What is the two-point calibration of a pH meter?
For precision pH meters, in addition to "positioning" and "temperature compensation" adjustments, there is also an electrode "slope" adjustment, which requires calibration with two standard buffer solutions. Generally, pH 6.86 or pH 7.00 is used for "positioning" calibration, and then pH 4.00 (acidic), pH 9.18, or pH 10.01 (alkaline) buffer solution is selected for "slope correction" based on the acidity and alkalinity of the test solution. The specific steps are as follows:
Wash and spin dry the electrode electrode, immerse it in a standard solution with pH 6.86 or pH 7.00, and place the temperature compensation knob at the solution temperature. After the indicated value is stable, adjust the positioning knob to make the indicated value of the instrument be the pH value of the standard solution.
Remove the electrode, wash and spin dry it, and immerse it in the second standard solution. After the indicated value is stable, adjust the slope knob of the instrument so that the indicated value of the instrument is the pH value of the second standard solution.
Wash and spin dry the electrode electrode, then immerse it in a standard solution with pH 6.86 or pH 7.00. If the error exceeds 0.02 pH, repeat steps (1) and (2). Until the correct pH value can be displayed without adjusting the knob in both standard solutions.
Take out the electrode, wash it and shake it dry. Adjust the pH temperature compensation knob to the sample temperature. Immerse the electrode in the sample solution, shake it, and let it stand still. After the display stabilizes, take a reading.
