cytiva pH/C-902 Manual de instrucciones - Página 4

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Calibrating the pH electrode
A proper laboratory routine is to calibrate the instrument once a day,
when the electrode is replaced and if the ambient temperature is
changed. The pH monitor is calibrated using standard buffer
solutions in a two point calibration. The two buffer solutions can have
any pH value as long as the difference between them is at least 1 pH
unit.
The calibration can be performed with the pH electrode either fitted
in or removed from the flow cell.
Calibrating with the electrode outside the flow cell
When calibrating the electrode out of the flow cell and changing from
one buffer to another, rinse the electrode tip with distilled water and
dab it carefully with a soft tissue to absorb the remaining water. Do
NOT wipe the electrode as this may charge it and give unstable
readings.
Step
Action
1
Remove the pH electrode from the flow cell and immerse the
electrode in the first standard buffer solution (normally pH 7.0).
Select System:Calibrate in the UNICORN System Control
2
module.
Result: The Calibration dialog box opens.
In the Monitor box, select pH_AIT121.
3
4
Enter the first reference pH value in the Reference value 1
box.
Click the Read Value 1 button.
5
Result: The measured value is displayed in the Measured
value box.
6
Rinse the electrode tip with distilled water and then immerse
the electrode in the second standard buffer solution (e.g. pH
4.0 or 9.0).
Enter the second reference pH value in the Reference value 2
7
box.
Click the Read Value 2 button.
8
Result: The measured value is displayed in the Measured
value box.
The Calibrated electrode slope box shows the slope of the
calibration curve where 100% corresponds to 59.16 mV per pH step at
25°C. The Asymmetry potential at pH 7 is shown as a mV value.
9
Before use, rinse the electrode using distilled water.
Calibrating with the electrode in the flow cell
Follow the same procedure as above. Ensure that the pH value has
stabilized before reading. Leave the pump running while calibrating.
Switch to the second standard buffer solution and repeat the
procedure.
4
Fig 3. Relationship between pH and the monitor output signal in mV
A new electrode typically has a slope of 95 – 102% and an asymmetry
potential within ±30 mV. As the electrode ages the slope decreases
and the asymmetry potential increases.
As a rule, an electrode should be replaced when it has an asymmetry
potential outside of ±60 mV and a slope lower than 80%, and no
improvement can be achieved by cleaning.
An electrode is still usable at lower slopes and higher asymmetry
potentials but the response will be slower and the accuracy
diminished.
Setting up pH temperature compensation
The relationship between pH and the output signal from the pH
electrode is temperature dependent. For more accurate
measurements during temperature changes, the pH measurement
can be temperature compensated. In normal applications, if the
temperature of the buffers and calibration buffers are identical,
temperature compensation does not need to be on.
When using temperature compensation it is important that the
temperature of the pH electrode is the same as that of the
conductivity flow cell since that is where the temperature is
measured.
The temperature compensation is activated in the UNICORN System
Control module:
Step
Action
Select System:Settings.
1
Result: The Instructions dialog box opens.
Select Monitors.
2
Select pH_AIA121_TC.
3
4
Click the ON radio button.
5
Click OK to close the dialog box.
Storage
CAUTION
Never leave the pH electrode in the flow cell for
any period of time when the system is not used
since this may cause the glass membrane of the
electrode to dry out.
pH/C-902 Instruction 14003958 AB