Metrohm 781 pH/Ion Meter - Manual (Page 149)

9 Annex
780/781 pH/Ion Meter, Manual
139
trolyte or an unsuitable reference system. The slope can also differ
slightly from UN.
As these variations describe the condition of the pH electrode and are
subject to variation with time this means that, depending on the accu-
racy required, a regular pH calibration is necessary. This calibration
provides a new description of the relationship between the electrode
potential and the measured pH value:
U/mV
4 7 9 pH
Slope
7
pH(0)
U
off
0
-100
100
200
Fig. 16: 3-Point pH calibration
pH(0) and slope are outputted by the pH/Ion Meter as calibration data.
In this case the slope (e.g. 99.5%) is a relative quantity which refers to
the temperature-specific value of the Nernst constant. It is given with a
positive sign, which is why the following explanations are extended by
including the negative sign.
Using this data the calibration curve can be described as follows:
( )
pH(0) pH U lope S U
N
− ⋅ ⋅ − =
resp.
N
U lope S
U
pH(0) pH
⋅
− =