A two-point calibration is a more accurate calibration technique than the one-point calibration. The two-point calibration adjusts the meter at two different pH values, thus the meter has been adjusted so that its response is accurate at more than one point along the linear equation. The second calibration point is selected on the basis of the type of solution (acidic or basic) to be measured so that the linear response can be established.
When measuring acidic solutions, use a buffer that is acidic, pH < 7 (a pH = 4.00 solution is common). When measuring basic solutions, use a buffer that is slightly basic, pH > 7 (a pH = 10.00 solution is common).
Two-Point Calibration Procedure
- Rinse the electrode with distilled water.
- Place the electrode in a pH = 7.00 buffer solution.
- Wait a for the meter to stabilize, then adjust the meter until it reads a pH of 7.00.
- Remove the electrode from the solution and rinse it with distilled water.
- Place the electrode in the second buffer solution (pH = 4.00 or 10.00 buffers).
- Wait a moment for the meter to stabilize, then adjust the meter until it displays the same pH as the buffer.
- Remove the electrode from the second buffer solution and rinse it with distilled water.
- Re-place the electrode in the pH = 7.00 buffer solution. If the reading does not return 7.00, then repeat the calibration procedure using both buffers.