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70
ION SELECTIVE ANALYSIS METHODS
Direct Analysis
This method is a simple procedure for measuring multiple samples. It should only be used in the
linear working regions of the sensor. A direct reading instrument such as the
HI5222 determines
concentration of the unknown by a direct reading after calibrating the instrument with the standards.
The instrument is calibrated as described in “ISE CALIBRATION & MEASUREMENTS” section, with two
or more freshly made standards that are in the measurement range of the unknowns. Ionic strength
adjustment is made to samples and standards. Unknowns are measured directly by the instrument.
At lower concentrations, in non-linear regions of the electrode response, multiple calibration points will
extend measurements to a practical detection limit. Calibrations must be performed more frequently
in these cases.
Incremental Methods
Incremental methods are useful for the measurement of samples whose constituents are variable
or concentrated. Incremental techniques can reduce errors from such variables as temperature,
viscosity, or pH extremes and will provide indirect analysis of ions for which there is no ISE sen-
sor for a direct measurement. There are four commonly used different incremental methods for
sample measurement. They are Known Addition, Known Subtraction, Analyte Addition and Analyte
Subtraction.
HI5222 allows the analyst to use these techniques as a simple routine procedure, thus
eliminating calculations or tables. The method once set up can be used for repetitive measurements
on multiple samples.
Known Addition and Known Subtraction
With Known addition, standard is added to a sample being measured. The standard and sample
contain the same ion. mV are taken before and after the standard addition. From the change in
mV, the sample concentration is determined.
E = E
o
+ S log(C)
C
SAMP
=
C
STD
· V
STD
·
(V
SAMP
+ V
ISA
)
(V
SAMP
+ V
STD
+ V
ISA
)·10
∆E
- (V
SAMP
+ V
ISA
) V
SAMP
S
In very dilute solutions γ approaches 1 so activity and concentration are the same.
Actual samples that are more concentrated have much smaller activity coefficients (γ < 1). The
addition of an inert background salt to standards and samples stabilizes the activity coefficient so
that concentration measurements may be made directly. Some of Hanna Instruments’s Ionic Strength
Adjuster formulations also may optimize pH and complex interferences, in addition to standardizing
the ionic strength.
The Nernst equation can be written:
ADDITIONAL INFORMATION
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