Electrodes have slightly different characteristics. Electrodes with different nominal values are produced by different manufacturers. The zero pH and the sensitivity will vary with time. Calibration matches the pH meter to the characteristics of the electrodes being used and continued calibration of the electrode on a regular basis corrects for continually changing characteristics during the lifetime of the electrodes. The most accurate calibration is performed using two different buffer solutions. This enables both pH¡Æ ( zero pH ) and the slope ( sensitivity ) to be determined.
A one-point calibration with only one buffer can be carried out if the last calibration was performed recently or if you are in a hurry. Only the pH¡Æ will be determined and the sensitivity previously found will be used.
The sensitivity is independent of temperature and is usually stated as a percentage of the theoretical value. The slope, when expressed as mV/pH, is directly dependent on temperature. The slope ( at 25¡Æ C ) is often used as an alternative to sensitivity in % ( 100% = 59 mV/pH ).
Zero pH ( pH¡Æ ) is generally used to describe electrode characteristics, though the potential at pH 0 or pH 7 at 25¡Æ C can also be given.
When performing a calibration, follow a consistent pattern. Use the same stirring speed, the same stability criteria, and the same waiting time. The buffers should be at the same temperature, preferably the temperature of the sample. For the most accurate measurements, the two buffers should bracket the value of the sample solution, that is, for samples between pH 4.0 and 7.0, use of the pH = 4.01 and pH = 7.00 buffers would be advisable. However, sample pH values close to, but outside this range, could be measured using these buffers.
Though there are many varieties of pH meters on the market, for example, those with knobs and dials for adjustment, the simplest and most convenient meters have an autocalibration, that is, buffer recognition, facility. These meters have a full range of buffer values, with corresponding temperature variations, preprogrammed in their memories. Based on definitive buffer recognition, the operator must be sure to use only the preprogrammed buffers, lest the meter be fooled into accepting a false buffer value, for instance, a pH 6.86 instead of a pH 7.00 buffer.