The Technique for Reducing PH Meter Measurement Error
(1) When using the pH meter for the first time, the composite electrode must be activated before use, that is, soak the electrode in a potassium chloride solution with a concentration of 3mol/L for more than 24 hours. Preheat the pH meter for 30 minutes before each acid rain measurement. The temperature of the precipitation sample must be balanced with the temperature of the standard buffer solution to ensure that the temperature difference is less than ±2°C. In order to shorten the temperature difference equilibrium time, the precipitation sample can be installed Put the beaker and the reagent bottles of two different standard buffer solutions into the water close to the room temperature at the same time. After the temperature balance, it should be measured immediately, and the temperature balance time generally does not exceed 2 hours. Immediately after the measurement, a protective cover filled with 3mol/L potassium chloride solution should be installed to keep the electrode ball wet, and the power should be turned off.
(2) Correct calibration of the pH meter. Before each measurement, a standard buffer solution with a known pH value must be used for positioning calibration, and the closer the value is to the measured value, the better. The two standard buffer solutions used for calibration in this station are neutral solution with pH=6.86 and acidic solution with pH=4.00. Generally speaking, when the instrument is in continuous use, it should be calibrated once a day by two-point calibration method. The correct way to calibrate is:
a. Set the "selection" switch to the ℃ position, and the digital display value is the solution temperature value;
b. Turn the slope adjustment knob clockwise to the end (that is, to the 100% position);
c. Insert the cleaned electrode into the neutral standard buffer solution of pH=6.86;
d. Adjust the positioning knob so that the reading on the instrument is consistent with the pH value of the buffer solution at the current temperature;
e. Clean the electrode with distilled water, then insert it into an acidic standard buffer solution with pH = 4.00, adjust the slope knob to make the reading displayed by the instrument consistent with the pH value of the buffer solution at the current temperature;
f. The instrument is calibrated. After calibration, the positioning adjustment knob and slope adjustment knob must not be changed in any way. Pay special attention to gently kneading and slow movements when adjusting the positioning and slope knobs.
(3) Preparing an accurate standard buffer solution can also reduce the measurement error of pH value. Therefore, you need to pay attention to the following points:
a. Chemical reagents used to prepare standard solutions should be stored in a dry place to prevent deliquescence;
b. The acid rain observation station must regularly measure the K value of the pure water used in the station every month, and it shall not exceed 10μS/cm;
c. When preparing, use a smaller beaker to dilute to reduce the pH standard solution sticking to the beaker wall. In addition to emptying out the plastic bags used to store chemicals, they should also be rinsed with distilled water several times, and then poured into the prepared pH standard solution together;
d. The storage period of the prepared standard solution shall not exceed 3 months, and if turbidity, mold or precipitation are found during the storage period, it cannot be used any longer.
3 Use the temperature compensation function to reduce measurement errors
Since the change of electrode potential is different at different temperatures, in order to adapt to the measurement of pH value under various temperature conditions, the pH meter is equipped with automatic and manual temperature compensation devices, and it is necessary to check the rear panel of the pH meter before measurement. Where is the temperature compensation switch? If the temperature automatic compensation function is used, it must be set to the "automatic gear" position. If the manual temperature compensation method is used, it must be set to the "manual gear" position to achieve the pH electrode. Full response, so that the measured acid rain pH data is true and reliable.
The temperature compensation on the pH meter is only the slope item of the compensation electrode. Therefore, in order to obtain accurate measurement results, the measured precipitation sample and the standard buffer solution should be measured at the same and constant temperature as much as possible.
In order to reduce the pH value measurement error caused by temperature, the temperature measuring probe should be calibrated once a month. Calibration should also be performed after the instrument is installed and repaired, and the difference between the temperature measuring probe temperature value and the standard temperature should not exceed 0.5°C.
In order to reduce the measurement error of the pH value of the precipitation sample, the observer should pay special attention to the temperature balance process of the measured precipitation sample from the collection to the laboratory and the standard solution during normal observation. Put a clean watch glass on it to prevent dust from falling in and contaminating the measured precipitation sample, which will cause errors in the measurement results.
