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pH
The pH of a solution is a measurement of its acidity or alkalinity. Acidic substances have a pH less than 7, whereas measurements above 7 indicate an alkaline solution; the further away a substance's pH is from 7, the more acidic or alkaline the substance. Because pH is a logarithmic scale, an acid that has a pH of 5, for example, is 10 times as acidic as something with a pH of 6.
pH Meters
A pH meter typically has three components: the pH sensor, an amplifier, and a transmitter or analyzer. The pH sensor contains a thermometer and two different electrodes. One electrode, known as the measuring electrode, registers an electrical current based on the pH of the solution. The other electrode is the reference electrode and provides a current that can be used as a reference point for the measuring electrode.
Why pH Meters Need Calibration
The measuring electrode of a pH meter is essentially a chemical battery. Over time, however, the characteristics of this electrode will change, causing a change in the voltages that different pHs will generate. As a result, these meters need to be periodically calibrated so that the transmitter or analyzer can compensate for these changes in the measuring electrode. This is especially important if a precise determination of pH is needed, such as within hundredths of a pH unit.
Calibration Using Buffers
Buffers are solutions that are specially formulated to have a stable pH over time. Most pH meters come with standardized buffers and buffers with a pH of 4, 7 and 10 are often used for calibration. Calibration of a pH meter using these buffers requires activating the calibration mode found on the meter and then allowing the meter to measure the voltages generated by each of the three buffers. The meter will then be able to use the voltages generated by the buffers as a reference point to more accurately measure the pH of an unknown solution.