Metrohm tiamo 2.2 (ProcessLab) - Manual (Page 898)

5.6 Commands ￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
882 ￭￭￭￭￭￭￭￭ tiamo 2.2 (ProcessLab)
5.6.4.2 Standard addition - Overview
Menu item: Method ▶ Insert ▶ New command... ▶ Measure
Principle
With the standard addition procedure an known amount of the substance
to determine is once or more than once added to the sample, whereby the
addition can be carried out manually or automatically. In contrast to the
normal ion measurement with ion-selective electrodes (see Chapter
5.6.5.6.1, page 1040), with the standard addition procedure it cannot be
differentiated between ions searched and interfering ions, because both are
present in the sample at the beginning. Only the sum can be determined.
Therefore a linear correlation of U and log(c
i
) is generally assumed for stand-
ard addition measurements.
This way, the regression line (linear regression) is determined iteratively
according to the method of least squares. This procedure provides the axis
intercept E(0), the slope s and the concentration of the measuring ion in the
diluted measuring solution c
A
.
The dilution is taken into account via the method parameters sample volume
V
S
(= Sample size in mL) and addition volume V
Add
, so that the end result
c
S
(concentration of the ion searched in the sample solution) determined by
tiamo can be calculated and directly displayed:
If instead of the sample volume the sample amount m
S
(= Sample size in
g) is entered, the result c
res
is calculated by tiamo.
In order to obtain the required end result c
S
(concentration of the ion
searched in the sample solution) out of this, this must be calculated with the
aid of a CALC command according to the following formula (
ρ = density in
g/mL):