Ion Selective Electrodes (ISE) Theory
What is an ISE and how do they work? The first in a series of posts to help you understand the principles behind ISE analysis.
Ion Selective Electrodes.(ISE)
Introduction.
An Ion selective electrode is an electrochemical sensor that
produces a potential in millivolts between itself and a reference electrode.
The potential varies when the concentration or activity of the ion being
measured changes. The potential
difference between the ISE and the reference electrode is recorded or displayed
on an Ion meter or a pH meter with a millivolt mode. The potential difference (mV) is proportional
to the Ionic concentration.
To make successful Ion measurements it is important to
control two factors that affect the activity of the ions in aqueous solutions.
Temperature.
A rise in temperature will increase the kinetic energy or
speed of the ions in solution. The ISE will therefore appear to sense the
presence of more ions and the mV potential will increase or decrease
accordingly even though concentration has not changed. The first rule of successful analysis is
therefore to ensure that samples and standard solutions are at the same
temperature.
Ionic Strength.
As the total number of ions in solution increases there will
be a greater number of ionic interactions which will cause the ions to move
less freely in solution. This has the same effect as reducing the temperature
in that the ISE will appear to sense fewer ions even though the concentration
has not changed.
The solution to this problem is the addition of an Ionic
Strength Adjustment Buffer (ISAB) to all of the standards and samples. This
buffer must be inert and of high ionic strength usually around a 2-5 Molar salt solution. A normal addition is around 2ml of ISAB to
every 50ml of standard and sample but this varies with the application
concerned. Once added the ionic strength is equalised in standards and samples.
Sensitivity.
The potential measured by the ISE depends on the charge of
the ion being measured. This is governed by the Nernst equation. This gives us
an indication of the response (in mV) we expect from changes in ion
concentration (the Slope). The slope will decrease over the lifetime of the ISE
and so knowing what to expect in theory will help decide if the sensor is still
in working order.
For ions with a single positive charge such as Sodium (Na+)
the slope or theoretical mV response to a 10 fold increase in concentration is
+59.16mV at 25 degrees C.
For ions with a single negative charge such as Fluoride (F-)
it is -59.16mV
For ions with a double positive charge such as Calcium
(Ca++) the perfect slope is halved and
is expected to be +29.56mV whilst the double negatively charged ion such as
Sulphide (S--) would be -29.56mV.
Making a
measurement.
A pH electrode is an ISE. It is reasonable to expect
therefore that measurements using other ISE’s will be as simple.
The most common form of measurement is called Direct Potentiometry.
Here you take two standard solutions of typically 100 and 1000ppm and add a
small amount of ISAB as described. Take the same volume of sample and add ISAB
in the same way.
Connect the ISE to the Ion meter and immerse the ISE
(combination ISE with built in reference electrode) in to the lowest standard
and calibrate. Repeat with the second, and more concentrated , standard to
complete the calibration.
After rinsing the ISE with deionised water simply measure
the sample. The concentration is given on the ion meter display.
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