Aquaprobe
®
AP-5000 Instruction Manual
10401-00906 Rev M
16. Optional ISE Electrodes Calibration and Maintenance
During calibration of the ISE electrodes, very special care must be taken to achieve the
specified temperatures. In order to help reduce the Probe's thermal mass and thereby
speed up temperature stabilisation, it is advisable to carry out ISE calibration with the
sleeve removed. In this condition, however, the individual electrodes are very vulnerable so
care should be taken in order to avoid damage.
16.1. ISE Electrode Limitations
All ion selective electrodes suffer from interference from ions which are similar in nature to
the target ion. For this reason, ISE Electrodes are not recommended for use in brackish or
salt water due to the high level of interfering ions.
In order to achieve accurate readings with ISE electrodes, the Probe needs to be either
placed in flowing water, or needs to be stirred or raised and lowered continuously to ensure
a minimum flow rate of 0.3m/s over the electrode.
If there is no water flow across the ISE
electrode, the ions in the immediate area of the electrode will be depleted and the
reading will start to fall. This also applies to calibration, where the probe should be
stirred at all times.
16.2. Calibration Points
All ISE electrodes have three calibration points. Careful calibration is essential in order to
ensure consistent and reliable results. Prior to initial calibration, all ISE Electrodes should
be soaked in their relevant Point 1 calibration solution for 20 – 30 minutes.
When an ISE electrode is first installed,
it MUST be calibrated at three points
in order to
establish the electrode's slope and thermal characteristics. Two of the calibration points
must be at the same temperature whilst the third must be at least 10ºC cooler.
Subsequently, a two-point calibration should be carried out weekly and a single point
calibration should be carried out daily. The ISE electrode should be replaced every 6-12
months.
16.3. Special Notes Concerning ISE Electrodes
The high ionic concentration of pH calibration
solutions (buffers), including RapidCal, can
cause significant offsets in ISE electrodes.
These offsets are temporary, but best avoided
because they can cause significant errors
during both calibration and normal operation.
For this reason all ISE electrodes are supplied
with a red rubber sealing cap.
The caps should be fitted to all ISE
Electrodes during pH calibration or when
using RapidCal
in order to protect the ISE
electrodes from the effects of the buffer solution.
At all other times, the ISE electrodes should be left uncovered.
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®
Ltd.
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