pH Adaptor & Electrode
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of a calibration curve is required to convert the mV reading to ppm or Log ion concentration reading.
Refer to the manufacturer’s guide for the ion-selective electrodes (ISE) and oxidation reduction
probes (ORP) for details of dilutions for calibration and mV slope values.
User calibration
If required the calibration constants of a pH electrode can be adjusted. The settings for an electrode
will be stored in the Adaptor as the User pH calibration.
Note: Mark the pH electrode and adaptor combination so they are used as a pair.
Standardised buffer solutions are used to adjust the sensor reading at either two or three points in
its range. A slope adjustment is made using these points and will affect the whole range, between
and beyond these points. The accuracy of the user calibration will depend upon the number of
calibration points used and their spacing. Ideally the buffer solutions used should encompass the
expected pH range and be as close as possible to the pH of the samples being measured.
A User calibration is best used when the:
experiment requires a very accurate calibration
electrode has aged to the point where its glass membrane has changed resistance
samples to be measured are at a lower or higher temperature than 25˚C. The buffer solutions
used to set values must be at the same temperature as the samples in the experiment. Buffers
values are temperature sensitive, enter a pH value for the buffer at that temperature.
Values of pH buffers a various temperatures:
Temperature ˚C
pH 4.0 buffer
pH 7.0 buffer
pH 10.0 buffer*
0˚
4.00
7.11
10.32
10˚
4.00
7.06
10.18
20˚
4.00
7.01
10.06
30˚
4.02
6.98
9.97
40˚
4.04
6.97
9.89
50˚
4.06
6.97
9.83
*Please note that high pH buffers are less stable as they tend to absorb atmospheric CO
2
which
lowers their pH. Only open the bottle of buffer to pour into a beaker, never leave the bottle open
.
How to calibrate
1.
Change the sensor’s range to User Cal.
2.
Select the Calibrate button.
3.
If only two samples of buffers are being used
select the down symbol for Calibration
Type, then Two point from the list.
4.
Type in the value of all the buffers being used
to set points into the appropriate boxes.
5.
Rinse the electrode in distilled water. Wipe off
the excess and suspend the electrode in the value
one buffer, stir and select Calibrate.
6.
After the 20 second count rinse the electrode in
distilled water, wipe off excess, suspend in the
value two buffer, stir and select Next.
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