57
• Press
to change the desired method parameters as indicated in the first step of the method.
• Press
to take the second mV reading.
• When the reading is stable, press
to store the second mV reading. The ISE measurement
results will be displayed on the LCD.
• Press
to log the current results into a ISE Method Report or press
to return to
ISE Measure mode.
• If pressing
, the method parameters can be changed and the sample concentration will be recalculated.
• Press
to start another measurement.
Note
: Press
at any time to stop the measurement and return to
ISE Measure mode.
KNOWN SUBTRACTION
To measure the concentration of a sample using the Known Subtraction incremental method:
• Press
and then
to select
ISE Measure mode for the selected channel.
• Select the Known Subtraction method (see
ISE Setup for details).
• Press
to start the measurement. The first step of the method will be displayed on the LCD in which the
user is notified to add the appropriate ISA Buffer Volume (if different from 0 mL) to the sample and to immerse
the electrode into the Sample Volume. The method parameters are also displayed on the LCD.
• Press
to set the method parameters. Press
/
to select next / previous
parameter to be edit, then press
and use
or
to set the desired parameter
value. Press
to save the modified value and then press
to exit method parameters edit
menu.
Note
: For the Known Subtraction method, the stoichiometric ratio (factor) between standard and sample
must also be set in order to take accurate measurements.
• Make sure that the set parameters values are the used ones and submerge the Ion Selective Electrode tip
approximately 4 cm (1½”) into the sample to be tested.
• Press
to take the first mV reading.
• When the reading is stable, press
to store the first mV reading. The second step of the method
will be displayed on the LCD in which the user is notified to add the Standard Volume of set concentration
to the sample. The method parameters are also displayed on the LCD.
• Press
to change the desired method parameters as indicated in the first step of the method.
• Press
to take the second mV reading.
• When the reading is stable, press
to store the second mV reading. The ISE measurement
60
ISE THEORY
ISE THEORY
ISE THEORY
ISE THEORY
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*K
-1
*mol
-1
);
T
- the temperature in degrees Kelvin;
F
- the Faraday’s constant (96,485 C*mol
-1
);
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;
- the activity coefficient;
C
- the concentration of the ion being measured.
In very dilute solutions approaches 1 so activity and concentration are the same.
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
–