Baseline Solutions
You will need solutions or buffers of at least two known pH values to perform the calibration. To calibrate a meter used to read acid solutions, the most common calibration solutions are 4.01 and 7.01 pH. To read alkaline solutions, you would use 7.01 and 10.01 pH. If you're reading both acid and alkaline solutions, use at least three solutions: 4.01, 7.01 and 10.01 pH. Some meters allow for calibration of up to five points on the pH scale.
Temperature Correction
Because its temperature affects the voltage produced by the solution, you'll need to correct for the temperature. Some pH meters perform this correction automatically, while others require that you enter the temperature into the meter using buttons or dials.
Neutral Calibration
Use the 7.01 pH baseline solution for this step. Rinse the electrode with water and dry it off, being careful not to rub hard enough to abrade the glass around the electrode. Submerge the electrode in the baseline solution, then turn the meter on and wait until the reading stabilizes. Once you set the meter to calibration mode, the more advanced meters detect the baseline solution and set the value themselves. If the meter doesn't do it automatically, you will be required to enter a pH of 7. Confirm the pH when prompted. The meter will associate the voltage being read with a 7 pH.
Two-Point Calibration
Use either the 4.01 or 10.01 pH baseline solution for this step, depending on whether you are calibrating for reading acid or alkaline solutions. Rinse the electrode with water again and dry it. Submerge it in the baseline solution and either accept or enter the desired value, 4 or 7 pH. Confirm the pH when prompted. This step associates the voltage being read with the pH of the baseline solution, either 4.01 or 10.01.
Three-Point Calibration
If you are measuring pH in both acid and alkaline ranges, you will need to perform a three-point calibration. Follow the steps outlined above. Repeat the two-point calibration step above, using whichever baseline solution you didn't use in the two-point calibration step. You can calibrate at additional points by repeating these steps with different baseline solutions.