As a precision pH measuring instrument, the accuracy of a desktop pH meter directly affects the reliability of experimental data. To ensure the authenticity and validity of measurement results, regular calibration should be carried out according to standardized procedures. The following is a standardized calibration operation guide:
1、 Preparation before calibration
1. Environmental control
-Ensure stable laboratory temperature, with an ideal range of 20-25 ℃, to avoid abnormal electrode response caused by temperature differences; Humidity ≤ 80%, to prevent condensation from affecting the circuit.
-Prepare at least three standard buffer solutions: pH 4.01 (potassium hydrogen phthalate), pH 7.00 (phosphate), and pH 10.01 (borax), covering the commonly used measurement range. The buffer solution should be prepared using high-purity reagents that have been certified by metrology, and the expiration date after opening should not exceed 3 months.
2. Electrode treatment
-Remove the protective cover of the composite electrode and gently rinse the surface of the glass film with deionized water. Do not use filter paper to wipe to prevent scratches. If there is oil contamination, it can be briefly soaked in 3% dilute hydrochloric acid and then rinsed.
-Immerse the electrode in a 3mol/L solution and activate it for 30 minutes to allow the reference electrode to fully permeate and ensure stable potential.
2、 Formal calibration steps
1. One point calibration (simple mode)
-Select a standard buffer solution with a pH similar to the sample solution to be tested, and pour it into a clean beaker until the electrode head is submerged. Start the calibration program and confirm storage after the reading stabilizes. Suitable for scenarios that only require rough verification.
2. Two point/three-point calibration (recommended mode)
-The first point of calibration is to use pH 7.00 neutral buffer solution as the reference. After installing the electrode, slowly stir the solution to eliminate bubble interference, and lock the data when the display screen value jumps by less than 0.02 pH/min. This step establishes a zero point reference frame.
-Second point calibration: Replace with pH4.01 acidic buffer solution and repeat the above operation. The system automatically calculates the electrode slope and corrects nonlinear errors.
-Third point calibration (optional): Use pH10.01 alkaline buffer to verify the linear response of the electrode in the high pH range. If the deviation exceeds ± 0.05pH, the electrode condition needs to be rechecked or the aging electrode needs to be replaced.
3. Temperature compensation settings
-Manually input or activate the automatic temperature probe based on the current ambient temperature, as the pH value of the buffer solution varies significantly with temperature (e.g. pH 7.00 is actually 6.86 at 25 ℃). Failure to perform temperature compensation may result in system errors above ± 0.1pH.
3、 Verification and maintenance after calibration
1. Performance verification
-After calibration is completed, use a standard solution close to the sample to be tested for backtesting, with an allowable error of<± 0.05pH. If it exceeds the tolerance, the cause should be investigated: dry electrode membrane, depleted filling solution, or poor cable contact.
2. Daily maintenance
-After calibration, store the electrodes in a dedicated storage solution and avoid prolonged exposure to air. Clean with deionized water once a week and replace the filling solution monthly.
3. Record management
-Complete records of calibration date, batch number of standard substances used, pre - and post calibration readings, and environmental temperature and humidity to form a traceable quality file.
By standardizing the calibration process, instrument system errors can be effectively eliminated, ensuring pH measurement accuracy of ± 0.01pH level. It is recommended to perform comprehensive calibration every quarter and shorten the calibration cycle for frequently used equipment. Strictly following the operating procedures is necessary to fully utilize the analytical and detection efficiency of the desktop acidity meter.