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7.6. pH-Calibration
7.6.1. General reference notes for pH-calibration
Combined electrodes are subject to a regular wear and tear and thus, should be
calibrated regularly. The pH-drift is compensated at the start of the analysis when
the automatic reading of the pH-value is compensated. However, as soon as the
reaction time of the electrode is > 30 s a check or a replacement of the electrode
is advisable.
A two point calibration with buffer solution pH 4 and pH 7 is done. The deviation
from zero is obtained with buffer solution pH 7 and the steppness with buffer so-
lution pH 4.
The values obtained are shown in [mV/pH] and [%], values accepted are a zero
point of 0 mV and a steppness of 59 mV/pH (100 %). The tolerance range of the
deviation from zero is +/- 20 mV. The steepness is accepted between 50 mV/pH
and 70 mV/pH. The ideal value with buffer solution pH 4 is at 177 mV and at a
pH-value of 7 at 0 mV.
Another criterion for the state of the electrode is the reaction time. If the electrode
reacts slowly during the calibration and if there is no stable measurement within 30
s, the calibration is not accepted. Should one of the tolerance ranges be exceeded
during the calibration, there is a respective error message and the calibration has
to be started again with a new buffer solution or with a new electrode.
Ideal temperature for buffer solution is 20 °C!
Reference note:
Usually the life span of the electrode is 1 year. If there is a slow measuring, make
sure to exchange the electrode. In order to guarantee a perfect function of the
instrument make sure to use original electrodes from C.Gerhardt only.
Make sure to read the reference notes in the instruction manual enclosed
which informs you abut handling and care of the electrode.
7.6.2. Run pH-calibration
From the menu bar select System > Titrator > pH-calibration or press the push
button "pH-Calibration", to open the following window.
Follow the instructions on the screen and immerse the electrode, which has been
rinsed with water and dried carefully (don’t rub it dry!!) in buffer solution pH7. Start
procedure by pressing key "Calibration" or by pressing of the Vapodest-function
button. The tension measured and the 30s stabilisation time counted backwards
are displayed. As soon as a measurement is stable for 2s, there is an automatic
switch to the second buffer solution.
7 System settings
Electrode
in Buffer pH7
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