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Technical questions
1) What do CNT Ion Selective
Electrode measure?
2) What is the difference
between activity and concentration?
3) What is ISAB and when is it
used?
4) What is meant by
Interference in CNT_ISE measurements?
5) What is new in NT Sensors reference electrode?
6) Do I need to replace or
top-up any electrode filling solutions?
7) What should I do if the
measured electrode slope is outside the specified limits?
8) After immersing the
electrodes, how long should I wait before taking a reading?
9) What precision can I expect
from a CNT_ISE measurement?
10) What is meant by "drift"
in ISE measurements?
11) What is the effect of temperature change on CNT ISE measurements?
12) How frequently should I re-calibrate the electrodes?
13) What is the concentration range and detection limit for a CNT_ISE
measurement?
14) How should I store the electrodes between analytical sessions?
15) What is the shelf life and operating life of a CNT_ISE?
16) Does the operating life of a CNT_ISE depend on the number of
samples
analysed?
17) Should I worry if the measured voltage is negative?
18) What are the advantages of using potentiometric
measures?
1)
What do CNT Ion Selective Electrode measure?
CNT ISEs measure the activity of free ions in aqueous solutions.
2) What is the
difference between
activity and concentration?
Activity is the effective
concentration - i.e. that portion of the ions which are free to take
part in a
given reaction - in this case coming into contact with the membrane
surface.
Activity is always less than concentration due to inter-ionic
interactions in
the solution, which inhibit the movement of ions and prevent some of
them from
reaching the membrane. It becomes proportionately less as the
concentration
increases. In practice this effect is negligible (within the error
limits of
the measurement) below about 0.01M for monovalent ions and 0.001M for
divalent
ions. The difference between activity and concentration is expressed as
the
Activity Coefficient.
3) What is ISAB and when is it used?
Ionic Strength Adjustment Buffer is added equally to
samples and
standards to minimise any errors due to differences in ionic strength
between
samples and standards which will cause differences in activity
coefficents
which can cause the concentration to be understimated by up to 50 or
60% in the
worst cases. In some cases ISABs can also include ingredients which
minimise
interference effects, and ensure that the pH is optimum for the CNT_ISE
measurement.
Furthermore, for some ions, the addition of ISAB can help to reduce the
time required to reach a stable reading after immersing the electrodes
in a new solution.
The addition of ISAB is not needed for the CNT_ISEs.For
highly accuracy measures, when the uncertainty required must be very
low, the activity of the ion must be taken into account, as factor of
the ionic strenght.
4) What is meant by
Interference in
CNT_ISE measurements?
Ion-selective
electrodes are not completely ion-specific. All are sensitive to some
other
ions to some extent. The degree of sensitivity to another ion is given
by the
Selectivity Coefficient.
5)
What is new in NT Sensors reference electrode?
CNT_ISE Multi Ion and CNT_ISE Combi Ion include NT Sensors reference
electrode, CNT_RE. It is an innovative reference electrode, which
combines the classic properties of a reference electrode with new
technologies. CNT_RE is compatible with all CNT_ISEs.
6) Do I need to replace or top-up any
electrode filling solutions?
No. All CNT_ISEs are all-solid-state and contain no liquid or gel
solutions - so there is nothing to replace.
The only need for CNT ISEs is to keep the protector cap in the sensor
to prevent faster aging.
7) What should I do if the measured
electrode slope is outside the specified limits?
If the slope is only a few millivolts outside specification but is
stable and reproducible then the CNT_ISEs can still be used
satisfactorily, although the lower the slope the higher the errors on
the measurement of activity (or concentration).
The aging of part of the samples will cause a progressive decrease in
the CNT ISE slope. It could be also due to membrane contamination. In
this case it has to be conditioned at least for 8 hours in a solution
of the target ion (0.1M / 1000 mg/L)
8) After immersing the electrodes, how
long should I wait before taking a reading?
Most electrode systems require about two minutes to reach a completely
stable reading. Nevertheless, CNT_ISE get to 90% of the final value in
less than thirty seconds, so it depends on your precision requirements
as to whether you wait for complete stabilisation or not.
9)
What precision can I expect from a CNT_ISE measurement?
Assuming that there are no systematic errors from interference or
activity coefficient effects, the precision depends on the error in the
measurement of the electrode potential (mV) and the slope of the
calibration line:- a 1mV error represents a ~4% error in the
concentration for monovalent ions (slope~54) and an ~8% error for
divalent ions (slope~27), when working in the normal linear range of
the electrodes.
Nevertheless, it has been shown that, under optimum conditions, a
precision of better than ± 2% can be achieved by making frequent
recalibrations and taking multiple readings of the electrode signals.
10)
What is meant by drift in ISE measurements?
Drift is the gradual change in electrode response over a period of
time. The rate and extent of drift can vary depending on which
particular CNT_ISE is being used and the age and degree of
contamination of the electrodes. If a series of samples are measured
repeatedly it will be observed that each successive reading of the same
solution will be slightly different from the previous one. This classic
problem in potentiometric measures can be overcome by frequently
measuring one calibration solution in between sample measurements, then
re-calibrating when the mV value has drifted beyond an acceptable level
- depending on precision requirements.
11)
What is the effect of temperature change on CNT_ISE measurements?
CNT ISE response depends on the temperature. Thus, to avoid any errors
due to temperature change it is advisable that standards were at the
same temperature as the sample solutions. If the sample temperature
deviates by more than about 2°C from the original calibration
temperature, it is advisable to recalibrate.
12)
How frequently should I re-calibrate the electrodes?
It depends on the precision requirements for the results and the rate
of drift of the electrode system. If only an order of magnitude
measurement is required then it may only be necessary to calibrate once
a day or even less frequently. Apart from drift, any temperature
changes (greater than 2°C) will cause a change in the calibration.
For the most precise results, it may even be beneficial to calibrate
between every sample.
13)
What is the concentration range and detection limit for a CNT_ISE
measurement ?
These figures are given in the specifications for each CNT_ISE (you can
find them here) and they are very similar
to classical potentiometric methods.
In general it can be noted that most ISEs cannot be used for
concentrations below about 0.1 ppm and many will only give a linear
response and reasonable precision above 1 or 2 ppm. The upper
concentration limit is usually quoted as 1Molar, but in practice it is
difficult to obtain reliable results above about 0.5Molar because of
uncertainties in the effect of high ionic strength on the activity
coefficient.
14)
How should I store the electrodes between analytical sessions?
In general, CNT_ISEs should always be rinsed with de-ionised water and
gently dabbed dry with a low-lint tissue. They should not be left more
than a few seconds in de-ionised water. Finally, the protective cap
should be replaced to protect the membrane from atmospheric
oxidation/corrosion.
15)
What is the shelf life and operating life of a CNT_ISE?
CNT_ISE are unusually robust and durable. The shelf life of a new
unused solid-state CNT_ISE is many years (providing that the protective
cap is kept in place to limit any atmospheric corrosion). They are
guaranteed for six months during normal operation, but in practice have
been found to last much longer than this.
16)
Does the operating life of a CNT_ISE depend on the number of samples
analysed?
Experience has shown that electrodes can be immersed continuously for
months without deteriorating. This demonstrates that, for individual
sample measurements with average immersion times of two or three
minutes, many thousands of samples can be measured with one electrode.
CNT_ISE operation is based on ion-exchange / charge-transfer
principles. Thus there is any consumption of materials or wear and tear
on components when samples are analysed. Assuming that the electrode is
not exposed to damaging chemicals, the only deterioration is due to the
gradual corrosion (oxidation, hydration, leaching) of the membrane, the
plastic body, and the adhesive used to attach the membrane to the body
(or to physical damage to the membrane surface or to the pin connecting
to the electrode head). The extent of the corrosion is affected by the
length of time the electrode is immersed or exposed to the atmosphere
rather than the number of samples analysed.
17)
Should I worry if the measured voltage is negative?
It is not necessarily a problem. The measurement range of a CNT_ISE can
be positive or negative, it depends on some elements.
18) What are the
advantages of using potentiometric measures?
It is a fast and efficient method to measure ions. This method is
economical, easy handled and it has a wide range of applications and
continued monotoring. It is used in
laboratories, industries, environmental, agriculture...Furthermore, it
doesn't require large volums of sample or reactives and it is a
non-destructive testing. It is possible to measure coloured
solutions or with presence of solid particles.
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