1 Potentiometry


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Potentiometry - ppt, Electro Analytical Method

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1 Potentiometry

  1. 1. POTENTIOMETRY by Mr. Shaise Jacob Faculty, Nirmala College of Pharmacy Muvattupuzha Kerala, India
  2. 2. <ul><li>Measuring the potential or emf of a solution </li></ul><ul><li>Using a set of indicator & reference electrode. </li></ul><ul><li>INDICATOR ELECTRODE </li></ul><ul><li>Responds to changes in emf or Ph of the solution </li></ul><ul><li>To indicate emf or ph </li></ul><ul><li>Example – Glass electrode, antimony electrode </li></ul>
  3. 3. REFERENCE ELECTRODE <ul><li>Which has a standard potential on its own </li></ul><ul><li>And its potential does not change to whichever solution it is dipped. </li></ul><ul><li>E.g. hydrogen electrode, saturated calomel electrode & silver-silver chloride electrode </li></ul><ul><li>Most commonly used is saturated calomel electrode </li></ul>
  4. 4. Nernst equation <ul><li>The Nernst Equation allows us to calculate the voltage produced by any electrochemical cell given Eo values for its electrodes and the concentrations of reactants and products. </li></ul>
  5. 5. <ul><li>The potential (E) of a metal electrode at 25 o C immersed into a solution of its own ions is given by </li></ul><ul><li>E o – Standard potential of the metal </li></ul><ul><li>N – valency of ions </li></ul><ul><li>C – concentration of ions </li></ul>
  7. 8. <ul><li>It can be used as indicator as well as reference electrode </li></ul><ul><li>Platinum coil coated with platinum black and has wire contacts through mercury </li></ul><ul><li>The assembly enclosed in a glass covering through which Hydrogen 99.8% is passed at 1 atm pressure </li></ul>
  8. 9. <ul><li>The standard H2 electrode potential is defined as the potential that is developed between the H2 gas adsorbed on the pt metal and H+ of the solution when the H2 gas at a pressure of 760 mm of Hg is in equilibrium with H+ of unit concentration </li></ul><ul><li>The magnitude of SHE potential is considered to be zero. It is used- </li></ul><ul><li>1) For the determination of electrode potential of metal electrode system. </li></ul><ul><li>2) For the determination of pH of the solution. </li></ul>
  9. 10. Limitations <ul><li>1) It is rather difficult to regulate the pressure of the H2 gas to be at exactly 1atm throughout the experiment. </li></ul><ul><li>2) If the solution contains any oxidizing agent, the H2 electrode cannot be used. </li></ul><ul><li>3) Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration decreases. In such a system, it is difficult to maintain the concentration of HCl at 1M. </li></ul><ul><li>4) Platinum foil gets easily poisoned by the impurities present in the gas and HCl. In fact, the attainment of equilibrium is ensured by trial and error. </li></ul>
  11. 15. <ul><li>It contains of an inner jacket and outer sleeve </li></ul><ul><li>Inner jacket has wire contact with Hg and plugged with a mixture of calomel Hg 2 Cl 2 & KCl </li></ul><ul><li>Outer sleeve – crystals of KCl & porous plug of asbestos </li></ul><ul><li>Space b/w inner jacket & outer sleeve is filled with either saturated KCl or 1N KCl or 0.1N KCl </li></ul>
  12. 16. <ul><li>Application </li></ul><ul><li>The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry. </li></ul><ul><li>This electrode and the silver/silver chloride reference electrode work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt. </li></ul><ul><li>The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode </li></ul>
  13. 17. MERITS OF SCE <ul><li>Ease of construction </li></ul><ul><li>Stability of potential </li></ul>
  14. 18. Silver- Silver chloride electrode <ul><li>The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, inexpensive, very stable and non-toxic. </li></ul><ul><li>it is mainly used with saturated potassium chloride (KCl) electrolyte, but can be used with lower concentrations such as 1 M KCl and even directly in seawater. </li></ul>
  15. 20. <ul><li>Silver wire coated electrolytically with silver chloride and dipped into KCl </li></ul><ul><li>It has the advantage that it is easy to use </li></ul><ul><li>Demer it is that it is difficult to prepare </li></ul>
  16. 22. Mercurous Sulphate Electrode <ul><li>Type – Reference electrode </li></ul><ul><li>♠ Similar in construction to the calomel electrode but utilizes dilute sulphuric acid saturated with mercurous sulphate. </li></ul><ul><li>Use - I t is used in solution, where silver or lead ions are present </li></ul>
  18. 24. INDICATOR ELECTRODES <ul><li>♠ Indicator electrode indicates the potential or Ph of a solution in comparison to a reference electrode of a known potential. </li></ul><ul><li>Hydrogen Electrode </li></ul><ul><li>Glass Electrode </li></ul><ul><li>Most widely used indicator electrode </li></ul>
  19. 26. <ul><li>It is selective to change in conc. of hydrogen ions . </li></ul><ul><li>It consists of a glass tube with a thin Ph sensitive glass bulb at its tip. </li></ul><ul><li>Ag- AgCl wire at the centre of the tube </li></ul><ul><li>Lower tip immerses into the 0.1N HCl Filled in the glass bulb. </li></ul><ul><li>Glass Membrane of the bulb is extremely thin & chemically made up of alumino silicate. </li></ul>
  20. 27. <ul><li>Potential of the glass electrode given by the following equation – </li></ul><ul><li>E = K + 0.0592 (Ph 1 – pH 2 ) at 25 o C </li></ul><ul><li>K = Constant for the electrode, depends on - </li></ul><ul><li>Thickness of glass bulb </li></ul><ul><li>Composition of solution </li></ul><ul><li>Ph 1 = pH of solution in bulb </li></ul><ul><li>pH 2 = pH of test solution </li></ul><ul><li>Now, pH 1 is constant for given electrode so, </li></ul><ul><li>E = K – 0.0592 pH 2 </li></ul>
  21. 28. ADVANTAGES <ul><li>Response is very rapid </li></ul><ul><li>Chemically resistant to oxidizing & reducing agents, dissolved gases, salts etc. </li></ul><ul><li>When Lithia -silica glasses are used, it can be used over the entire Ph range. </li></ul>
  22. 29. Use – for pH measurement <ul><li>DISADVANTAGES </li></ul><ul><li>It is extremely fragile </li></ul><ul><li>Minute abrasions on the surface of the tip, damages the electrode </li></ul><ul><li>It cannot be used with simple potentiometers, because of the high resistance. </li></ul>
  23. 30. Antimony – Antimony Oxide Electrode <ul><li>it consists of a antimony rod dipped into a solution, whose potential or Ph to be determined. </li></ul><ul><li>Antimony oxide is formed on exposure to air </li></ul><ul><li>ADVANTAGES </li></ul><ul><li>It can be used from Ph 3 to Ph 8. it can be used even up to Ph 12 </li></ul><ul><li>It is not easily poisoned or damaged </li></ul><ul><li>Can be used even with viscous fluids </li></ul>
  24. 31. DISADVANTAGES <ul><li>1. This electrode cannot be used in presence of dissolved oxygen, </li></ul><ul><li>oxidizing agents, complexing agents etc.. </li></ul>
  25. 32. ION SELECTIVE ELECTRODE <ul><li>a ) General </li></ul><ul><li>i. electrodes based on determination of cations or anions by the selective adsorption </li></ul><ul><li> of these ions to a membrane surface. </li></ul><ul><li>ii. Often called I on S elective E lectrodes (ISE) </li></ul><ul><li>iii. Desired properties of ISE’s </li></ul><ul><li>‚ minimal solubility – membrane will not dissolve in solution during </li></ul><ul><li> measurement </li></ul><ul><li>– silica, polymers, low solubility inorganic compounds </li></ul><ul><li>‚ Need some electrical conductivity </li></ul><ul><li>‚ Selectively binds ion of interest </li></ul>
  27. 34. ION SELECTIVE ELECTRODE <ul><li>This is a extension of concept of glass electrode </li></ul><ul><li>Electrodes which is specifically permeable to specific types of ions only & other ions are resisted. </li></ul><ul><li>Now-a-days verities of ion selective electrodes are available </li></ul><ul><li>SOLID MEMBRANE ISE </li></ul><ul><li>solid substance of crystal structure is used to select any ion </li></ul>
  28. 36. <ul><li>Specifically designed liquid is filled in specific plastic material or resin material, - semi permeable membrane, highly selective for particular ions. </li></ul>
  29. 37. LIQUID MEMBRANE ISE <ul><li>LIQUID IS FILLED IN PLASTIC OR RESIN MATERIAL , WHICH HAS SEMI PERMIABLE MEMBRANE - SELECTIVE FOR PARTICULAR ION . </li></ul><ul><li>Membrane” usually consists of organic liquid (not soluble in sample) held by porous disk between aqueous reference solution and aqueous sample solution. </li></ul><ul><ul><li>‚ Membrane has ability to selectively bind ions of interest </li></ul></ul>
  30. 38. Example : Calcium dialkyl phosphate Liquid membrane electrodes
  31. 40. ENZYME ELECTRODE <ul><li>Enzyme is chemically bonded to some inner surface </li></ul><ul><li>They are very selective for reaction with substrate </li></ul><ul><li>Thus, this electrode is useful for measurement of insulin or adrenaline etc. </li></ul>
  32. 42. QUINHYDRONE ELECTRODE <ul><li>This electrode is used as a substitute of hydrogen electrode to overcome its disadvantages. </li></ul><ul><li>Bright platinum wire dipped into the test solution which has been saturated with quinhydrone . </li></ul>
  33. 43. PLATINUM ELECTRODE <ul><li>MOST SIMPLE ELECTRODE </li></ul><ul><li>It has a platinum plate or wire or ring </li></ul><ul><li>Platinum is the most inert & non reactive metal – which is a good character for stable electrode </li></ul><ul><li>USE – </li></ul><ul><li>It is used in all redox titrations </li></ul>
  34. 46. pH METERS <ul><li>Same as potentiometers </li></ul><ul><li>pH meters consists of two electrodes </li></ul><ul><li>1. Saturated Calomel electrode </li></ul><ul><li>2. Glass electrode </li></ul><ul><li>☼ USE – they are used to measure pH. </li></ul><ul><li>* here instead of platinum electrode, glass electrode is used </li></ul>
  35. 47. <ul><li>Bze for measurement of Ph, we want to measure selectively only H + </li></ul><ul><li>Ion conc. & glass electrode is H ion electrode. </li></ul><ul><li>FEATURES / ADVANTAGES OF pH METERS </li></ul><ul><li>Due to the following features </li></ul><ul><li>temperature control knob </li></ul><ul><li>Calibration knob </li></ul><ul><li>Temperature display </li></ul><ul><li>Internal calibration, without external calibration using buffers </li></ul>
  36. 50. POTENTIOMETRIC TITRATIONS <ul><li>End point of titrations can be determined by measuring changes in potential of a solution caused by addition of titrant. </li></ul><ul><li>ADVANTAGES </li></ul><ul><li>Colored solutions, dil. Solutions or turbid suspensions can be titrated. </li></ul><ul><li>Titration can be automated </li></ul><ul><li>Mixture of components can be titrated </li></ul><ul><li>Inexpensive & more accuracy </li></ul><ul><li>Reference electrode potential need not be known - constant </li></ul>
  37. 51. APPARATUS
  38. 52. Method of detecting end point <ul><li>Indicator method not suitable </li></ul><ul><li>Potentiometer – determining end point graphically by using – </li></ul><ul><li>Normal titration curve </li></ul><ul><li>Emf vs vol. of titrant </li></ul>
  39. 54. 2. First derivative curve <ul><li>A plot of ▲E/▲V VS Vol. of titrant </li></ul>
  40. 55. Second derivative curve <ul><li>A plot of ▲ 2 E/▲ 2 V VS Vol. of titrant </li></ul>
  41. 56. <ul><li>At the end point , the rate change of potential is maximum. </li></ul><ul><li>APPLICATIONS </li></ul><ul><li>Following types of titrations can be done by potentiometry. </li></ul><ul><li>Acid base titrations </li></ul><ul><li>Redox titrations </li></ul><ul><li>Diazotisation titrations </li></ul><ul><li>Precipitation titrations </li></ul><ul><li>Complexometric titrations </li></ul>
  42. 57. Dead stop end point technique ( Biamperometry) <ul><li>E.g. – determination of water ( moisture content) by KARL FISCHER REAGENT </li></ul><ul><li>It contains two platinum electrodes </li></ul><ul><li>b/w which Small emf is applied </li></ul><ul><li>No current flows till the solution is free from polarizing substances </li></ul><ul><li>Current flows only when both electrodes are depolarized </li></ul>