The role of the ‘frit’ (or wick):
For a pH meter with a single combination electrode to work properly there
must be an electrical connection between the electrode filling solution
(usually potassium chloride – which is electrically conductive) and the
sample solution. This is achieved via the use of a porous frit or wick in
the glass wall separating the inner filling solution chamber from the
sample (Fig 1.8a, 1.8b). Both these devices allow the free (very slow)
flow of the filling solution into the sample solution. The most common
cause of pH meter failure is due to blockage of this porous frit or wick.
Dry storage of
electrodes
Dry storage causes dehydration of both the
glass electrode and precipitation of salts within the interstices of the
frit itself (Fig 1.8b). The consequences are reading drift, slow response
times and loss of accuracy and precision. To avoid these problems the
electrode tip must be permanently stored in a specially formulated storage
solution (e.g. Flairform's
pH Electrode Storage) - see Fig 1.7.
Note, pH buffers and distilled water are
not suitable for storing pH electrodes as they can upset electrode
calibration.
Electrode contamination
The ‘frit’ and glass tip of pH meter
electrodes usually becomes contaminated with impurities (Fig 1.8b) causing
similar symptoms to dry storage. Contamination may be so severe that
re-calibration is not possible without prior cleaning.
Removing contamination: When testing
nutrient solutions, both ‘organic’ and ‘inorganic’ contaminations tend to
co-occur. It is therefore important to use an all-purpose cleaner such as
Flairform’s ‘Electrode Cleaner’. Be aware that most cleaners usually only
remove inorganic contaminants (i.e. “salts”).
Preventing contamination:
1. Immediately clean the
electrode in distilled water after use (Fig 1.6).
2. Avoid placing electrodes in
oily solutions as they can “coat” the electrode.
3. Avoid testing (or leaving
electrodes for too long) harsh chemicals. For example, highly caustic
(e.g. pH Up, silica additives) or concentrated solutions (e.g. raw
nutrient).
4. Filter potentially problem
(i.e. greasy or highly turbid) water samples prior to performing a pH
measurement.
