To ensure their accuracy, pH meters must be
regularly 'calibrated' (i.e. tested and adjusted for accuracy) using
standard pH buffer solutions which are made to an internationally agreed
recipe (Fig 1.9). These are stable solutions that possess a specific pH value.
When calibrating, it is essential to use at
least two different pH buffers. For nutrient solutions,
pH buffer 4.0 and 7.0 are the
ideal calibration standards to use.
Note: Some manufacturers claim their
pH meters only require calibration using pH buffer 7.0. However, that
claim is simply not justifiable if accuracy is desired. The use of two pH
buffer standards ensures that the correct calibration "slope" is obtained.
Calibration procedure
To calibrate, either refer to your meter's
instructions, or do the following:
Step 1. Rinse electrode in
distilled water then gently shake off the excess water.
Step 2. Calibrate first with pH
buffer 7.0 followed by pH buffer 4.0: Immerse electrode in pH buffer 7.0
solution for at least 2 minutes before adjusting so that it reads 7.0.
To prevent contamination
of the pH buffers, decant a portion into a smaller container to perform
the above calibration, and discard once used.
Step 3. Repeat the above
procedure with pH buffer 4.0 adjusting so that it reads 4.0.
Step 4. Remove electrode from pH
buffer solution then rinse with distilled water. The meter is now ready to
be used.
