Drift; Ageing Of The Glass Electrode; Checking The Performance Of The Glass Electrode; Aging Of The Reference Element - ABB Sonde Ammoniac 8002 Manuel D'instructions

3.11 Drift

Drift of the potential of the probe at a rate greater
than 1mV in 12 hours should not be
experienced. If greater drift does occur it is
probably due to:
• inadequate temperature control;
• osmotic effects, i.e. total concentration of
dissolved species in samples too high;
• loss of ammonia from the sample;
• use of the probe in very strong ammonia
solution immediately prior to measurement;
• puncture of the membrane (usually visible);
• interference;
• ageing of the glass electrode – see Section
3.12.

3.12 Ageing of the Glass Electrode

After the probe has been in use for some
months, the performance of the glass electrode
may eventually deteriorate due to its continued
use in weakly buffered solutions at near neutral
pH. The response of the probe becomes
sluggish and the response slope drops. The
electrode may frequently be restored to its initial
condition by soaking for 12 hours in 0.1M
hydrochloric acid. The performance of the
electrode may be checked as described below.
3.13 Checking the Performance of
the Glass Electrode
The glass electrode
independently of the probe with a laboratory
calomel reference electrode (e.g. Model 1370
210) by testing the pair in pH buffer solutions.
The glass electrode should only be immersed to
a depth of 5 to 10mm. There should be no liquid
contact with the reference element.
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 90% of
the theoretical slope value.
may be checked
3 PROBE MEASUREMENTS

3.14 Aging of the Reference Element

The silver chloride coating of the reference
element gradually dissolves after many months
use. If drift is experienced in an old electrode it
could be due to this effect. Either replace the
inner electrode (Part
CAREFULLY follow
procedure in Section 7.1.
No. 8002-650) or
the chlorodisation
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