Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 93)

178
Appendix: ISE Theory
PH6500, PC6500 and PCD6500 meters
Technology Made Easy ...
Incremental Methods
These procedures can reduce errors caused by temperature variations, complex matrices, and
complexation. They are also useful for applications where only occasional samples are analyzed.
Incremental methods include: Known Addition, Known Subtraction, Analyte Addition, and Analyte
Subtraction.
Known Addition and Known Subtraction
With the Known Addition and Known Subtraction methods, the initial millivolt output of the
electrode in the sample solution is measured. Then an aliquot of standard solution is added, and
a final millivolt potential is measured. The concentration of the sample can then be computed
from the observed change in the millivolt readings before and after the addition. The techniques
differ in that with Known Addition the standard added is the same ionic species as that being
measured. With Known Subtraction, the measured ion is not the same as the one being 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
s
= C std V std___
(V
s
+ V
std
)10
∆E/S
– V
s
Known Subtraction
C
s
= C std V std____
V
s
– (V
s
+ V
std
)10
∆E/S
where
C
s
is the concentration of the sample; C
std
is the concentration of the standard; V
s
is the sample
volume; V
std
is the standard volume; ∆E 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.