One point calibration and two point calibration of pH meters
Any pH meter must be calibrated with a pH standard solution before measuring the pH value of the sample. For samples with measurement accuracy below 0.1pH, a one point calibration method can be used to adjust the instrument, generally using pH 4.01 or pH 7.00 standard buffer solution. Some instruments have an accuracy of only 0.2pH or 0.1pH, so they only have a positioning adjustment knob. The specific operation steps are as follows:
(1) Measure the temperature of the standard buffer solution, check the table to determine the pH value at that temperature, and adjust the temperature compensation knob to that temperature.
(2) Rinse the electrodes with pure water and shake dry.
(3) Immerse the electrode in the buffer solution and shake it before standing still. After the reading is stable, adjust the positioning knob to make the instrument display the pH value of the standard solution.
(4) Remove the electrode, rinse and shake dry.
(5) Measure the sample temperature and adjust the pH meter temperature compensation knob to this temperature value.
(6) Immerse the electrode into the sample solution, shake it and let it stand still, and display a stable reading.
What is two-point calibration of pH meters?
For precision pH meters, in addition to "positioning" and "temperature compensation" adjustments, there is also electrode "slope" adjustment, which requires calibration with two standard buffer solutions. Generally, pH 4.01 or pH 7.00 is used for "positioning" calibration first, and then pH 4.01 (acidic) or pH 9.21 and pH 0.01 (alkaline) buffer solutions are selected for "slope" calibration based on the acidity and alkalinity of the test solution. The specific operation steps are:
(1) Wash the electrode and shake it dry, immerse it in a standard solution of pH 4.01 or pH 7.00, and place the instrument 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.
(2) Take out the electrode, wash and shake dry, and immerse it in the first 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.
(3) Take out the electrode, wash it and shake it dry, then immerse it in a pH 4.01 or pH 7.00 buffer solution. If the error exceeds 0.02pH, repeat steps (1) and (2) until the correct pH value can be displayed in both standard solutions without adjusting the knob.
(4) Take out the electrode and shake it dry. Adjust the pH temperature compensation knob to the temperature of the sample solution. Immerse the electrode in the sample solution, shake it, and let it stand still until it shows a stable reading.
