3
permeable to the ion exchanger, but impermeable to water. It allows the sensing
cell to contact the sample solution and separates the internal filling solution
from the sample. The membrane module has a shelf life of 12–24 months and is
replaceable.
The voltage developed between the sensing and reference electrodes is a
measure of the concentration of the reactive ion being measured. As the
concentration of the ion reacting at the sensing electrode varies, so does the
voltage measured between the two electrodes.
As described in the Nernst equation, ISE response is a linear equation:
E
=
Eo
+
m
(
ln a
)
where
E
is the measured voltage,
Eo
is the standard potential for the
combination of the two half cells,
m
is the slope,
ln
is natural log, and
a
is the
activity of the measured ion species.
Assuming the ionic strength is fairly constant, the Nernst equation may be
rewritten to
describe the electrode response to the concentration,
C
, of the measured ionic
species:
E
=
Eo
+
m
(
lnC
)
Troubleshooting
For troubleshooting and FAQs, see
Repair Information
If you have watched the related product video(s), followed the troubleshooting
steps, and are still having trouble with your Potassium Ion-Selective Electrode,
contact Vernier Technical Support at [email protected] or call 888-837-
6437. Support specialists will work with you to determine if the unit needs to
be sent in for repair. At that time, a Return Merchandise Authorization (RMA)
number will be issued and instructions will be communicated on how to return
the unit for repair.
Accessories/Replacements
Potassium ISE Replacement Membrane Modules
The Potassium ISE has a PVC membrane module with a limited life expectancy.
The module is warranted to be free from defects for a period of twelve (12)
months from the date of purchase. It is possible, however, that you may get
somewhat longer use than the warranty period. If you notice a reduced response
(e.g., distinctly different voltages or voltage ranges during calibration), it is
probably time to replace the membrane module.
Important
: Do not order
membrane modules far in advance of the time you will be using them; the
process of degradation takes place even when they are stored on the shelf.
l
Long-term storage of the ISE (longer than 24 hours): Moisten the sponge in
the bottom of the long-term storage bottle with distilled water. When you
finish using the ISE, rinse it off with distilled water and blot it dry with a
tissue. Loosen the lid of the bottle and insert the ISE.
Note
: The tip of the
ISE should NOT be touching the sponge. Check to be sure the reference
mark is inside, rather than outside the bottle or under the grommet. Tighten
the lid. This keeps the electrode in a humid environment, which prevents the
reference junctions from completely drying out.
l
Short-term, wet storage (less than 24 hours): Fill the short-term soaking
bottle 3/4 full with High Standard. Loosen the cap, insert the electrode into
the bottle, and tighten.
Maintaining and Replacing the ISE Standard Calibration Solutions
Having accurate standard solutions is essential for performing good calibrations.
The two standard solutions that were included with your ISE can last a long
time if you take care not to contaminate them. At some point, you will need to
replenish your supply of standard solutions.
Vernier sells replacement standards in 500 mL bottles. Order codes are:
l
K-LST: Potassium Low Standard: 10 mg/L
l
K-HST: Potassium High Standard, 1000 mg/L
To prepare your own standard solutions, use the information in the table below.
Note
: Use glassware designed for accurate volume measurements, such as
volumetric flasks or graduated cylinders. All glassware must be very clean.
Standard
Solution
Concentration
(mg/L or ppm)
Preparation Method using High
Quality Distilled Water
Potassium (K
+
)
High Standard
1000 mg/L K
+
1.907 g KCl / 1 L solution
Potassium (K
+
)
Low Standard
10 mg/L K
+
Dilute the High Standard by a factor
of 100 (from 1000 mg/L to 10 mg/L)
Do not wrap the cable tightly around the sensor for storage. Repeatedly doing
so can irreparably damage the wires and is not covered under warranty.
How the Sensor Works
The Vernier Potassium Ion-Selective Electrode (ISE) is a membrane-based
electrode that measures a specific ion (K
+
) in an aqueous solution. When the
membrane of the electrode is in contact with a solution containing the specific
ion, a voltage, dependent on the level of that ion in solution, develops at the
membrane. The ISE is a combination style electrode. The voltage develops in
relation to an internal Ag/AgCl reference electrode. The ISE measures for the
specific ion concentration directly. Samples need to be aqueous to avoid
contaminating or dissolving the membrane. The Vernier Potassium Ion-Selective
Electrode has a solid polymer membrane. The membrane is a porous plastic disk,
