What is the relationship between one point calibration or two point calibration?
One point calibration of pH meters
The pH value of the sample can only be measured after calibration with a pH standard solution. For samples with measurement accuracy below 0.1 pH, a one point calibration method can be used to adjust the instrument. Generally, pH 4.01 or pH 7.00 standard buffer solution is used. Some instruments have an accuracy of only 0.2 pH or 0.1 pH, 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 pH sensor with pure water and shake dry.
(3) Immerse the pH sensor in the buffer solution and shake it, then place it 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 pH sensor, rinse and shake dry.
(5) Measure the sample temperature and adjust the pH meter temperature compensation knob to this temperature value.
(6) Immerse the pH sensor into the sample solution, shake it and let it stand still, and display a stable reading.
Two point calibration of pH meter?
For precision pH meters, in addition to temperature compensation adjustment, 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, and then pH 4.01 (acidic) or pH 9.21 and pH 10.01 (alkaline) buffer solutions are selected for "slope" calibration based on the acid-base condition of the test solution. The specific operation steps are:
(1) Wash and dry the pH sensor, 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 displayed value stabilizes, press the button to make the instrument display the pH value of the standard solution.
(2) Remove the pH sensor, wash and shake dry, and immerse it in the first standard solution. After the reading stabilizes, press the button to set the instrument reading to the pH value of the second standard solution.
(3) Wash the pH sensor and shake it dry, then immerse it in a pH 4.01 or pH 7.00 buffer solution. If the error exceeds 0.02 pH, 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 pH sensor and shake it dry. Adjust the pH temperature compensation knob to the temperature of the sample solution, immerse the pH sensor in the sample solution, shake it, and then stand still. The reading will be stable after the display.
