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Potentiometry (Ch 15-1 to 15-3)

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Potentiometry (Ch 15-1 to 15-3) Potentiometry (Ch 15-1 to 15-3) • To describe the construction and half-cell reactions of a Ag|AgCl and a calomel reference electrode (Do Exercise 15-1 & 15-2) • To determine ionic activities using a metal indicator electrode in combination with a reference electrode (Do Exercise 15-9) • To explain the cause of junction potential (Do Exercise 15-13) Chem215/P.Li/Potentiometry/P 1 Classification of electroanalytical techniques Electroanalytical chemistry Non-faradaic methods Faradaic methods potentiometry potentiometric conductometry voltammetry electrogravimetry coulometry titration or electrodeposition at non-stationary at stationary electrode electrode (e.g. polarography) (e.g. cyclic voltammetry) Chem215/P.Li/Potentiometry/P 2 1 Potentiometry Potentiometry uses electrodes to measure voltages which are related to chemical information. Indicator electrode or working electrode: responds to analyte activity. Reference electrode: maintains a fixed (reference) potential. Ag(s) AgCl(s)Cl - (aq) Fe3+ (aq), Fe2+ (aq) Pt(s) 144424443 144424443 reference electrode indicator electrode Chem215/P.Li/Potentiometry/P 3 Reference electrodes Ag½AgCl reference electrode See Fig. 15-3 for a commercial single-junction Ag½AgCl reference electrode See Fig. 15-4 for a commercial double-junction Ag½AgCl reference electrode There is an additional compartment to contain KCl solution or any other solution (e.g. KNO3) that is more compatible with the analyte solution. At 25 °C, E° = + 0.222V AgCl(s) + e- Ag(s) + Cl- (aq) E(saturated KCl) = + 0.197V 0.05916 E = E o - log[Cl - ] 1 0.197 = 0.222 - 0.05916log[Cl - ] \[Cl - ] = 2.65M The advantage of using saturated KCl solution is the [Cl-] does not change even if some water evaporates. Chem215/P.Li/Potentiometry/P 4 2 Reference electrodes Calomel electrode (calomel is Hg2Cl2) - - ½Hg2Cl2(s) + e Hg(l) + Cl (aq) Fig. 15-5 for a commercial double-junction saturated At 25 °C, E° = + 0.268V calomel electrode (S.C.E.) E(saturated KCl) = + 0.241V How to convert voltages between different reference scales? Fig. In Sec 15-1, voltage scale with reference to standard hydrogen electrode. S.C.E. A -0.461 B -0.417 Ag|AgCl +0.033 -0.011 S.H.E. -0.220 0 +0.197 +0.230 +0.241 Chem215/P.Li/Potentiometry/P 5 Indicator electrodes A) metal electrode: inert metals (e.g. Pt, Au and carbon); electroactive metals (e.g. Ag, Hg). B) Ion-selective electrode (I.S.E.): This is not based on redox processes. Instead, selective migration of one type of ion across an electrode membrane generates an electric potential (e.g. pH electrode, F- electrode). Chem215/P.Li/Potentiometry/P 6 3 Indicator electrodes + - o Ag + e Ag E+ = 0.799V - - ½Hg2Cl2(s) + e Hg(l) + Cl (aq) E-° = 0.241V 1 - + Hg(l) 2 Hg2Cl2 (s) Cl (aq) Ag (aq) Ag(s) é 0.05916 1 ù + E = E+ - E- = ê0.799- log + ú - 0.241 = 0.558+ 0.05916log[ Ag ] ë 1 [Ag ]û So the voltage of the cell provides a direct measure of [Ag+]. Chem215/P.Li/Potentiometry/P 7 Junction potential Junction potential is developed when 2 dissimilar electrolyte solutions are in contact. Fig. 15-7: development of junction potential caused by unequal mobilities of Na+ and Cl-. Since Cl- diffuses faster than Na+, excess negative charge is developed on the side containing excess Cl-. KCl and KNO3 are usually used in salt bridge because of the similar mobilities of + - - K , Cl and NO3 . Since the junction potential is usually not known, its presence puts a fundamental limitation on the accuracy of direct potentiometric meaurements. Chem215/P.Li/Potentiometry/P 8 4.
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