98
99
The Nernst equation can be rewritten:
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
HI 5522
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” 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 sensor 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.
HI 5522
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.
With Known subtraction,
a known standard is added to an ionic sample
being measured. The standard reacts
with the measured Ion in the sample in a known manner, thus removing measured ions from the solution.
From the change in mV, the concentration of the sample is determined.
E
=
E
o
+
S
log(
C
)
C
SAMP
=
C
STD
·
V
STD
·
f
·
(
V
SAMP
+
V
ISA
)
(
V
SAMP
+
V
ISA
)
- (
V
SAMP
+
V
STD
+
V
ISA
)·10
∆
E
V
SAMP
S
C
SAMP
=
C
SAMP
·
V
STD
·
(
V
SAMP
+
V
ISA
)
(
V
SAMP
+
V
STD
+
V
ISA
)·10
∆
E
- (
V
SAMP
+
V
ISA
)
V
SAMP
S
ISE THEORY
An Ion Selective Electrode (ISE) is an electrochemical sensor that changes voltage with the activity or concentration
of ions in solutions. The change in voltage is a logarithmic relationship with concentration, and is expressed by
the Nernst equation:
where:
E
- the measured voltage;
E
o
- standard voltage and other standard system voltages;
a
- the activity of the Ion being measured;
S
- the Nernst slope factor and is derived from thermodynamic principles:
R
- the universal gas constant (8.314 J/Kmol);
T
- the temperature in degrees Kelvin;
F
- the Faraday’s constant (96,485 C/mol);
n
- the Ion charge.
The slope may be positive or negative depending upon the Ion charge (n).
Activity and concentration are related by an “activity coefficient”, expressed as:
where:
a
- the activity of the Ion being measured;
γ
- the activity coefficient;
C
- the concentration of the Ion being measured.
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’s Ionic Strength Adjuster formulations also may optimize pH and complex
interferences, in addition to standardizing the ionic strength.
ADDITIONAL INFORMATION
S
E
I
C
E
P
S
E
P
O
L
S
)
e
d
a
c
e
d
/
V
m
(
n
o
it
a
c
t
n
e
l
a
v
o
n
o
M
6
1
.
9
5
+
n
o
i
n
a
t
n
e
l
a
v
o
n
o
M
6
1
.
9
5
–
n
o
it
a
c
t
n
e
l
a
vi
D
8
5
.
9
2
+
n
o
i
n
a
t
n
e
l
a
vi
D
8
5
.
9
2
–
E
=
E
o
+
S
log(
a
)
S
=
2.303
RT
nF
a
=
γ
·
C
w
w
w
.
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om
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rm
at
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1.
81
87
