Aquaprobe
®
AP-7000 Instruction Manual
10401-00894 Rev M
13.4. Calibrating pH
pH electrodes should be calibrated fully at least once a week to ensure optimum accuracy.
Full calibration involves calibrating at pH 7.00 first, then at pH 4.01 and/or pH 10.00. The
AP-7000 allows for both two and three point pH calibration. Should you decide to carry out
just a two point calibration, the probe will automatically calculate and save a calibration
value for the uncalibrated third point in order to maintain the electrode's linearity over the
full range of 0 – 14.
For best results, calibrate all three points as close to 25
C as possible.
13.4.1.
Special Notes Concerning ISE Electrodes during pH Calibration
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
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.
13.4.2.
Calibrating the First Point (pH 7.00)
Due to the way in which pH calibration works,
the Probe must be calibrated at pH7.00
before calibrating at pH 4.01 or pH 10.00
.
Never calibrate at pH 4.01 or pH 10.00
before first calibrating at pH7.00.
To calibrate the pH electrode follow these steps:
1. Pour 400mL of fresh pH 7.00 solution or RapidCal in a calibration cup, remove the
storage cap from the pH electrode, wash the Probe in distilled water, then gently
lower the Probe in all the way.
2. If you do not have any ISE caps fitted, activate the probe cleaning feature in order to
remove any air bubbles that may be clinging to the electrodes. To do this, press the
MENU key and select CLEAN PROBE.
If you have ISE caps fitted, agitate the
Probe to eliminate air. Do not activate the cleaning mechanism as the brush
will jam against the ISE caps.
3. Wait until the temperature and pH measurements are completely stable.
4. Ensure the temperature of the solution is between 5ºC and 40ºC (41ºF – 104ºF).
5. Press the
MENU
key then select
Calibration.
The following screen will be displayed.
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®
Ltd.
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