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...3 PREPARATION FOR USE
3.4 Standardisation Redox
A method of checking redox electrode pairs
The normal method of checking a redox pair is to immerse the
electrodes in solutions of known redox potential. The
standards used are quinhydrone/pH solutions prepared by
making up standard ABB Kent-Taylor 4 and 7pH buffer
solutions and saturating both by adding excess quinhydrone.
A redox electrode pair is then immersed in the solutions in turn
and the readings noted. The values of the e.m.f. as measured
at 25°C are shown below. Readings should be within ±15mV.
Table 3.1 E.m.f. values for Pt vs Ag/AgCl Reference
Electrode
S
l o
t u
o i
n
4
p
H
7
p
H
t P
v
A
g
A /
g
l C
+
2
5
9
m
V
+
8
2
m
V
4 ROUTINE MAINTENANCE
This consists of keeping the system topped up with salt
bridge solution, checking the standardisation of the
electrodes against buffer solutions, and making periodic
checks to ensure that the electrode membrane and ceramic
plug are clean.
4.1 Salt Bridge Solution
The saturated KCl salt bridge solution should be topped up
weekly. The level must not be allowed to fall below the
transparent perspex section, otherwise it could fall undetected
below the entry tube of the reference element.
4.2 Standardisation
Standardisation should be carried out at regular intervals. The
precise interval will depend on the use of the electrode and
cannot be stated definitely, but normally should take place
weekly, or more frequently if conditions demand it. At first,
carry out daily checks, and then extend the interval in the light
of experience.
4.3 Cleaning
a) The need for cleaning the electrode becomes apparent
when the response becomes sluggish, with possibly a
falling off in calibration.
b) If the cleaning procedures described below do not
eliminate these symptoms, the electrode assembly and/or
ceramic plug should be changed.
c) During cleaning, avoid rough handling of the electrode
membrane. Wiping with a cloth soaked in suitable solvent
for the deposit, or washing with a strong jet of water, are
preferred methods.
d) Where the cleaning process for the electrode or ceramic
plug necessitates soaking for a time, it is advisable to
exchange the electrode system with a standby system, so
that there is least interruption to the measurements.
Warning. DO NOT USE ORGANIC SOLVENTS THAT
MAY ATTACK THE ABS PLASTIC MATERIAL USED IN
THE
CONSTRUCTION
ELECTRODE ASSEMBLY.
4.3.1 Glass Electrodes
Methods of removing various types of deposit are given below:
General sludge and loosely adhering matter:
Direct a strong jet of water onto the glass membrane.
Light Inorganic Deposits:
Wipe the glass with cotton wool soaked in 0.1M HCl and then
wash in water before standardising.
Greasy Organic Deposits:
Wipe the glass with cotton wool soaked in a non-ionic
detergent and rinse thoroughly. If sluggishness is still
observed, soak the electrode for a few hours in 0.1M HCl and
then wash in water before standardising.
OF
THE
SYSTEM
AND
3
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