Potentiometry

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Potentiometry

  1. 1. Potentiometry (Chapter 14) Potentiometry Ecell w/o current flow  info abt. a chemical system • endpoint in a titration • Measure [ion] Rapid; Simple; Inexpensive Need Reference electrode Indicator electrode Potential measuring device
  2. 2. Potentiometry Typical Cell Reference electrode / salt bridge / analyte soln / Indicator electrode Eref Ej Eind Reference electrode: Anode by convention Ecell = Ecath - Eanode = Eind - Eref + Ej
  3. 3. Potentiometry Cell:
  4. 4. Potentiometry A. Junction Potential (Ej) – Potential that develops across the boundary between 2 electrolyte solns of different composition – Fundamental problem in Potentiometry – Limits accuracy – Small unk. value A junction potential of ~ 3 mV produces an error of ~ 0.05 pH units (12% error in [H+])
  5. 5. 0.01 M HCL - + 1 M HCl examples Skoog etal
  6. 6. Caused differences in mobility of the ions
  7. 7. Potentiometry B. Reference Electrodes Why is the SHE not useful as a reference in potentiometry a. Limited practical use b. Difficult to prepare electrode surfaces c. Difficult to control the activity of H+ d. All of the above Which one of the following is a arbitrary reference electrode for measuring electrode potentials a. Ag/AgCl b. Saturated Calomel Electrode c. Normal Hydrogen electrode d. All of the above
  8. 8. 1. Ag/AgCl Ag / AgCl, KCl (x M) // AgCls + 1e-  Ags + Cl- E0 = 0.222 V Potential determined by [Cl- ] Sat’d, E = 0.197 V Two common Reference Electrodes Ag/AgCl SCE
  9. 9. 2. calomel electrode Hg / Hg2Cl2 (sat’d), KCl (x M) // Hg2Cl2(s) + 2e-  2Hgl + 2Cl- Potential determined by [Cl- ] Standard conditions: E0 = 0.268 V Sat’d calomel electrode (SCE) E = 0.241 V Why are sat’d solutions used?
  10. 10. Voltage Conversions Between Different Reference Scales p. 331 SCE to SHE; SHE to SCE SHE to AgCl; AgCl to SHE
  11. 11. C. Indicator Electrodes Electrode that responds directly to the analyte Ideal: Responds rapidly, reproducibly Selective Two main types 1- metallic 2- membrane
  12. 12. Potentiometry 1. Metallic Electrodes Example, Pt in a solution of Fe2+ /Fe3+ Potential determined by the Nernst Equation Eind = E0 – 0.0592 log [Fe2+ ]/[Fe3+ ] Ecell = Eind – E AgCl + Ej Not selective
  13. 13. Potentiometry 2. Membrane Indicator Electrodes – Ion selective electrodes (pIon electrodes) – Respond “selectively” to one species in solution Inside: soln containing the ion of interest, const. A Outside: soln containing the ion of interest, var. A Measure potential difference across the membrane Thin membrane that separates the sample from the inside of the electrode
  14. 14. pH measurement with a glass electrode Glass combination electrode
  15. 15. pH electrode Cell schematic Ag l AgCl l Cl- (x M) ll H+ (outside) l H+ (inside), Cl- (x M) l AgCl l Ag Cations (Na+ ) bind oxygen in SiO4 structure
  16. 16. pH Electrode Membrane must be hydrated
  17. 17. Functions by exchange of ions at the surface H+ Gls = H+ aq + Gl- s Glass 1 Soln 1 H+ Gls = H+ aq + Gl- s Glass 2 Soln 2 Eb = E1 – E2 = 0.05916 log Ain / Aout Eb = L – 0.0592pH Position of these two equil. are determined by aH+ in soln on the two sides of the membrane
  18. 18. Glass Electrode Potential Three components 1- boundary potential 2- potential of internal Ag/AgCl reference electrode 3- small asymmetry potential Eind = Eb + Eref2 + Easy Eind = L’ + 0.0592 Log A1 + Eref2 + Easy Eind = K + 0.0592 Log [H+] Eind = K + β0.0592 Log [H+] Electromotive efficiency
  19. 19. Activity becomes important when a. Ions have divalent or trivalent charges b. Ionic strength of the solution is high c. All of the above d. None of the above Activity vs Concentration
  20. 20. Calibrating a glass electrode Eind = K + 0.0592 Log [H+] For every 10 fold change in activity, the potential should change by 59.2 mV Slope = ?
  21. 21. Potentiometry Errors (limitations) in pH measurements 1- must calibrate electrode 2- junction potential and drift 3- alkaline (sodium) error 4- acid error 5- allow membrane to equilibrate 6- membrane must be hydrated 7- Temperature
  22. 22. Other examples of ISE Eind = K + (0.0592 / 2)Log [Ca2+ ] Liquid-based ISE
  23. 23. E = K – 0.0592 Log [F-] Slope = ? Solid State ISE Linear range = ?
  24. 24. Potentiometry Equation written from the point of view that the membrane electrode responds to only one ion, maybe more Full expression E = const. + 2.303RT / zF log (Ai + Ki,jAj zi/zj ) K  ranges from 0 to values greater than 1 Selectivity coefficient KA,X = response of X / response of A The smaller, the less interference by X
  25. 25. Potentiometry Monovalent cations (assume k= 0) E = const + 0.0592 log Ax+ Monovalent anions (assume k = 0) E = const – 0.0592 log Aa-
  26. 26. Constant made up of several constants Determine experimentally 1- meas. Ecell for std sol of known conc. 2- meas Ecell for unknown conc. make assumption that K is unchanged
  27. 27. Advantages (Figures of Merit) and other characteristics a.Linear response over a wide range b.Non destructive c.Non contaminating d.Short response time (min) e.Unaffected by color and turbidity f.Precision: OK (1% at best) g.Sensitivity/Detection limits: (10-6 to 10-9*** M) h.Standard addition method often used (Why ?)
  28. 28. Problems: Questions to ask: Membrane or Metal ISE?? If membrane, Ecell = Emem – Eref Ememb = const +/− 0.0592 log a If metal, Ecell = Ecathode – Eref Ecathode  Nernst Eq
  29. 29. Calculate the potential of the following cell when the aqueous solution is 7.40 x 10-3 M Hg2+ SCE // aq soln / Hg SCE is the saturated Calomel electrode, E0’ = 0.244 V
  30. 30. Example A pH glass/calomel electrode was found to develop a potential of –0.0412 V when used with a buffer of pH 6.00. With an unk soln, the potential was –0.2004 V. Calculate the pH of the solution.

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