Indirect Methods
The standard addition technique with one addition is very
suitable for analysing samples in which the ionic strength and
interferences are moderate to high and variable, or when the
sample contains lead ion complexing agents. With the standard
addition method, the electrodes are immersed in the sample and
the potential (E) is measured. An aliquot of standard solution
containing lead ions is added to the sample and the potential (E
is measured. As a general rule, the addition of standard should
double the concentration. For Pb
change between 5 and 15 mV will normally be sufficient.
Upon completion of the measurement, the concentration of Pb
in the sample is calculated as follows (with the PHM250 the
calculations are done automatically):
c
= ±
smp
(V
+ V
smp
Where:
c
=
concentration of Pb
smp
c
=
concentration of Pb
std
V
=
sample volume (mL)
smp
V
=
total volume of standard added (mL)
std
E
=
potential measured before first addition/
subtraction (mV)
E
=
potential measured after addition/
1
subtraction (mV)
S
=
electrode sensitivity (mV/pC)
In addition, the standard subtraction technique can be per-
formed. This involves the addition of known amounts of reagents
which react with the Pb
negative sign) is used for result calculation.
Analate addition techniques can also be applied. During an
analate addition measurement, the sample is added to the
standard instead of vice versa.
2+
c
• V
std
std
[E
- E / S]
) • 10
1
std
2+
in sample
2+
in standard
2+
in the sample. The same formula (with
9
determination, a potential
- V
smp
)
1
2+