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pH meters are among the most commonly used instruments in laboratories for measuring the acidity or alkalinity of a solution. However, the electrode gradually loses sensitivity, and without regular calibration the readings can be misleading.
Why Is Calibration Necessary?
A pH electrode relies on ion exchange at the surface of the glass. Over time the electrode surface becomes contaminated and ages, and the reference potential drifts. A measurement taken with an uncalibrated instrument can deviate from the true value by 0.1–0.5 pH units — an unacceptable error, especially in quality control and in pharmaceutical or food production.
Three-Point Calibration Steps
1. Prepare the Buffer Solutions: Standard buffers of pH 4.01, 7.01 and 10.01 are usually used. Always check the expiry date of the solutions.
2. Rinse the Electrode: Rinse the electrode with distilled water at every buffer change and gently blot it dry with a soft tissue — do not rub it.
3. Immerse in Order: Start with pH 7 (neutral), then move on to the pH 4 or pH 10 buffer closer to your sample. Wait until the reading stabilizes.
4. Record and Date It: Write the calibration date and the buffer lot numbers in a calibration logbook — this record is requested during audits.
How Often Should You Calibrate?
Daily calibration is recommended for heavy daily use, and calibration before each use for weekly use. If the electrode response time has become longer (stabilization takes more than 30 seconds), the electrode may need to be cleaned or replaced regardless of the calibration interval.
