at room temperature to ensure that there are not too many potassium chloride crystals. Prevent clogging of the path to the solution being measured.

6) 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.

7) When the surface of the glass electrode is contaminated, it needs to be treated. If there is adsorption of inorganic salts and scaling, warm dilute hydrochloric acid can be used to dissolve it; for insoluble scaling such as calcium and magnesium, EDTA disodium solution can be used to dissolve it; if it is stained with oil, it can be cleaned with acetone. After the electrode is treated with 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.

8) Precautions for the use of composite electrodes:

a. When not in use after measurement, the electrode protective cap should be put on, and a small amount of KCl solution with a concentration of 3mol/L should be placed in the cap. To keep the electrode bulb moist. If it is found to be dry, it should be soaked in 3mol/L KCl solution or slightly acidic solution for several hours before use to reduce the asymmetric potential of the electrode.

b. After removing the protective cap, be careful not to let the sensitive glass bulb in the plastic protective grid come into contact with hard objects. Any damage or scratches will cause the electrode to fail.

c. The external reference supplementary solution for the composite electrode is 3mol/L KCl solution. The supplementary solution can be added from the small hole at the upper end.

d. Avoid immersing the electrode in distilled water, protein, acidic fluoride solution for a long time, and avoid contact with silicone grease.

e. After long-term use, if the percent theoretical slope of the electrode is found to be slightly reduced, the lower end of the electrode can be soaked in (4 96) HF for 3 to 5 seconds, washed with distilled water, and then in 0.1 Soak it in mol/L HCl for several hours, rinse it with deionized water to refresh it.