Potentiometric titration
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Potentiometric titration






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Potentiometric titration Presentation Transcript

  • 2. 1. Introduction:  Potentiometric method include two type of measurement these are ; 1. Direct measurement of an electrode potential from which concentration of an active ion may be found 2. Change of E.M.F. of an electrode cell brought about by the addition of titrant
  • 3.  Both methods are based on quantitative measurement of E.M.F. of cell is given by E cell = E reference + E indicator +E junction  As the reference electrode potential is independent of solution and junction potential is constant so the cell potential is measure of indicator electrode potential and can give information on the nature and concentration of substance under test.
  • 4.  In the potentiometric titration the titration reaction is followed by measurement of concentration of one or more species potentiometrically. The beaker or flask becomes one of the half cells and the reference electrode is the other half cell.
  • 5.  There is a different between titration reaction and cell reaction; in titration reaction the reactant and products be in the same half cell and the titration reaction always at equilibrium while in cell reaction is not at equilibrium.
  • 6.  In potentiometric titration the change in electrode potential upon the addition of titrant are noted by the volume of titrant added. At the end point the rate of change of potential is maximum  The potentiometric end point has been applied to all types of chemical reaction. It can be used with colored or opaque solution
  • 7. 2. Instrumentations: There are three types of instrumentations which are used for measurement of potentials these are: 1. Non – electronic instruments 2. Electronic instruments 3. Automatic instruments
  • 8. 1.  Non – electronic instruments A potentiometer for titrations can be made from simple instrument of battery for current supply, two dry cell, a resistance and voltmeter; the instrument can be operated by dipping the electrodes in the sample solution and record the voltmeter reading. 2. Electronic instruments  These instruments have many advantages over non- electronic instruments
  • 9. 3. Automatic instruments  The use of manual instrument to locate the end point and to draw a titration curve is time consuming and boring job; so automatic instrument for recording and performing titration curve provides a logical solution
  • 10. 3. Types of Potentiometric titration:  Potentiometric titrations may be applied to different type of reactions of these are; acid – base, oxidation reduction, precipitation and complexmetirc
  • 11. 1.  Acid – Base titrations: The neutralization of acid or base is always accompanied by the changes the concentration of H+ and OH- ions.  In these reactions hydrogen electrode is used as indicator electrode and N- calomel electrode as a reference electrode.
  • 12.  A known volume of the acid titrant is kept in a beaker with continuous stir; the hydrogen and N- calomel electrode are connected by the salt bridges and connected to a potentiometer which record the EMF of the solution, into the beaker the after the addition of base from burette the values of EMF are plotted against volume of titrant added and a curve are obtained.
  • 13.  The potential of a hydrogen electrode is given by : E+ E⁰-0.0591 log a H+ Where E⁰ is standard electrode potential and pH is - logaH+ E+ E⁰+ 0.0591 pH  As the standard electrode potential is constant so the cell potential or EMF is proportional to the change of pH during the reaction.  The point where E.M.F increased rapidly is the end point.
  • 14.  Amore sensitive and precise method for measure end point is to plot the slope of curve against volume as the slope is maximum at equivalence point, the maximum value give the end point
  • 15. 2. Complexmetric titration  A metal electrode is used whose ions are involved in the complex formation, example silver electrode is used to measure cyanide ion with standard solution of silver Ag++ 2CN- (Ag (CN)2)- K= (Ag+)(CN-)2 (Ag(CN)-2)
  • 16.  In this case, solid silver cyanide begins to get precipitated soon after the equivalence point. The further addition of silver neither changes the concentration of the complex nor changes the silver ion to any extent, so that the curve has an almost horizontal portion shortly after the equivalence point
  • 17.  In many Complexmetric reactions the situation cannot be handled so easily because more than one complex is formed. Thus, the reactions in the case of mercuric ion with cyanide are: Hg2++3CNHg2++4CN Hg (CN)3Hg (CN)42- However, these situations have became widely used because of the discovery of the metal chelating agents such as EDTA
  • 18. 3. Oxidation-reduction titrations  Redox reactions can be followed by an inert indicator electrode. The electrode assumes a potential proportional to the logarithm of the concentration ratio of two oxidation states of the reactant or the titrant whichever is capable of properly poising the substance being oxidized to substance being reduced.
  • 19.  For example , Ca4+ + Fe2+ Ce3+ + Fe3+ It is generally considered that such a reaction consists essentially of two half reactions whose standard potentials may be used to calculate the standard potential of the reactions.
  • 20.  Fe+  Ce4+  Ce4+ + Fe2+  Fe3+ + e, Eo = -0.67 V ..……. (i) e Ce3+ , Eo = +1.61 V …….. (ii) Fe3++ Ce3+, Eo = +0.85 V… (iii) The equilibrium constant , K, of any reaction may be calculated from the following formula : Eo = log10 K ……. (iv)  Where Eo is the number of equivalent of electricity associated with one molar unit of reaction .
  • 21.  If an acidic ferrous solution is titrated with a standard ceric solution at 25o C, the potential of a platinum electrode in contact with the solution will be given by either of the following equations: E=Eo ce4+/ ce3+ - 0.0591 log10 [ce3+] …. (v) 1 [ce4+] E=Eo Fe3+/ Fe2+ - 0.0591 log10 [Fe3+] … (vi) 1 [Fe2+]
  • 22.  It would be more convenient to use the equation (vi) before the equivalence point, as the right hand term of this equation could be easily found from the known extent of the titration . if equation (v) is used , then the [Ce3+] / Ce4 ) ratio has to be calculated by means of the equilibrium constant, After the equivalence point, calculations are done by means of equation (v).
  • 23.  From equation (vi) it is evident that the potential that the potential at the start of the titration should be—co, because Fe3+ ions are the only ions present and there are no Fe3+ ions.  At the mid-point of the titrations, where [Fe2+]=[Fe3+], equation (vi) because : E=Eo Fe3+/ Fe2+ ……(vii)
  • 24.  At the equivalence point, the concentration of unchanged ferrous ions will be equal the concentration of the unchanged ceric ions. Similarly , the concentration of cerous ions will be equal to the concentration of ferric ions.  Thus, it can be concluded that; [Fe2+] =[Ce4+] ……(viii) [Fe3+] [Ce3+]
  • 25. Now, k = [Fe3+][Ce3+] ……(ix) [Fe2+][Ce4+]  At equivalence point, [Fe3+][Ce3+]= k……(x) [Fe2+][Ce4+]
  • 26.  On combining equations (v) and (vi) with equation (x) , we get Eep = EoCe4+ / Ce3+ - log10 K …..(xi) Eep = EoFe3+ / Fe2+ + log10 K  ……(xii) On adding, equations (xii), we get Eep = EoFe3+/ce3++EoFe3+/Fe2+ 2  where Eep= End point potential .
  • 27.  Oxidation-reduction titration may be used in procedures such as monitoring of cyanide wasters from metal plating industries or chlorine compounds in bleach compounds manufacturing, and the used of these bleach compounds in paper man fracturing. They are also extensively used in water pollution, sewage treatment, agricultural and biochemical studies.
  • 28. 4. Precipitation titrations. Any precipitation titration that involves insoluble salts of metals such as mercury, silver, lead and copper may be followed potentiometrically . The indicator electrode may be made of the metal involved in the reaction or may be an electrode whose potential is governed by the concentration of the anion being precipitated
  • 29.  The magnitude of the potential change at the end point depends on the solubility of the substance being precipitated as well as the concentration involved. The titration of chloride ions with a standard solution of silver nitrate using a silver metal indicator electrode is an example of a precipitation titration.
  • 30.  The other electrode to complete the cell is unimportant, provided that it is a true reference electrode, i.e., it maintains a constant potential. In the above case it will be assumed that the normal hydrogen electrode (N H E.) is used and this assumption is convenient because standard potential may be used directly. The potential of silver electrode will be governed by the appropriate Nernst equation :
  • 31. EAg+/ Ag = Eo Ag+/ Ag + log10 [Ag+] …..(i)  As soon as enough silver nitrate to precipitate Cl as AgCl has been added, the following equilibrium is established, AgCl  Ag+ + Cl- ………….(ii) The equilibrium constant for the above reaction is KAgCl = [Ag+][Cl-]=10-10 …………..(iii)
  • 32.  If 0.1 N sodium chloride is titrated against 0.1 N silver nitrate, the silver ion concentration may be considered to be 10-9 N as soon as few drops of silver nitrate have been added. Equation (i) can be used to calculate the indicator electrode potential.  EAg+/Ag=0.08 V + 0.0551 log10 10-9 = 0.2681 …….. (iv)
  • 33.  Similarly, half wave through the titration will be when the chloride ions concentration has been reduced to 0.033 N.  EAg+/Ag=0.08 V+0.0591 log 10 (3 10-9) = 0.30 V  At the equivalence point, [Ag+]=[Cl-] = 10-5N  EAg+/Ag=0.08 V+0.0591 log 10 10-5 = 0.50 V
  • 34. 4. Non – aqueous titration  The potentiometric method has been found to be useful for carrying out titrations in nonaqueous solvents. The ordinary glass- calomel electrode system can be used  Generally the millivolt scale of the potentiometer rather than the pH scale should be employed because the potential in nonaqueous titration may exceed the pH scale.
  • 35.  Advantages of Potentiometric titrations over 'classical' visual indicator methods are: 1. Can be used for coloured, turbid or fluorescent analyte solution. 2. Can be used if there is no suitable indicator or the colour change is difficult to ascertain.
  • 36. 3. Can be used in the titration of polyprotic acids, mixtures of acids, mixtures of bases or mixtures of halides. loured, turbid or fluorescent analyte solution 4. The apparatus required is inexpensive, reliable and readily available. 5. It is easy to interpret the titration curve.