421-821-chapter-23

421-821-chapter-23 - Potentiometric Methods A Introduction...

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Potentiometric Methods A.) Introduction : 1.) Potentiometric Methods: based on measurements of the potential of electrochemical cells in the absence of appreciable currents (i o 0) 2.) Basic Components: a) reference electrode: gives reference for potential measurement b) indicator electrode: where species of interest is measured c) potential measuring device
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B.) Reference Electrodes : 1.) Need one electrode of system to act as a reference against which potential measurements can be made relative comparison. Desired Characteristics: a) known or fixed potential b) constant response c) insensitive to composition of solution under study d) obeys Nernest Equation e) reversible
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B.) Reference Electrodes : 2.) Common Reference Electrodes used in Potentiometry a) Calomel Electrode (Hg in contact with Hg 2 Cl 2 & KCl) i. S aturated C alomel E lectrode (SCE) very widely used SCE
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b) Silver/Silver Chloride Electrode - most widely used reference electrode system - Ag electrode in KCl solution saturated with AgCl ½ cell: Ag/AgCl (satd), KCl (xM)|| Advantage one advantage over SCE is that Ag/AgCl electrode can be used at temperatures > 60 o C Disadvantage – Ag reacts with more ions c) Precautions in the Use of Reference Electrodes - need to keep level of solution in reference electrode above level in analyte solution - need to prevent flow of analyte solution into reference electrode ° can result in plugging of electrode at junction erratic behavior Vycor plug
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C.) Indicator Electrodes : 1.) Detects or Responds to Presence of Analyte Three Common Types: a) Metallic Indicator Electrodes b) Membrane Indicator Electrodes c) Molecular Selective Electrode
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2.) Metallic Indicator Electrode ( Four Main Types) a) Metallic Electrodes of the First Kind i.
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