2 calibration for ion measurements, Calibration for ion measurements – Metrohm 781 pH/Ion Meter User Manual
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9.2 Evaluation
142
780/781 pH/Ion Meter, Manual
9.2.2
Calibration for ion measurements
Just as for pH measurement, ion measurement using an ion-selective
electrode (ISE) also requires regular calibration. In contrast to pH
measurement, a linear relationship between the measured potential Ui
and the logarithm of the required ion concentration ci cannot always be
assumed. The influence of the interfering ions which is responsible for
this can be described by the Nikolsky equation. This is an extended
Nernst equation which takes the selectivity of the ISE into consideration.
For the ISE calibration with the pH/Ion Meter it is used in the following
form:
(
)
)
blank
(
c
c
log
z
U
)
0
(
E
U
i
N
i
+
⋅
+
=
The Nernst constant UN represents, when divided by the charge z of
the ion being measured, the theoretical slope of the calibration function.
For anions it is given a negative sign, for cations a positive sign.
The influence of the interfering ions is given as the 'blank value concen-
tration' c(blank). A significant blank value reduces the lower measuring
range and therefore gives a poorer detection limit.
This means that during the calibration three unknown quantities, i.e.
E(0), Slope and c(blank) must be determined for the calibration func-
tion. In order to be able to calculate all the parameters properly this
means that at least three standard measurements are required. A high-
er number of standards increases the reliability of the mathematical
calculation of the calibration function.
As in a pH calibration, the pH/Ion Meter determines the calibration data
for the measurement of the concentration according to the number of
standard solutions used:
1-Point calibration
Slope:
The existing value is retained, if available. Otherwise the
theoretical slope
U
N
is used.
E(0)
=
)
c
log(
Slope
U
1
1
⋅
+
c(blank): Is set to zero and not shown.
2-Point calibration
Slope
=
)
c
log(
)
c
log(
U
U
1
2
1
2
−
−
E(0)
=
)
c
log(
Slope
U
⋅
+
;
)
c
log( , U= mean value
c(blank): Is set to zero and not shown.