Two-point calibration method is universally adopted for pH meter calibration.
Despite there being various types of pH meters, all of them adopt the two-point calibration method. Two standard buffer solutions are chosen: one is pH 7 standard buffer solution, and the other is pH 9 or pH 4 standard buffer solution.
First, position the pH meter with pH 7 standard buffer solution, then select the second standard buffer solution according to the acidity and alkalinity of the solution to be tested. Select pH 4 standard buffer solution for acidic test solutions, and pH 9 standard buffer solution for alkaline test solutions.
For manually regulated pH meters, repeat the operation between the two standard buffer solutions several times until the instrument can accurately display the pH values of both solutions without adjusting the zero and slope knobs. The calibration is finished. Do not adjust these knobs again in subsequent measurements.
Intelligent pH meters need no repeated adjustment. They store built-in pH values of multiple standard buffer solutions for automatic identification and calibration. Nevertheless, correct selection and accurate preparation of standard buffer solutions are essential. Generally, intelligent 0.01-grade pH meters are pre-programmed with three to five types of standard buffer solutions.
Besides, pay close attention to the temperature of the test solution before calibration to select proper buffer solutions and turn the temperature compensation knob on the instrument to be consistent with the sample temperature. The pH values of standard buffer solutions differ at different temperatures, as shown in the table below:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━
(℃)┄┄ pH7 ┄┄ pH4 ┄┄ pH9.2
━━━━━━━━━━━━━━━━━━━━━━━━━━━━
10┄┄┄┄┄┄6.92 ┄┄ 4.00 ┄┄9.33
15┄┄┄┄┄┄6.90 ┄┄ 4.00 ┄┄9.28
20┄┄┄┄┄┄6.88 ┄┄ 4.00 ┄┄9.23
25┄┄┄┄┄┄6.86 ┄┄ 4.00 ┄┄9.18
30┄┄┄┄┄┄6.85 ┄┄ 4.01 ┄┄9.14
━━━━━━━━━━━━━━━━━━━━━━━━━━━━
After calibration, frequently used pH meters do not need recalibration within 48 hours. Recalibration is required in the following cases:
There is a great temperature difference between the test solution and calibration temperature;
The electrode is exposed in the air for more than half an hour;
The positioning or slope adjustment knobs are moved accidentally;
After measuring strongly acidic solutions (pH＜2) or strongly alkaline solutions (pH＞12);
After replacing the electrode;
The pH value of the measured solution is far away from pH 7 and beyond the range of the two calibrated buffer solutions.
