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Ion specific electrodes
Ion selective electrodes (ISEs) are important tools in measuring the activity of metal ions, such as Na+, K+, and Ca2+, crucial for various cellular processes. They operate on principles similar to pH electrodes, incorporating an ion-selective membrane to detect specific ions and come in various types, including glass and solid-state electrodes. ISEs have diverse applications, including in diagnostics and studying ion transport in biological systems, and they are favored for their wide response range, low cost, and insensitivity to color or turbidity in samples.
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Ion specific electrodes
- 1. Ion specIfIc electrodes TapeshwarYadav (Lecturer) BMLT, DNHE, M.Sc. Medical Biochemistry
- 2. 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.
- 3. ISE • Another metalions, Ca2+ is known to act as a minatory intracellular message stimulating such diverse process as Insulin secretion Chemotaxis Endocytosis Cellular proliferation
- 4. Basic principles • AnISE 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
- 5. Contd… On to thiselectrode 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 glass 2.An ion exchanger dispersed in an innert matrix
- 6. Contd… 3. A crystal 4.A liquid ion exchanger The external reference electrode is either a calomel or a Ag/Agcl electrode The potential developed a cross the ion selective electrode can be measured on a milli volt scale available in a PH meter.
- 7. Contd… This is proportionalto 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.
- 8. Types of ISE 1.Glass membrane electrodes 2. Solid-state ion exchanger electrodes 3. Solid-state crystal electrodes 4. Liquid-membrane electrodes
- 9. 1. Glass membraneelectrodes • 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 : pH ii. Type : Cation sensitive iii.Type : Sodium sensitive Note: 2nd two subtypes are in general responsive to monovalent cations and are more or less unresponsive to anions.
- 10. Contd… • Appropriate adjustmentsof 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.
- 11. Contd… • Glasses witha 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.
- 13. Applications of Glasselectrodes: 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.
- 14. 2. Solid-State IonExchange 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.
- 15. Contd… • Membrane havegood 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.
- 16. Contd… Solid-state ion exchangerelectrodes have 2 drawbacks: 1.Relatively short working life. 2.Concerns itself with the extremely high resistance of silicone robber & other matrices.
- 18. 3. Solid-State CrystalElectrodes: • 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 fluoride Silver 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
- 20. 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.
- 22. Interferences 2 maintypes of interferences are encountered by ISEs. They are- 1.Method interference 2.Electrode interference
- 23. 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.
- 24. 2. Electrode interference: Theseare 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.
- 25. Applications of ISE Activitymeasurements 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.
- 26. Contd… It can havemany 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
- 27. Contd… • Stimulating biologicalapplication 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.
- 28. Advanteages - Exhibit wideresponse - 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)
- 29. - Made froman 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)
- 30. - 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)
- 31. - Responds preferentiallyto one species in solution Internal reference electrode Ion-selective membrane Internal (filling) solution ION-SELECTIVE ELECTRODES (ISE)