Ch15-TCF

Ch15-TCF - Ch 15 Electrodes and Potentiometry In...

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1 Ch. 15 Electrodes and Potentiometry In potentiometry , electrodes are used to measure voltages that provide chemical information about a specific analyte. An indicator electrode or working electrode responds to changes in the activity of the analyte. A reference electrode (self-contained half cell) produces a constant reference potential. Ion-selective electrodes ( e.g. pH glass electrode) respond mainly to one ion that selectively interacts with a specialized membrane of the electrode. The potential difference across the membrane ( E ) depends on the activity ( A ) of the analyte target ion (of charge n ) in the external analyte solution. At 25°C:
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Reference Electrodes E cell = E + E Measures [Fe 2+ ] [Fe 3+ ] 2 Anode: Ag + Cl AgCl + e Cathode: Fe 3+ + e Fe 2+ Ag( s ) | AgCl( s ) | Cl ( aq , satd) || Fe 3+ ( aq ), Fe 2+ ( aq ) | Pt( s )
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3 Silver-silver chloride reference electrode Wire lead Air inlet to allow electrolyte to drain slowly through porous plug Ag wire bent into a loop Aqueous solution saturated with KCl and AgCl AgCl paste Solid KCl plus some AgCl Porous plug for contact with external solution (salt bridge) AgCl( s ) + e Ag( s ) + Cl ( aq ) ||Cl ( aq )|AgCl( s )|Ag( s ) E ( satd KCl) = +0.197 V ( cf . E o = +0.222 V; 1 M Cl ) Double-junction reference electrode
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4 Saturated calomel electrode (S.C.E.) 1/2 Hg 2 Cl 2 ( s ) + e Hg( l ) + Cl ( aq ) Mercury (I) chloride (calomel) || KCl( aq )|Hg,Hg 2 Cl 2 ,KCl(paste)|Hg( l )|Pt( s ) E ( satd KCl) = +0.241 V ( cf . E o = +0.268 V; 1 M Cl ) Pt wire Hole to allow drainage through porous plug Hg( l ) Hg, Hg 2 Cl 2 , KCl Glass wool Opening Saturated KCl solution KCl( s ) Porous plug (salt bridge)
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5 Relating reference electrodes A working electrode, A , has a potential w.r.t. a saturated calomel electrode (SCE) of 0.436 V. What would this voltage be w.r.t. SHE or silver-silver chloride electrodes?
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This note was uploaded on 03/28/2011 for the course CHEM 300L at USC.

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Ch15-TCF - Ch 15 Electrodes and Potentiometry In...

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