D664_8 (17a) 04-2018
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When a silver/silver
chloride reference electrode
is used for the titration and
it contains an electrolyte which
is not 1 molar-3 molar
lithium chloride in ethanol,
replace the electrolyte.
Drain the electrolyte
from the electrode.
Wash away all the salt, if
present, with water, and then
rinse with ethanol.
Rinse several times with
the lithium chloride
electrolyte solution.
Finally, replace the sleeve
and fill the electrode
with the lithium
chloride electrolyte
to the filling hole.
When refitting
the sleeve, ensure
that there will be a free flow
of electrolyte into the system.
A combination electrode shall
be prepared in the same manner.
The electrolyte in a
combination electrode
can be removed with the
aid of a vacuum suction.
Test the meter
electrode combination
when first put into use or when
new electrodes are installed
and retest at
intervals thereafter.
Rinse the electrodes with
solvent, then with water,
and dip them into a pH 4
aqueous buffer solution.
Read the millivolt value
after stirring one minute.
Remove the electrodes
and rinse with water.
Dip the electrodes into
a pH 7 aqueous buffer.
Read the millivolt value
after stirring one minute.
Calculate the
millivolt difference.
A good electrode system
will have a difference
of at least 162
millivolts, 20 degrees
Celsius to 25 degrees Celsius.
If the difference is
less than 162 millivolts,
lift the sleeve of the electrode
and ensure electrolyte flow.
Repeat the measurements.
If the difference is still
less than 162 millivolts,
clean or replace the electrodes.
When the sensing electrode
and the reference electrode
are separate, one
pair of electrodes
shall be considered as one unit.
If one or the other
is changed, it
shall be considered as different
pair and shall be retested.
Maintenance and
storage of electrodes--
cleaning the
electrodes thoroughly,
keeping the ground glass joint
free of foreign materials,
and regular testing
of the electrodes
are very important in
obtaining repeatable potentials
since contamination
may introduce
uncertain erratic and
unnoticeable liquid contact
potentials.
While this is of
secondary importance
when the endpoints are
chosen from inflection
points in the
titration curve, it
may be quite serious
when endpoints
are chosen at arbitrarily
fixed cell potentials.
See appendix X1 of
the written standard
for a possible
procedure to check
the electrode performance.
Clean the glass electrode
at frequent intervals
based on use and type of
samples being analyzed,
not less than once every
week during continual use
by immersing in
non-chromium-containing,
strongly-oxidizing
cleaning solution.
The reference electrode
shall be cleaned periodically
when in use or when a new
electrode is installed.
Drain the reference electrode
at least once each week
and refill with a
fresh lithium chloride
electrolyte as far
as the filling hole.
Ensure that there
are no air bubbles
in the electrode liquid.
If air bubbles are
observed, hold the electrode
in a vertical position
and gently tap
it to release the bubbles.
Maintain the electrolyte level
in the reference electrode
above that of the liquid
in the titration beaker
or vessel at all times.
Prior to each titration,
soak the prepared electrodes
in water, pH 4.5 to 5.5,
for at least 5 minutes.
Rinse the electrodes with
isopropyl alcohol immediately
before use and then with
the titration solvent.
When not in use,
immerse the lower half
of the reference electrode in
lithium chloride electrolyte.
When the glass
electrode is used,
store it in water that
has been acidified
with hydrochloric acid
to a pH of 4.5 to 5.5.
Do not allow electrodes to
remain immersed in titration
solvent for any
appreciable period of time
between titrations.
While the electrodes are
not extremely fragile,
handle them carefully
at all times.
Electrode life--
typically, electrode usage
is limited to three to six
months, depending upon usage.
Electrodes have a
limited shelf life
and shall be tested before use.
See 8.2 of the written standard.