Proper calibration is the foundation of accurate results. If your pH meter does not know which voltage to associate with which pH, accurate readings will be impossible to obtain.
pH meters
The use of a pH meter is a convenient, automated way to quickly determine the pH of a solution. A pH meter measures a voltage created by the ions in the solution that pass through the meter's membrane, and associate this voltage with a particular pH.
Calibration of a pH meter
Most pH meters are calibrated by using a set of standard solutions that are guaranteed to be at a particular pH. Three of these points are the minimum number needed to establish a calibration curve with which the meter will work to associate each voltage reading with a particular pH. These standards are usually color-coded for ease of use.
Electrode Drift
Many pH electrodes drift over time from their calibrated settings. This will cause inaccurate readings to occur. Therefore, it is necessary to recalibrate the meter periodically to ensure continued accurate results. Drift can be caused by a number of factors, mostly related to the meter's electronics.
Testing Standards
It is useful from time to time to test standards of known pH values. This will help you to determine if it is time for you to repeat the calibration procedure. If the meter's reading of the known value of a pH standard solution is off by two tenths or more of a pH unit, you should recalibrate.
Manufacturer Calibration
Some electrodes can be sent back to their manufacturer for calibration. This is the best way to ensure accuracy. However, this can be too time-consuming to be practical.
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