Thermo Fisher Scientific CyberScan PC 5000 - Manual (Page 167)

Instruction Manual CyberScan PC5500/ 5000
162
Known addition and Known Subtraction
With the Known Addition and Known Subtraction methods, the initial
millivolts output of the electrode in the sample solution is measured.
Then an aliquot of standard solution is added, and a final millivolts
potential is measured. The concentration of the sample can then be
computed from the observed change in the millivolts readings before
and after the addition.
The techniques differ in that with Known Addition the standard added
via the standard. For example, a chloride ISE can be immersed in
sample for a chloride measurement. If a silver standard is added to
the sample, AgCl precipitates, effectively removing or subtracting
chloride ion from the sample.
Equations for known addition and known subtraction calculations are
as follow:
Known Addition
C V
(V +V )10 - V
std
s std 5
UE/S
std C =
s
Known Subtraction
C V
V - ( V )10
std
s std
UE/S
std
V +
s
C =
s
C
s
is the concentration of the sample; C
std
is the concentration of the
standard; Vs is the sample volume; V
std
is the standard volume;
UE
is the difference in millivolt readings from the electrode; and S is the
slope of the electrode determined in a previous standardization.
Whereas the direct method is suitable for samples with fairly
uncomplicated ionic matrices, the addition methods are
advantageous with samples that have complex matrices, samples in
which no ISA is added, samples in which excess complexing species
are present, and samples whose concentration is in the non-linear
region on the electrode response curve.