What is a two-point calibration of a pH meter?
For precision-grade pH meters, in addition to "positioning" and "temperature compensation" adjustments, there is also an electrode "slope" adjustment, which requires two standard buffers for calibration. Generally, pH=6.86 or pH=7.00 is used for "positioning" calibration first, and then according to the acid-base condition of the test solution, pH=4.00 (acidic) or pH=9.18 or pH=10.01 (alkaline) buffer solution is selected for "slope correction" , the specific operation steps are:
(1) Wash and dry the electrode, immerse it in the pH=6.86 or pH=7.00 standard solution, and place the temperature compensation knob at the solution temperature. After the indication value is stable, adjust the positioning knob so that the instrument indication value is the pH value of the standard solution.
(2) Take out the electrode, wash it and spin it dry, and immerse it in the second standard solution. After the indication value stabilizes, adjust the slope knob of the instrument so that the indication value of the instrument is the pH value of the second standard solution.
(3) Wash the electrode and spin it dry, then immerse it in the pH6.86 or pH7.00 standard solution. If the error exceeds 0.02pH, repeat steps (1) and (2). Until the correct pH value can be displayed in the two standard solutions without adjusting the knob.
(4) Take out the electrode, wash it and spin it dry, adjust the pH temperature compensation knob to the sample temperature, immerse the electrode in the sample solution, shake it and place it still, and the reading will be stable.
