Ion selective electrodes


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Ion selective electrodes

  1. 1. Ion Selective ElectrodesGuided by:- Prepared by:-Dr. Falgun A Mehta Shweta Singh(HOD pharmaceutical analysis) (Ist sem analysis) 1
  2. 2. INDEX:• The principle of the I.S.E.• Advantages and limitations.• Types of I.S.E.• Application. 2
  3. 3. Principle:-• An ideal I.S.E. consists of a thin membrane across which only the intended ion can be transported.• The transport of ions from a high conc. to a low one through a selective binding with some sites within the membrane creates a potential difference. 3 Ref:-
  4. 4. • 4Ref:-
  5. 5. ADVANTAGE:- When compared to many other analytical techniques, Ion- Selective Electrodes are relatively inexpensive and simple to use and have an extremely wide range of applications and wide concentration range. The most recent plastic-bodied, all-solid-state or gel- filled models are very robust and durable and ideal for use in either field or laboratory environments. Under the most favourable conditions, when measuring ions in relatively dilute aqueous solutions and where interfering ions are not a problem, they can be used very rapidly and easily (e.g. simply dipping in lakes or rivers, dangling from a bridge or dragging behind a boat). They are particularly useful in applications where only an order of magnitude concentration is required. 5 ref:-
  6. 6.  They are particularly useful in biological/medical applications because they measure the activity of the ion directly, rather than the concentration. They are unaffected by sample colour or turbidity. ISEs can be used in aqueous solutions over a wide temperature range. Crystal membranes can operate in the range 0°C to 80°C and plastic membranes from 0°C to 50°C. ISEs are one of the few techniques which can measure both positive and negative ions. Non-destructive: no consumption of analyte. Non-contaminating. Short response time: in sec. or min. useful in industrial applications. 6ref:-
  7. 7. LIMITATION:-• Precision is rarely better than 1%.• Electrodes can be fouled by proteins or other organic solutes.• Interference by other ions.• Electrodes are fragile and have limited shelf life.• Electrodes respond to the activity of uncomplexed ion. So ligands must be absent. 7 ref:-
  8. 8. Types of I.S.E :-  Glass electrodes  Liquid ion exchanger membrane electrodes  Solid state membrane electrodes  Neutral carrier membrane electrodes  Coated wire electrodes  Field effect transistor electrodes  Gas sensing electrodes  Air gap electrodes  Biomembrane electrode 8
  9. 9. Glass electrode:-By altering the composition of the glass, itis possible to make the electrode selective fordifferent ions. Usually the glasses contain 60 to 75 mole% SiO2, 2 to 20 % Al2O3 or LaF3, 0 to 6 %BaO and CaO, and a variable amount of agroup 1A oxide. The mixtures of the oxides is melted andcooled to form the glass. Monovalent cations in the threedimensional glass structures are relatively 9mobile.
  10. 10. Consequently, monovalent cations from a solution into which the glass is dipped can penetrate into the surface of the glass and be cation exchanged net negatively charged sites in the glass. Because the concentration of the analyzed ion in the sample solution differs from that in the internal reference solution, a potential difference develops across the membrane. Glass membranes are selective for monovalent cations because polyvalent ions cannot easily penetrate the surface of the membrane. 10
  11. 11. Evidently the selectivity of glass electrodes is related both to the ability of the various Monovalent cations to penetrate into the glass membrane and to the degree of attraction of the cations to the negative sites within the glass. Glass electrodes which are selective for H+ (pH electrode),Li+, Na+, K+, Cs+ ,Ag+, Ti+ and NH4+ are commercially available 11
  12. 12. Liquid ion exchanger membrane electrodes:-The inner compartment of theelectrode contains a referenceelectrode and an aqueous referencesolution. The outer compartment containsan organic liquid ion exchanger. The liquid ion exchanger isinsoluble in the solvent in which theelectrode is to be used (water) and isnonvolatile at room temperature. 12
  13. 13.  The ion exchanger is dissolved in a relatively high molecular weight solvent such as dioctylphenyl phosphonate. Liquid ion exchanger consists of polar ionic sites attached to a relatively large non polar organic molecule. The ionic sites are negative in a cation exchanger and positive in an anion exchanger. Typical liquid ion exchangers are (RO)2PO2- (for Ca2+ and Mg2+) and RSCH2COO- (for Cu2+ and Pb2+). R in the ion exchangers can be any of several organic groups, for example p-(1,1,3,3-tetramethylbutyl)phenyl, p-(n- octyl)phenyl, and decyl. The S and O- in ion exchanger of the RSCH2COO- type selectively form chelate rings with a certain ions, e.g., Cu2+ or Pb2+. 13
  14. 14.  The ion exchangers and reference solutions are held in place inside the electrodes by a porous membrane. Although the membrane can be made from polyvinyl chloride (PVC), it is usually constructed from some form of cellulose, e.g., from cellulose acetate. The membrane is prepared to have a pore diameter of about 100 nm. 14
  15. 15. Chemical treatment makes the membrane hydrophobic. The membrane is in physical contact with the liquid ion exchanger and becomes permeated with it. Because the membrane is hydrophobic, water from the internal reference solution and from the sample solution is prevented from mixing with the liquid ion exchanger. A second type of liquid ion exchanger membrane utilizes a polymeric membrane to permanently hold the ion exchanger solution in place within the membrane. 15
  16. 16. That type of membrane does not need to be in direct contact with a solution of liquid ion exchanger. In either case the shell of the electrode is made from an inert material such as glass or an organic polymer. Liquid ion exchanger membrane electrodes owe their selectivity to their ability to selectively exchange ions. Upon contacting the membrane, an ion from the aqueous solution exchanges with an ion on a polar site in the ion exchanger.The newly created ion and ion exchanger combination can freely diffuse throughout the membrane. 16
  17. 17. The ionic conductivity of the membrane results from the mobility of the ion within the membrane. The potential across the membrane is related to the ionic conductivity within the membrane.Liquid ion exchanger membrane electrode have been used for the assay of :- Ca2+, K+, Li+, Na+, Mg2+, Ni2+, Zn2+, Ti+, Ag+, Hg2+, water hardness (Ca2+ + Mg2+), Cu2+,Pb2+, Cl-, BF4-, NO3-, ClO4-, Cr2O72-, benzoates, SCN- and other ions. 17
  18. 18. Solid state membrane electrode:-A solid state membrane electrodecan be a single crystal, a pellet madefrom a sparingly soluble salt, or asparingly soluble salt embedded in aninert matrix, e.g., rubber.Because the single crystal and pelletmembranes are homogenous,electrodes containing them are referredto as homogenous membraneelectrodes.The membrane consisting of thesparingly soluble salt in the inertbinding material is a heterogeneousmembrane electrode. 18
  19. 19.  The lanthanum fluoride (LaF3) membrane is the only single crystal membrane that is widely used in ion selective electrodes. In the process called as “doping”, the resistance of the LaF3 crystal is decreased by replacing a relatively small number of La3+ ions in the crystals with Eu2+ ions. Fluoride ions migrate from vacancy to vacancy in the defective LaF3 crystal. As a fluoride ion abandons one position in the crystalline structure, it leaves a hole into which another fluoride can migrate. The result is a crystal which exhibits ionic conductivity. 19
  20. 20. The conductance to the membrane, as well as the potential across the membrane, can be related to the analyte concentration for many solid state membrane electrodes.Vacancies in the crystalline structure have exactly the proper size, charge, and shape to hold a fluoride ion. Because fluoride can selectively migrate to the crystal, the lanthanum fluoride membrane is selective for fluoride. If no fluoride is present in the sample solution, the LaF3 membrane electrode can be used to assay for La3+. 20
  21. 21. A heterogeneous membrane consists of an active ingredient dispersed throughout an inert binding material. The inert binder provides the physical properties that are required of the membrane, and the active ingredient provides the membrane selectivity. Wax, silicon rubber, polyvinyl chloride and several other polymeric substances are used as inert binders in ion selective electrode. 21
  22. 22.  Silicon rubber and PVC are most popular binders. After mixing binder with active ingredient, the membrane is formed into thin sheet of appropriate size and attached to the end of the electrode body. The active ingredient is often a sparingly soluble substance similar to the substance in homogenous membranes. 22
  23. 23. Neutral-carrier membrane electrodes:- They have the same design as liquid-ion-exchanger membrane electrodes. The liquid-ion-exchanger is replaced in neutral-carrier membranes with a neutral complexing agent (a neutral carrier) such as crown ether, which is dissolved in a highly water insoluble organic solvent.The neutral carrier complexes with the analyte at membrane-sample interface to form a charged complex which is extracted from the aqueous solution into the organic solvent in the membrane. The selectivity of the membrane for a particular ion depends upon the ability to extract the ion into the membrane, which in turn depends upon the ability of the ion 23 to form a complex with the neutral carrier.
  24. 24. After complexation and extraction, the species in the neutral-carrier membrane has the same charge as the extracted ion.The solvent in which the neutral carrier is dissolved is usually a high boiling organic compound such as nitrobenzene (used in Ba2+ selective electrodes), dibutylsebacate (used in K+ selective electrode) and o- nitrophenyl-n-octylether (used in a Ca2+ selective electrode).The physical support for the neutral carrier and solvent is usually a cellulose membrane, more commonly, a PVC membrane. In addition to K+, Ca2+, Ba2+, neutral-carrier membrane electrodes are also selective for Li+, H+, Mg2+, NH4+ , Sr2+. 24
  25. 25. Coated wire electrodes:-They are considerably smaller thanother forms of ion selective electrodesbecause the internal filling solution iseliminated and the ion selectivemembrane is coated directly on theinternal electrode wire. The ion selective membranesutilized in coated wire electrodesconsists of either an ion exchanger orneutral carrier immobilized in apolymeric material that is coated onthe electrode. They are more sturdy than otherISEs and can be constructed withsmall tips. 25
  26. 26. Method of preparation of the electrode:- First the metal on the interior of the electrode is sealed into a glass or some other suitable material so that several mm or less of the wire is exposed. The exposed wire is successively dipped into a solution of the polymeric material and then into a solution of the ion exchanger or neutral carrier. 26
  27. 27. After the electrode has air-dried, the dipping procedure is repeated, if necessary, until the membrane coating on the wire is the desired thickness. Alternatively, the wire can be dipped into a single solution containing both the membrane material and the polymerizer. The polymeric matrix can be any of materials including PVC, polymethyl acrylate(PMM) or epoxy. The internal electrode can be constructed from metals like platinum, copper, silver wire and graphite rods. 27
  28. 28. Ion selective field effect transistors (ISFETs):-The electrode consists of an ionselective membrane deposited orcoated on the gate of a field effecttransistor (FET). The membrane canbe a sparingly soluble compound suchas silver bromide (solid statemembrane) or some other type ofmembrane such as an ion exchanger orneutral carrier in a PVC matrix.Often membranes in a PVC matrixare used. Membranes in a PVC matrixcan be forced to adhere to the gate ofthe FET by placing a polyimide meshover the gate prior to coating it with 28the membrane.
  29. 29. The potential at the membrane is partially determined by the activity of the analyte in solution. That potential determines the flow of current through the drain of the FET. The drain current consequently varies with the activity of the analyte and is the monitored factor. 29
  30. 30. Gas-sensing electrodes:-They are used to assay the gasesdissolved in aqueous solutions. It is constructed by enclosing theglass pH membrane in a second, gas-permeable hydrophobic membrane. A thin layer of an electrolytesolution is held between the twomembranes. They also have a small referenceelectrode enclosed within the gas- 30permeable membrane.
  31. 31.  The potential between the internal ISE and the reference electrode within the outer membrane is monitored. The gas permeable membrane holds a constant volume of solution around the internal ISE into which the gaseous analyte can diffuse. The hydrophobic gas-permeable membrane can be composed of substance which allows passage of dissolved gas but prevents the solution within the membrane from escaping. The materials used are silicon rubber, Teflon polypropylene, fluorinated ethylene propylene, polyvinylidene fluoride etc… Gas from the sample solution passes through the submerged gas- permeable membrane and equilibrates in the electrolyte solution between the two membranes. 31
  32. 32.  The gas reacts reversibly with the electrolyte solution to form an ion to which the ion selective electrode responds. Because the activity of the ion that is formed between the two membranes is proportional to amount of gas dissolved in sample, the electrode response is directly related to the activity of the gas in the sample. The gases (primarily NH3, SO2 and CO2) which are detected by gas sensing electrodes based on the pH electrode equilibrate with the electrolyte solution to alter its pH: • NH3 + H2O = NH4+ + OH- • SO2 + H2O = HSO3- + H+ • CO2 + H2O = HCO3- + H+ H2S, HCN, HF and chloride can be assayed by using internal homogenous membrane electrode containing the appropriate silver salt. Disadvantage possesses relatively long response time i.e; require 1-7 minutes after insertion in to a sample solution to reach equilibrium. 32
  33. 33. Air-Gap electrodes:- They are another form of gas sensing electrodes invented by Ruzicka and Hansen. A very thin layer of anappropriate electrolyte solution isadsorbed on the surface of themembrane of the glass electrode. The electrolyte solution isadsorbed on glass membrane whenmembrane comes in contact withthe sponge containing theelectrolyte solution and a wettingagent. 33
  34. 34. The reference electrode makes contact with the adsorbed electrolyte layer through a small, porous, ceramic salt bridge. The air gap electrode is used to assay ionic species which can be chemically converted to gases, e.g. HCO3- The HCO3- solution is placed in the sample holder and an acid is added to convert HCO3-(aq) to CO2 (g).The sample holder is placed in position under the electrode and stirred with a magnetic stirrer and stirrer bar. Carbon dioxide which is emitted during the chemical reaction equilibrates with the electrolyte solution on the glass membrane and alters the pH of the solution. 34
  35. 35. The glass electrode measures the pH of the resulting solution. The electrolyte solutions used with air gap electrode are the same as those used with other gas-sensing electrodes.The air-gap electrode has a faster response time due to the thinner layer of electrolyte solution and a longer lifetime than most of the other types of sensing electrodes. A typical response time for an air-gap electrode is less than a minute. Air-gap electrode is primarily used for analysis of NH4+, HSO3-. As an example they can be used for the determination of urea in blood. 35
  36. 36. Biomembrane electrodes:- It is an ion selective electrode which is coated with an enzyme-containing acrylamide gel.  The gel and enzyme are held in place on the surface of the ion selective electrode by an inert physical support.  The design is same as gas-sensing electrode.  The support is a sheet of cellophane or a piece of gauze made from dacron or nylon.  The physical support is wrapped around the electrode membrane and tied in place. 36
  37. 37.  The acrylamide gel containing the enzyme is coagulated on the support-electrode combination. Enzymes are highly selective biochemical catalysts.The selectivity of Biomembrane electrode is due to the selectivity of the enzymes that are used in electrodes. Here the enzyme-catalyzed reaction of the analyte yields an ionic reaction product which is monitored by the internal ion-selective electrode.The operation of the urea-selective electrode will serve to illustrate the operation of Biomembrane electrodes. 37
  38. 38. The glass membrane of an ammonium-sensitive glass electrode is coated with an acrylamide gel layer containing the enzyme urease. When the electrode is dipped into a solution containing urea, the following reaction occurs to yield NH4+ : CO(NH2)2 + H2O 2NH4+ + CO2 The NH4+ formed during the reaction is measured at the ammonium-selective electrode.A working curve is prepared by plotting the potential of the electrode in standard urea solutions as a function of the logarithm of urea concentration. 38
  39. 39.  The urea concentration in the sample is obtained from the working curve. Unfortunately the enzymes used in Biomembrane electrodes gradually decay and the enzyme containing gel must be periodically replaced.The Biomembrane of urea electrode lasts about 2 weeks. Biomembrane electrodes have long response time of 5 or more minutes. 39
  40. 40. APPLICATION:-• Ion-selective electrodes are used in a wide variety of applications for determining the concentrations of various ions in aqueous solutions. The following is a list of some of the main areas in which ISEs have been used.• Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents, and natural waters.• Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material, fertilisers and feedstuffs.• Food Processing: NO3, NO2 in meat preservatives.• Salt content of meat, fish, dairy products, fruit juices, brewing solutions.• F in drinking water and other drinks. 40ref:-
  41. 41. • K in fruit juices and wine making.• Corrosive effect of NO3 in canned foods.• Detergent Manufacture: Ca, Ba, F for studying effects on water quality.• Paper Manufacture: S and Cl in pulping and recovery- cycle liquors.• Explosives: F, Cl, NO3 in explosive materials and combustion products.• Biomedical Laboratories: Ca, K, Cl in body fluids (blood, plasma, serum, sweat).• F in skeletal and dental studies.• Education and Research: Wide range of applications.• Ca in dairy products and beer. 41
  42. 42. References:-••• .PDF 42
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