Metrohm tiamo 2.2 (ProcessLab) - Manual (Page 1057)

￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭ 5 Method
tiamo 2.2 (ProcessLab) ￭￭￭￭￭￭￭￭ 1041
The influence of interfering ions can be described by the Nikolski equation.
This is an extended Nernst equation which considers the selectivity of the
ISE. For the ISE calibration with tiamo it is used in the following 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 calibration
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 concentration
c
B
. A significant blank value reduces the lower measuring range and there-
fore causes a deterioration of the detection limit.
For the calibration, there are thus three unknown quantities to be calculated,
namely E(0), s and c
B
. In order to unambiguously calculate all parameters,
at least three standard measurements are thus necessary. A higher number
of standards increases the reliability of the mathematical calculation of the
calibration function.
tiamo determines the calibration data for the concentration measurement
as for the pH calibration depending on the number of the standard solutions
used.
￭ 1 point calibration
– s: the existing value is kept if available. Otherwise the theoretical
slope U
N
/ z is used.
–
– c
B
: is set equal to 0.
￭ 2 point calibration
–
–
– c
B
: is set equal to 0.