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Potentiometry - ppt, Electro Analytical Method

Potentiometry - ppt, Electro Analytical Method

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  • 1. POTENTIOMETRY by Mr. Shaise Jacob Faculty, Nirmala College of Pharmacy Muvattupuzha Kerala, India
  • 2.
    • Measuring the potential or emf of a solution
    • Using a set of indicator & reference electrode.
    • INDICATOR ELECTRODE
    • Responds to changes in emf or Ph of the solution
    • To indicate emf or ph
    • Example – Glass electrode, antimony electrode
  • 3. REFERENCE ELECTRODE
    • Which has a standard potential on its own
    • And its potential does not change to whichever solution it is dipped.
    • E.g. hydrogen electrode, saturated calomel electrode & silver-silver chloride electrode
    • Most commonly used is saturated calomel electrode
  • 4. Nernst equation
    • 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.
  • 5.
    • The potential (E) of a metal electrode at 25 o C immersed into a solution of its own ions is given by
    • E o – Standard potential of the metal
    • N – valency of ions
    • C – concentration of ions
  • 6. HYDROGEN ELECTRODE
  • 7.  
  • 8.
    • It can be used as indicator as well as reference electrode
    • Platinum coil coated with platinum black and has wire contacts through mercury
    • The assembly enclosed in a glass covering through which Hydrogen 99.8% is passed at 1 atm pressure
  • 9.
    • 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
    • The magnitude of SHE potential is considered to be zero. It is used-
    • 1) For the determination of electrode potential of metal electrode system.
    • 2) For the determination of pH of the solution.
  • 10. Limitations
    • 1) It is rather difficult to regulate the pressure of the H2 gas to be at exactly 1atm throughout the experiment.
    • 2) If the solution contains any oxidizing agent, the H2 electrode cannot be used.
    • 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.
    • 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.
  • 11. SATURATED CALOMEL ELECTRODE
  • 12.  
  • 13.  
  • 14.  
  • 15.
    • It contains of an inner jacket and outer sleeve
    • Inner jacket has wire contact with Hg and plugged with a mixture of calomel Hg 2 Cl 2 & KCl
    • Outer sleeve – crystals of KCl & porous plug of asbestos
    • Space b/w inner jacket & outer sleeve is filled with either saturated KCl or 1N KCl or 0.1N KCl
  • 16.
    • Application
    • The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry.
    • 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.
    • The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode
  • 17. MERITS OF SCE
    • Ease of construction
    • Stability of potential
  • 18. Silver- Silver chloride electrode
    • The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, inexpensive, very stable and non-toxic.
    • 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.
  • 19.  
  • 20.
    • Silver wire coated electrolytically with silver chloride and dipped into KCl
    • It has the advantage that it is easy to use
    • Demer it is that it is difficult to prepare
  • 21.  
  • 22. Mercurous Sulphate Electrode
    • Type – Reference electrode
    • ♠ Similar in construction to the calomel electrode but utilizes dilute sulphuric acid saturated with mercurous sulphate.
    • Use - I t is used in solution, where silver or lead ions are present
  • 23. INDICATOR ELECTRODES
  • 24. INDICATOR ELECTRODES
    • ♠ Indicator electrode indicates the potential or Ph of a solution in comparison to a reference electrode of a known potential.
    • Hydrogen Electrode
    • Glass Electrode
    • Most widely used indicator electrode
  • 25.  
  • 26.
    • It is selective to change in conc. of hydrogen ions .
    • It consists of a glass tube with a thin Ph sensitive glass bulb at its tip.
    • Ag- AgCl wire at the centre of the tube
    • Lower tip immerses into the 0.1N HCl Filled in the glass bulb.
    • Glass Membrane of the bulb is extremely thin & chemically made up of alumino silicate.
  • 27.
    • Potential of the glass electrode given by the following equation –
    • E = K + 0.0592 (Ph 1 – pH 2 ) at 25 o C
    • K = Constant for the electrode, depends on -
    • Thickness of glass bulb
    • Composition of solution
    • Ph 1 = pH of solution in bulb
    • pH 2 = pH of test solution
    • Now, pH 1 is constant for given electrode so,
    • E = K – 0.0592 pH 2
  • 28. ADVANTAGES
    • Response is very rapid
    • Chemically resistant to oxidizing & reducing agents, dissolved gases, salts etc.
    • When Lithia -silica glasses are used, it can be used over the entire Ph range.
  • 29. Use – for pH measurement
    • DISADVANTAGES
    • It is extremely fragile
    • Minute abrasions on the surface of the tip, damages the electrode
    • It cannot be used with simple potentiometers, because of the high resistance.
  • 30. Antimony – Antimony Oxide Electrode
    • it consists of a antimony rod dipped into a solution, whose potential or Ph to be determined.
    • Antimony oxide is formed on exposure to air
    • ADVANTAGES
    • It can be used from Ph 3 to Ph 8. it can be used even up to Ph 12
    • It is not easily poisoned or damaged
    • Can be used even with viscous fluids
  • 31. DISADVANTAGES
    • 1. This electrode cannot be used in presence of dissolved oxygen,
    • oxidizing agents, complexing agents etc..
  • 32. ION SELECTIVE ELECTRODE
    • a ) General
    • i. electrodes based on determination of cations or anions by the selective adsorption
    • of these ions to a membrane surface.
    • ii. Often called I on S elective E lectrodes (ISE)
    • iii. Desired properties of ISE’s
    • ‚ minimal solubility – membrane will not dissolve in solution during
    • measurement
    • – silica, polymers, low solubility inorganic compounds
    • ‚ Need some electrical conductivity
    • ‚ Selectively binds ion of interest
  • 33. ION SELECTIVE ELECTRODE
  • 34. ION SELECTIVE ELECTRODE
    • This is a extension of concept of glass electrode
    • Electrodes which is specifically permeable to specific types of ions only & other ions are resisted.
    • Now-a-days verities of ion selective electrodes are available
    • SOLID MEMBRANE ISE
    • solid substance of crystal structure is used to select any ion
  • 35.  
  • 36.
    • Specifically designed liquid is filled in specific plastic material or resin material, - semi permeable membrane, highly selective for particular ions.
  • 37. LIQUID MEMBRANE ISE
    • LIQUID IS FILLED IN PLASTIC OR RESIN MATERIAL , WHICH HAS SEMI PERMIABLE MEMBRANE - SELECTIVE FOR PARTICULAR ION .
    • Membrane” usually consists of organic liquid (not soluble in sample) held by porous disk between aqueous reference solution and aqueous sample solution.
      • ‚ Membrane has ability to selectively bind ions of interest
  • 38. Example : Calcium dialkyl phosphate Liquid membrane electrodes
  • 39.  
  • 40. ENZYME ELECTRODE
    • Enzyme is chemically bonded to some inner surface
    • They are very selective for reaction with substrate
    • Thus, this electrode is useful for measurement of insulin or adrenaline etc.
  • 41.  
  • 42. QUINHYDRONE ELECTRODE
    • This electrode is used as a substitute of hydrogen electrode to overcome its disadvantages.
    • Bright platinum wire dipped into the test solution which has been saturated with quinhydrone .
  • 43. PLATINUM ELECTRODE
    • MOST SIMPLE ELECTRODE
    • It has a platinum plate or wire or ring
    • Platinum is the most inert & non reactive metal – which is a good character for stable electrode
    • USE –
    • It is used in all redox titrations
  • 44.  
  • 45.  
  • 46. pH METERS
    • Same as potentiometers
    • pH meters consists of two electrodes
    • 1. Saturated Calomel electrode
    • 2. Glass electrode
    • ☼ USE – they are used to measure pH.
    • * here instead of platinum electrode, glass electrode is used
  • 47.
    • Bze for measurement of Ph, we want to measure selectively only H +
    • Ion conc. & glass electrode is H ion electrode.
    • FEATURES / ADVANTAGES OF pH METERS
    • Due to the following features
    • temperature control knob
    • Calibration knob
    • Temperature display
    • Internal calibration, without external calibration using buffers
  • 48.  
  • 49.  
  • 50. POTENTIOMETRIC TITRATIONS
    • End point of titrations can be determined by measuring changes in potential of a solution caused by addition of titrant.
    • ADVANTAGES
    • Colored solutions, dil. Solutions or turbid suspensions can be titrated.
    • Titration can be automated
    • Mixture of components can be titrated
    • Inexpensive & more accuracy
    • Reference electrode potential need not be known - constant
  • 51. APPARATUS
  • 52. Method of detecting end point
    • Indicator method not suitable
    • Potentiometer – determining end point graphically by using –
    • Normal titration curve
    • Emf vs vol. of titrant
  • 53.  
  • 54. 2. First derivative curve
    • A plot of ▲E/▲V VS Vol. of titrant
  • 55. Second derivative curve
    • A plot of ▲ 2 E/▲ 2 V VS Vol. of titrant
  • 56.
    • At the end point , the rate change of potential is maximum.
    • APPLICATIONS
    • Following types of titrations can be done by potentiometry.
    • Acid base titrations
    • Redox titrations
    • Diazotisation titrations
    • Precipitation titrations
    • Complexometric titrations
  • 57. Dead stop end point technique ( Biamperometry)
    • E.g. – determination of water ( moisture content) by KARL FISCHER REAGENT
    • It contains two platinum electrodes
    • b/w which Small emf is applied
    • No current flows till the solution is free from polarizing substances
    • Current flows only when both electrodes are depolarized
  • 58.  
  • 59.  
  • 60.  
  • 61.