Lecture 22. Potentiometry Pt1

Lecture 22. Potentiometry Pt1 - Chapter 15...

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Unformatted text preview: Chapter 15: “Potentiometry” Chapter Metal Electrodes (e.g. Ag/Ag+ ) E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + Therefore a plot of E vs. log(1/aAg+) is linear! Chapter 15: “Potentiometry” Chapter Metal Electrodes (e.g. Ag/Ag+ ) E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + Therefore a plot of E vs. log(1/aAg+) is linear! Metal- Metal Salt Electrodes (e.g. Ag/AgCl ) E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + with a Ag + = K sp a Cl E=E 0 AgCl, Ag 2.303RT log aCl F Therefore a plot of E vs. log aCl- is linear! Therefore Potentiometric Response of a AgCl Electrode Potentiometric Response of a AgCl Electrode Lecture notes; W. Simon, Swiss Federal Institute of Technology, Zurich. Chapter 15: “Potentiometry” Chapter Metal Electrodes (e.g. Ag/Ag+ ) E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + Therefore a plot of E vs. log(1/aAg+) is linear! Metal- Metal Salt Electrodes (e.g. Ag/AgCl ) E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + with a Ag + = K sp a Cl E=E 0 AgCl, Ag 2.303RT log aCl F Therefore a plot of E vs. log aCl- is linear! Therefore Junction Potential Junction Ecell = (Ecathode – Eanode) + Econst+ Ejunction Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Work Functions (CRC Handbook of Chemistry and Physics) Work Functions Measured by: TE: Thermoionic emission PE: Photoelectric effect FE: Field emission CPD: Contact potential difference Junction Potential Junction Ecell = (Ecathode – Eanode) + Econst+ Ejunction Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Freeman and Company, New York, 2001. Freeman Junction Potential Junction Daniel C. Harris, Exploring Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. Freeman Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Daniel Freeman and Company, New York, 2001. Freeman The Liquid Junction Potential The Electrochemical Methods: Fundamentals and Applications, 2nd ed., Allen J. Bard, Larry R. Faulkner; Wiley, New York, 2001. Daniel C. Harris, Exploring Chemical Analysis, 2nd ed., W.H. Daniel Freeman and Company, New York, 2001. Freeman Junction Junction Potential Daniel C. Harris, Exploring Daniel Chemical Analysis, 3rd ed., W.H. Freeman and Company, New York, 2005. New Gary D. Christian, Analytical Gary Chemistry, 6th ed., John Wiley Chemistry, & Sons, Inc., U.S.A., 2004. Sons, The Liquid Junction Potential (I will not ask you to use this equation, but you must know that liquid junction (I potentials can be calculated reasonably easily IF you know the concentrations of all ions. ) concentrations If it is assumed that If * ci = ai * linear concentration profile left to right E junction å = å i i z i ui zi z i ui [Ci,right - Ci,left ] [Ci,right - Ci,left åz RT ln ]F åz i i i i ui Ci,left ui Ci,right Henderson equation Morf, W. E. The Principles of IonThe Selective Electrodes and of Membrane Selective Transport; Elsevier: New York, 1981. Transport Two Tricks to Minimize Liquid Junction Potentials Potentials • Use a salt bridge with ions of matched mobility. E.g.: Equal mobility for an anion and cation of the same charge). Why? Why? • Use a salt bridge with a high salt concentration. Why? Another Pitfall that Prevents Linear Calibration Curves Calibration E right = E 0 Ag + , Ag 2.303RT 1 log F a Ag + • Theory clearly shows that there is a linear relationship between E and the logarithm of the activity, not the logarithm of concentration. activity, • E will only depend linearly on the logarithm of the concentration if… concentration (1) … (2) … The Liquid Junction Potential The Common Salt Bridges: KCl (3 M) LiOAc (1 M) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Morf, The Elsevier: New York, 1981. Elsevier: ...
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