Metrohm NH3-selective electrodes - Manual (Page 17)

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6 Performing measurements
NH
3
-selective electrodes
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6.4 Performing standard addition
Standard addition is a considerably more convenient method for the mea-
surement of ion concentrations with an ion-selective electrode (ISE). If you
need to measure only a few samples, then this method is very simple,
because you do not need to incorporate a calibration curve. Furthermore,
all matrix effects are eliminated, because the electrode is calibrated in the
sample solution.
Observe the following notes when performing standard additions:
￭ Add defined volumes of a standard solution of the measurement ion to
the sample in a series of steps.
The concentration in the sample solution is calculated on the basis of
the resulting change in voltage and the initial voltage prior to the addi-
tion of the standard solution. Modern ion meters, such as the 781 pH/
Ion Meter or the 867 pH Module, can perform a standard addition
automatically when a corresponding dosing device is attached to the
instrument.
￭ In order to guarantee a reliable evaluation of the standard addition,
care must be taken to ensure that the buret volume and the concentra-
tion of the standard solution are adjusted to the respective measuring
conditions. The individual addition volumes must be selected in such a
way that the potential difference is at least 15 mV after each additive
step. At least four volume additions should be carried out.
Table 3 Recommended concentrations of the standard solution for the
standard addition
Buret volume in mL c
Standard solution
: c
Sample
5 40 : 1
10 20 : 1
20 10 : 1
50 5 : 1
Example of a standard addition
NOTE
This sample is valid for standard additions with any number of ion-
selective electrodes.
Sample concentration 5 mg/L