Metrohm tiamo 2.4 Manual - Manual (Page 953)

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5 Method
tiamo 2.4
￭￭￭￭￭￭￭￭ 937
Total volume 20 mL
Factor c
Standard solution
/c
Sam-
ple
20
This results in a sample concentration in the measuring solution of 2.5
mg/L. The optimum concentration of the standard solution is thus 2.5
mg/L · 20 = 50 mg/L. Please note that this is merely to be considered a
guideline for standard additions. Even if you deviate from this recommen-
dation, precise measurements will still be possible.
Principle
With the standard addition procedure, a known amount of the substance
to be determined is added to the sample once or more. This addition can
take place manually or automatically. As opposed to normal ion measure-
ment with ion-selective electrodes (see Chapter 5.6.5.6.1, page 1113),
the ions of interest and interfering ions cannot be distinguished in the
standard addition procedure, as both are present in the sample at the
beginning. Only the sum can be determined. Therefore, a linear correla-
tion between U and log(c
i
) is generally assumed for standard addition
measurements.
The regression line (linear regression) is thus determined iteratively accord-
ing to the method of least squares. This procedure provides the axis inter-
cept 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 vol-
ume 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) deter-
mined by tiamo can be calculated and directly displayed:
If instead of the sample volume the sample quantity m
S
(= Sample size in
g) is entered, the result c
res
is calculated by tiamo:
In order to obtain also the required end result c
S
(concentration of the ion
of interest in the sample solution) out of this, this must be calculated with