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Ion
Selective Electrodes: Measurement Considerations
This article is designed to apply, in general,
to all specific ion electrodes. Typically, the ISE (Ion Selective
Electrode) that is purchased will come with its own instruction
manual that pertains to that particular type of electrode. It
is best to read all instructions thoroughly before using the electrode.
When the ISE is received, it is advised that you open up the package
immediately and check all the parts of the electrode. Most ISE's
have a pre-treatment procedure that should be followed prior to
operation.
Before
beginning your measurements the following are a few basic facts
that will aid in designing your analysis procedure.
Agitation
When carrying out selective ion measurements, it is important
to have good agitation. This allows a fresh supply of ions to
be exposed to the sensing portion of the ISE. It is best to select
a speed that keeps a constant, smooth motion. A turbulent rate
should be avoided.
Sample
pH
Most ISE's require a specific pH range for optimum performance
of the electrode. Some ISE's require a precise adjustment. Consult
OMEGA's pH Engineering staff for details on purchasing a pH adjustment
reagent formulated for your particular electrode.
Response
Time
ISE's require a much longer time for the readings to stabilize.
At least fifteen minutes should be allowed for equilibrium to
be established when measuring your standard solutions.
Establishing
a Calibration Curve
It is recommended to use three standard solutions when establishing
a calibration curve. To choose the concentrations of the standard
solutions it is helpful to know the approximate values of your
unknown solutions. For example, if your unknown solutions are
in the 100 ppm range, the choice of standards may include a 10
ppm, a 100 ppm, and a 1000 ppm solution.
Rinsing
It is necessary to rinse the ISE between measurements to insure
accurate readings. Use a steady stream of deionized or distilled
water. Take care not to rub the electrode with a cloth to dry
the probe. It is usually best to "shake off" any excess water.
Take care not to hit the probe against anything while shaking
the electrode.
Conditioning
The ISE needs to remain moist at all times even when not in use.
Consult the operator's manual that accompanies your electrode
for details on cleaning, conditioning and storing the ISE. Common
Questions and Answers About Ion-Selective Electrodes
What
is an Ion-Selective Electrode?
An Ion Selective Electrode measures the potential of a specific
ion in solution. (The pH electrode is an ISE for the Hydrogen
ion.) This potential is measured against a stable reference electrode
of constant potential. The potential difference between the two
electrodes will depend upon the activity of the specific ion in
solution. This activity is related to the concentration of that
specific ion, therefore allowing the end-user to make an analytical
measurement of that specific ion. Several ISE's have been developed
for a variety of different ions.
What
Type of Equipment is Needed for an ISE Measurement?
A pH meter that also measures millivolts can be used to interface
with an ISE. Most ISE's are combination electrodes that have the
reference electrode built into the body of the ISE, however, some
ISE's require a separate reference electrode. If this is the case,
the pH/mV meter must have a pin-connector to connect the reference
electrode.
How
Does the mV Reading Correspond to the Concentration?
Standard solutions of known concentrations must be accurately
prepared. These solutions are then measured with the pH/mV meter.
The mV reading of each solution is noted and a graph of concentration
vs. mV reading must be plotted. Now the unknown solution can be
measured. The mV value of the unknown solution is then located
on the graph and the corresponding solution concentration is determined.
Please see the graph shown above as an example.
What
is Ionic Activity?
Ionic activity is a thermodynamic measurement that represents
the energy available due to free and reactive ions in solution.
The activity coefficient is the ratio between the free ions and
the total ionic concentration in the solution.
Why
is pH Adjustment Necessary?
I n many cases pH control is necessary for accurate, repeatable
measurements. Certain ions exhibit different activity when different
concentrations of hydrogen ions are present in solution. This
occurrence will not only alter the potential due to the specific
ion that is measured, it may also allow other ions in solution
to become active that otherwise were not. This increased activity
from the other ions will interfere with the ability to evaluate
the ion of interest.
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