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Ion specific electrodesTapeshwar Yadav
(Lecturer)BMLT, DNHE,
M.Sc. Medical Biochemistry
Ion Selective Electrodes
• It is a well known fact that metal ions have a profound effect on cellular processes
• The importance or the role that ions play in cellular activity can be gauged by the fact that most cells maintain a very critical Na+ & k+ balance between the extracellular and the intracellular spaces.
• Any distribution in this critical balance is to the cellular metabolism through a drastic change in the osmotic pressure resulting in cellular swelling.
ISE
• Another metal ions, Ca2+ is known to act as a minatory intracellular message stimulating such diverse process as
Insulin secretionChemotaxisEndocytosisCellular proliferation
Basic principles
• An ISE operates an exactly the same principles as a PH electrode
In fact, a PH electrode is a type of ion selective electrode sensitive to hydrogen ion.
Just like a PH electrode, the electrode body contains a reference solution and an metal reference electrode
Contd…
On to this electrode body is sealed an ion selective membrane which act as the ion sensor
Four different types of ion selective membranes are in use
They are:-1.Specially formulated glass2.An ion exchanger dispersed in an innert matrix
Contd…
3. A crystal4. A liquid ion exchangerThe external reference electrode is either a
calomel or a Ag/Agcl electrodeThe potential developed a cross the ion
selective electrode can be measured on a milli volt scale available in a PH meter.
Contd…
This is proportional to the activity of ion in the sample
More sophisticated instruments employ specific ion meters (high impedance mill volt meters) which have readout scales directly calibrated in concentration.
Types of ISE
1. Glass membrane electrodes2. Solid-state ion exchanger electrodes3. Solid-state crystal electrodes4. Liquid-membrane electrodes
1. Glass membrane electrodes• Glass electrode consists of a stem of non-cation
responsive, high resistance glass on which is fused a thin walled bulb of cation responsive glass.
• Selectivity of a glass electrode is a function of the comp. of the glass.
• 3 subtypes:i. Type : pHii. Type : Cation sensitiveiii.Type : Sodium sensitiveNote: 2nd two subtypes are in general responsive to monovalent
cations and are more or less unresponsive to anions.
Contd…• Appropriate adjustments of glass composition
change the degree of electrode selectivity.• Thus, glass can be made more responsive to
cations by adding to it elements which have co-ordination numbers greater than oxidation numbers to alkali metal-silicate glasses (20% Na2O – 10% CaO – 70% SiO2).
• It usually imparts an excessive negative change to the glass making it suitable to attract cations having a proper charge-size ratio.
Contd…
• Glasses with a composition of 27% Na2O – 15% Al2O3 – 68% SiO2 show a general cation response.
• If the above composition is modified to 11% Na2O – 18% Al2O3 – 71% SiO2 , the glass becomes highly sodium selective as compared to other alkali metal ions.
Applications of Glass electrodes:
Measurement of sodium, lithium, or silver ions because of their high specificity for these ions.
Due to its relative inertness it can be used in non-aqueous media, organic solvents & also in the presence of lipid soluble or surface active molecules.
2. Solid-State Ion Exchange Electrodes
• In these electrodes, the glass membrane is replaced with a solid-state ionically conducting membrane.
• Ion responsive material is an insoluble or sparingly soluble salt dispersed in an inert matrix.
• Often used inert matrices include silicon rubber, polyvinyl chloride & other polymeric materials.
Contd…• Membrane have good mechanical properties and
give reproducible potentials.• The membrane so prepared is then cemented to a
glass or epoxy resin electrode body.• The body holds an internal-reference solution and
a reference electrode .• Sometimes, the back of an ion responsive
membrane is coated with mercury and platinum wire is connected to it which works as the reference electrode.
Contd…Solid-state ion exchanger electrodes have 2
drawbacks:1.Relatively short working life.2.Concerns itself with the extremely high
resistance of silicone robber & other matrices.
3. Solid-State Crystal Electrodes:• Crystal of an insoluble salt can act as ion sensitive
elements operating in much the same way as the salt dispersed in an inert matrix.
• Crystal should not have high resistance.• Eg.Lanthanum fluoride electrode:- measures fluorideSilver chloride electrode:- measures chloride These electrodes have a life of about 1-2 years.However, if used at high temp., their life gets shortened
considerably 1-3 months
4. Liquid-Membrane Electrodes:
• Sensing element of these electrodes is a layer of organic solvent in which an ion exchanger is dissolved.
• Most available electrodes use a porous diaphragm (glass or ceramic disc) which separates the inner ion exchanger sol. From the test sol.
• Ion exchange sol. Keeps the disc always saturated.• This type of electrodes are used for the measurement
of calcium, nitrate, perchlorate & other ions.
Interferences
2 main types of interferences are encountered by ISEs. They are-
1.Method interference2.Electrode interference
1. Method interference: It can occur when either or all of such factors at the pH,
ionic strength or temp. of the sample being tested are not properly mixed.
Thus, while measuring fluoride ions, a low pH might give very low values for the activity of this ion.
This is so since fluoride ions form complexes with the hydrogen ions and consequently lose their measurable activity.
The method interference can therefore be done away with by fixing the measurement conditions diligently.
2. Electrode interference:These are mostly due to the passage of an interfering
ion into the membrane, being measured. Thus, high barium ion conc. In a sample being
measured for calcium can be detrimental to the experiment as barium ions can compete with calcium ions for passage into the membrane.
Another type of electrode interference occurs chiefly with crystal electrodes.
Crystal electrodes consequently loses its sensitivity to the desired ion.
Applications of ISE
Activity measurements are valuable because the activities of ions determine rates of reactions and chemical equilibria.
Ion electrodes have been used for predicting corrosion rates, extent of precipitation, degree of acidity , formation of complexes, solution conductivities and effectiveness of electroplating bath solutions.
Contd…
It can have many diagnostic application in biology. For e.g., the chloride electrode is being used to assay chloride ion activity in the sweat of babies as a diagnostic test for cystic fibrosis.
It is used to study the control of ion transport by tissues and cells
Contd…
• Stimulating biological application of ion selective electrodes is that they can be converted into enzyme electrodes.
• These electrodes can be measure the activity of an enzyme in a given sample
• For e.g., the ammonium ion electrode can be converted into an enzyme electrode measuring the activity of ureas.
Advanteages
- Exhibit wide response
- Exhibit wide linear range
- Low cost
- Color or turbidity of analyte does not affect results
- Come in different shapes and sizes
ION-SELECTIVE ELECTRODES (ISE)
- Made from an ion-conducting membrane(ion-exchange material that allows ions of one electrical sign to pass through)
- Reference electrode is inbuilt
- Internal solution (solution inside electrode) contains ion of interest with constant activity
- Ion of interest is also mixed with membrane
- Membrane is nonporous and water insoluble
ION-SELECTIVE ELECTRODES (ISE)
- Selective (preferential) ion is C+
- Membrane is made of poly(vinyl chloride) (PVC)
- Membrane is impregnated with nonpolar liquid
- Membrane contains ligand L (ion-selective ionophore)
- Membrane contains the complex LC+
- Membrane contains hydrophobic anion R- (ion exchanger)
ION-SELECTIVE ELECTRODES (ISE)
- Responds preferentially to one species in solution
Internal reference electrode
Ion-selective membrane
Internal (filling) solution
ION-SELECTIVE ELECTRODES (ISE)
