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...8 MAINTENANCE
Connect the electrodes to a pH meter and calibrate with pH
buffers in the usual way. The glass electrode may be found to
be somewhat slower in response than a conventional bulb–
type electrode, but adequate scale length should be obtained,
typically better than 98% of the theoretical slope value.
8.5.3 Reference Element
During the lifetime of the probe, the internal reference element
may show signs of deterioration, indicated by the removal of
the grey/brown silver chloride coating, exposing the buff/light
grey silver element beneath. This is a normal process, the rate
of removal depending on probe operating conditions and
maintenance.
When new and in good condition this element is completely
and evenly coated. Loss of a large part of this coating causes
probe drift, but can be restored by re–chloridising the element,
using the following procedure:
8.5.4 Procedure for Chloridising the Reference
Element
Requirements –
•
Plating solution – hydrochloric acid (HCl) 0.1M, 500 ml.
Note. Do not use acid preserved with mercuric ions.
•
Cleaning solutions – Ammonia solution (NH
vol., 200 ml.
To prepare, dilute 100 ml analytical reagent grade
concentrated ammonia solution, s.g. 0.88, with 100 ml
distilled water and stir.
•
Nitric acid (HNO
), 25% vol./vol. 200 ml.
3
To prepare; cautiously pour 50 ml analytical reagent
grade, concentrated nitric acid, s.g. 1.42, into 150 ml
distilled water, stirring continuously. Allow to cool before
use.
Warning. Stored ammonia solutions and concentrated
nitric and hydrochloric acids are irritants and very
corrosive. Take great care not to inhale the fumes of either
or to get any on the skin or clothing. Any slight spillage on
skin, clothing or working surface must be washed away
with plenty of water.
•
Constant current supply, 2 mA d.c. output.
•
Silver wire (counter electrode).
•
1 beaker.
Cleaning –
a) Immerse the lower 50 mm of the combination electrode
(i.e. so that the silver reference element is covered) in the
ammonia solution for about 1 minute. Remove and rinse
with distilled water.
b) Immerse the electrode to the same depth in the 25% nitric
acid until the silver element is a uniform creamy white. This
process usually takes about 1 minute, but the element
must be inspected frequently as prolonged immersion is
detrimental.
36
c) If the element is not uniform in colour repeat the ammonia/
nitric acid process.
d) When clean, rinse with distilled water and immediately
transfer to the plating solution such that the reference
element is completely immersed.
Chloridisation –
a) Attach the screen of the combination electrode cable to the
negative terminal of the constant current supply and
immerse the lower 50 mm of the electrode in the plating
solution so that the reference element is covered. Immerse
a silver counter electrode in the plating solution and
connect to the positive terminal of the supply.
b)
Pass a current of 2 mA for approximately 30 seconds.
Carefully tap the electrode to remove all bubbles and then
reverse the connections to the supply (i.e. the electrode to
be plated is connected to the positive terminal).
c)
Pass a current of 2 mA for 30 minutes after which
time the electrode will be a dark brown or grey colour. A
more uniform coating may be obtained by employing
moderate stirring during the process.
d)
Remove the electrode from the plating solution, rinse
with distilled water and dry with a tissue.
) 50% vol./
3
8.6 Microprocessor Unit Error Messages
The instrument incorporates an automatic self-diagnostic
checking facility for detection of input and output errors. If such
a fault occurs, one of the error messages detailed below is
shown on the dot matrix display.
'CHAN 1 INPUT ERROR'
This error message is usually caused by an open circuit
sensor input allowing the input to drift outside the range of
±400 mV. This may be due to any of the following:
a) Low level of filling solution in the probe.
b) Open
connection.
c) Broken electrical connections between the probe and
the Microprocessor Unit sensor input.
d) Sensor coaxial plugs not correctly inserted.
'TEMP INPUT ERROR'
'PRT OUT OF LIMITS'
'TEMP REF. ERROR'
'THIRD LEAD ERROR'
The above four error messages refer to electrical problems
with the temperature sensor input – usually an open or short-
circuited connection.
circuit
electrode
internally,
e.g.
broken
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