pH electrode cleaning and
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 (Fig 10.5, 10.8) in the glass wall that
separates the inner filling solution chamber from the sample. Both these
devices allow the free yet very slow flow of the filling solution into the
sample solution. The blockage of this frit is the most common cause of pH
meter failure and is a consequence of either dry storage or contamination
- see below.
storage of pH electrodes: Dry storage causes dehydration of the
glass electrode and the precipitation
of salts within the interstices of the frit itself (Fig 10.8). 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 (Fig 10.6).
Note, pH buffers 7.0 or
4.0 and distilled water are not suitable for storing pH electrodes as they
can upset the electrode calibration.
contamination: Unless the ‘frit’ and glass tip of a pH meter
electrode is properly cleaned after use, it will invariably become
contaminated with impurities (Fig 10.7). This causes similar symptoms to
dry storage. Contamination may be so severe that re-calibration is not
possible without prior cleaning or replacement.
contamination: Further points to help maintain electrode
performance and longevity:
1. Experience indicates that real solutions
must generally be regarded as being unfriendly to electrodes. Hence, keep
their contact time with electrodes to a minimum. The following specific
precautions should be taken:
Avoid measuring harsh chemicals such as highly caustic solutions (e.g. pH
Up and silica additives), or concentrated solutions such as raw nutrient.
- Avoid placing the electrode in oily
solutions as they can “coat” the electrode.
- Filter water samples that are potentially a
problem (i.e. greasy) prior to performing a pH measurement.
2. Clean the electrode regularly using an
electrode cleaner. Be aware that most cleaners are only designed to remove
inorganic contaminants. Therefore if the nutrient solution contains
all-purpose cleaner may be required.