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...4 ROUTINE MAINTENANCE
Heavy Non-Greasy Deposits: (e.g. rust, chalk)
Wipe` with cotton wool soaked in concentrated HCl to remove
the deposit, and rinse thoroughly. Soak the electrode for a few
hours in 0.1M HCl and then wash in water. An alternative
treatment is to wipe the electrode and soak it overnight (or
longer if necessary) in a 10% solution of E.D.T.A. followed by
reactivation in 0.1M HCl and rinsing in water.
Heavy deposits formed during effluent neutralisation are often
removed by using a 1:1 mixture of concentrated HCl and
water.
4.3.2 Platinum Electrodes:
In general, the same procedures may be used as for glass
electrodes. They may also be cleaned cathodically.
4.3.3 Antimony Electrodes
Emery paper or a file should be used
4.3.4 Ceramic Plug:
The same procedures may be used as for glass electrodes,
but lengthy soaking should not be necessary. In general, the
ceramic plug need not be removed. A heavily fouled ceramic
plug may still perform satisfactorily, but if found to be the
source of reduced system performance the plug and seal
should be replaced together. The seal is removed by careful
easing out with a spanner or the special wrench provided.
4.4 Care of Plugs and Sockets
Plugs and Sockets should be examined every time they are
disconnected. Dirt or grease should be removed by wiping with
a clean cloth moistened with a solvent e.g. propanol. The
centre contact of sockets should also be examined to make
sure that the two halves have not spread too far apart to grip
the plug contact firmly. Gentle pressure with pliers should be
used to bring the contacts together again.
4
5 SERVICING
5.1 Removal of Electrode Assembly
The procedure is as follows:
a) Disconnect the waterproof plug on the coaxial cable
connected to the electrode system from the coaxial socket
connected to the measuring instrument.
b) Fit the waterproof cap to the coaxial plug (when supplied)
c) Remove the system tube from the sampling point, ensuring
that the process solution is not under pressure. KEEP
TUBE UPRIGHT.
d) Rinse off the solution so that the electrode assembly may
be handled.
e) Remove the split rubber bung holding the coaxial cable in
place in the upper end of the system.
f) Unscrew the cap in the upper end of the system and empty
the salt bridge solution into a clean vessel (the solution
may be kept and used for refilling).
g) Hold the end of the electrode assembly in one hand and
the lower part of the system tube in the other. Unscrew the
electrode assembly anticlockwise until it is free of the
system.
h) Remove the screw cap from the coaxial cable by passing it
over the plug end.
i) Gently draw the coaxial cable and plug through the system
tube so that the electrode assembly and cable are free.
j) Hold the system tube upside down and wipe the inside,
where the electrode assembly fits, with absorbent paper.
This prevents a layer of KCl crystals forming as any salt
bridge solution left dries out.
5.1.2 Replacement of Electrode assembly
Carry out the procedure described under 3 PREPARATION
FOR USE, removing the cap from the reference element if the
assembly is a new one.
5.2 Fault Finding
The location of a fault is best found by fitting a replacement
electrode assembly and carrying out a standardisation check.
If changing the electrode assembly clears the fault, then the
assembly is faulty and should be checked to see if it can be
restored to working order. If, after cleaning and reactivating the
electrode assembly, cleaning or changing the ceramic plug
and refilling with fresh salt bridge solution, there is no
improvement, then the old electrode assembly should be
discarded.
If the fault is in neither the electrode assembly nor the ceramic
plug, it must be assumed that the measuring instrument or its
connections are faulty.
The following is a short (and by no means exhaustive) list of
fault symptoms and the possible causes due to faulty
electrodes.
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