Generally, three buffer solutions are used to calibrate the pH meter, one of the buffer solutions has neutral pH i.e. 7.00 and the second buffer solution is slightly acidic pH i.e. say pH 4.00 and another it is basic, i.e. pH 9.20. It is best to compare the pH range in which the pH should be measured. The counter's gain and offset settings are frequently adjusted as the probe is alternately placed over two or three calibration settings until the solutions obtain accurate readings. Modern devices have fully automated this process, requiring only one or two soaks in each solution.
Before calibrating the pH meter, clean the electrode as follows.
Immerse the pH electrode in 0.1 M HNO3 or 0.1 M HCl for 15 min, followed by immersion in storage solution for 30 min.
Calibration procedure for a pH meter:
- Select buffer solutions that vary with the likely pH of the sample. Buffers should be pH 07.00, pH 04.00 and pH 09.20.
- Make sure the buffer solutions are at room temperature. If the buffers are at different temperatures, temperature compensation is recommended and the sample should be measured at 250C.
- Wash the electrodes using distilled water and wipe them with soft tissue paper.
- Keep the electrode in pH 07.00 buffer. Expect a stable screen. Adjust the instrument to the pH value of the buffer at the measurement temperature.
- Wash the electrodes using distilled water and wipe them with soft tissue paper.
- Repeat the same process with the pH 04.00 and pH 09.20 buffer solutions as follows for pH 07.00.
- Wash the electrodes using distilled water and wipe them with soft tissue paper.
- Keep the electrode in the sample solution. While the display is stable, read the pH.
- Clean the electrode with distilled water and store it in 3 M KCl/Saturated KCl/pH 04.00 buffer solution. (Do not store the pH electrode in deionized or distilled water.)
Periodically check electrode performance with standardization of two or three buffers. If the grade is less than 92% or you read pad drift, follow the standard cleaning procedure.
