Metrohm tiamo 2.4 Manual - Manual (Page 1130)

5.6 Commands
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1114 ￭￭￭￭￭￭￭￭
tiamo 2.4
Parameters
The parameters for the CAL LOOP Conc command are configured in the
following dialog window:
￭ CAL LOOP Conc - Properties
Principle
For calibrating ion-selective electrodes (ISE), a linear correlation of the
measured potential U
i
and the logarithm of the sought ion concentration
c
i
cannot always be assumed. In ion measurements, interfering ions have
an influence on the measurement and that is why this effect is perceptible
by a flattening of the curve, especially in low concentration ranges.
The influence of interfering ions can be described by the Nikolsky equa-
tion. This is an extended Nernst equation which takes the selectivity of the
ISE into account. For the ISE calibration with tiamo, it is used in the fol-
lowing form:
( )
B i
N
i
c c
z
U
E U + + = log ) 0 (
The Nernst constant U
N
represents the theoretical slope s of the calibra-
tion function, divided by the charge z of the measuring ion. With anions it
has a negative sign, with cations a positive one.
The influence of the interfering ions is indicated as blank value concentra-
tion c
B
. A significant blank value reduces the lower measuring range and
therefore causes a deterioration of the limit of detection.
For the calibration, there are thus three unknown quantities to be calcu-
lated, namely E(0), s and c
B
. In order to unambiguously calculate all
parameters, at least three standard measurements are therefore neces-
sary. A higher number of standards increases the reliability of the mathe-
matical calculation of the calibration function.