What is
Ion Concentration ?
Definition of Ion: ion - An atom or
molecule that has a net electrical charge from gaining or losing
electrons.
Chemical sensors are miniaturized analytical
devices, which can deliver real-time and on-line information in the
presence of specific compounds or ions in complex samples. Usually
an analyte recognition process takes place followed by the
conversion of chemical information into an electrical or optical
signal. Among various classes of chemical sensors, ion-selective
electrodes (ISE) are one of the most frequently used potentiometric
sensors during laboratory analysis as well as in industry, process
control, physiological measurements, and environmental monitoring.
The principle of ion-selective electrodes operation is quite well
investigated and understood.
An ion-selective membrane is the key
component of all potentiometric ion sensors. It establishes the
preference with which the sensor responds to the analyte in the
presence of various interfering ions from the sample. If ions can
penetrate the boundary between two phases, then an electrochemical
equilibrium will be reached, in which different potentials in the
two phases are formed. If only one type of an ion can be exchanged
between the two phases, then the potential difference formed between
the phases is governed only by the activities of this target ion in
these phases. When the membrane separates two solutions of different
ionic activities (a1 and a2) and provided the
membrane is only permeable to this single type of ion, the potential
difference (E) across the membrane is described by the Nernst
equation:
E = RT/zF . ln (a2/a1)
If the activity of the target ion in phase 1
is kept constant, the unknown activity in phase 2 (a1 = ax)
is related to (E) by:
E = RT/zxF .
ln (ax/a1) = const + S . log (ax)
where S=59.16/z [mV] at 298 K and zx
- the charge of the analyte. The potential difference can be
measured between two identical reference electrodes placed in the
two phases. In practice the potential difference i.e. the
electromotive force is measured between an ion selective electrode
and a reference electrode, placed in the sample solution. An
examplary set-up for the measurement of electromotive force is
precented in figure 1. It is important to note that this is a
measurement at zero current i.e. underequilibrium conditions.
Equilibrium means that the transfer of ions from the membrane into
solution is equal to the transfer from the solution to the membrane.
The measured signal is the sum of different potentials generated at
all solid-solid, solid-liquid and liquid-liquid interfaces.
The above text was copied from pages from
Warsaw University of Technology and was prepared by Wojciech
Wroblewski (Copied from http://www.ch.pw.edu.pl/~dybko/csrg/tutorials/ise/index.html)
How Can
Ion Concentration be
Measured?
Ion Concentration can be measured in the laboratory using a bench
Ion Meter, in outdoor and field applications a portable Ion Meter
would be used.
What products do R.M.S Ltd
offer for Ion Measurement?
Portable Hand Held
Instruments:
The R.M.S EcoScan Ion 6 portable Ion meter
is ideal for water hardness testing, ions in feed or wastewater,
sewage treatment, soil testing, fertilizer testing, plant tissue
testing, food processing (sodium, potassium levels), geological
testing. This
competitively priced hand held Ion meter offers a user friendly, low-cost
instrument ideal
for measuring the Ionic strength of a wide range of applications.
Please Click on the picture below to
view in detail:
Outdoor & Field
Applications:
The CyberScan 300 & 310 Meters are an ideal choice for anyone requiring
to take Ion measurements in rugged outdoor field work or in situ
Plant and Process applications - being waterproof and robustly
built, the chance of accidental damage is virtually eliminated.
Not only is this instrument waterproof but in the event of
accidentally dropping it into liquid, it will float to facilitate
easy retrieval.
Please Click on the picture below to
view in detail:
Laboratory Applications:
The high specification,
economically priced Ion 510 Bench Ion
Meter
offering direct Ion concentration
measurement with use of many Ion Selective Electrodes,
is the idea choice for routine laboratory Ion measurements. With
advanced features to suit the demands
of today's scientist, sophisticated yet easy-to-use functions entail
this microprocessor-based meter makes laboratory Ion testing
procedures routine.
Please Click on the picture below to
view in detail:
Replacement Sensors and
Ancillaries:
At
R.M.S Ltd we can supply replacement Ion Selective Electrodes that
are compatible with all modern instruments. All of our
electrochemical sensors can be supplied with or with out temperature
compensation. All we need is the make and model of your Ion meter so that we can determine the correct
connector etc - please
email,
phone or
fax us with your requirements and one of our team will contact you.
Please Click on the picture below to
view in detail:
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