221-chapter-15

221-chapter-15 - Electrodes and Potentiometry Introduction 1 Potentiometry Use of Electrodes to Measure Voltages that Provide Chemical Information

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Electrodes and Potentiometry Introduction 1.) Potentiometry Use of Electrodes to Measure Voltages that Provide Chemical Information - Various electrodes have been designed to respond selectively to specific analytes Use a Galvanic Cell - Unknown solution becomes a ½-cell - Add Electrode that transfers/accepts electrons from unknown analyte - Connect unknown solution by salt bridge to second ½-cell at fixed composition and potential Indicator Electrode : electrode that responds to analyte and donates/accepts electrons Reference Electrode : second ½ cell at a constant potential Cell voltage is difference between the indicator and reference electrode
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Binding generates potential difference. Electrodes and Potentiometry Introduction 2.) Example A Heparin Sensor - Voltage response is proportional to heparin concentration in blood - Sensor is selective for heparin Negatively charged heparin binds selectively to positively charged membrane. heparin Potential is proportional to [heparin]
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Electrodes and Potentiometry Reference Electrodes 1.) Overview Potential change only dependent on one ½ cell concentrations Reference electrode is fixed or saturated doesn’t change! Reference electrode, [Cl ] is constant Potential of the cell only depends on [Fe ] & [Fe ] Pt wire is indicator electrode whose potential responds to [Fe ]/[Fe ] { } ] Cl log[ . . ] Fe [ ] Fe [ log . . E cell - + + - - - = 05916 0 222 0 1 05916 0 771 0 3 2 Unknown solution of [Fe ] & [Fe ]
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Electrodes and Potentiometry Reference Electrodes 2.) Silver-Silver Chloride Reference Electrode E o = +0.222 V Activity of Cl - not 1 E (sat,KCl) = +0.197 V
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Electrodes and Potentiometry Reference Electrodes 3.) Saturated Calomel Reference Electrode (S.C.E) E o = +0.268 V Activity of Cl - not 1 E (sat,KCl) = +0.241 V
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Electrodes and Potentiometry Reference Electrodes 4.) Observed Voltage is Reference Electrode Dependant The observed potential depends on the choice of reference electrode - Silver-silver chloride and calomel have different potentials Use Reference Scale to convert between Reference Electrodes Observed potential relative to SCE Observed potential relative to Ag|AgCl Observed potential relative to SHE
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Electrodes and Potentiometry Junction Potential 1.) Occurs Whenever Dissimilar Electrolyte Solutions are in Contact Develops at solution interface (salt bridge) Small potential (few millivolts) Junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements - Don’t know contribution to the measured voltage Again, an electric potential is generated by a separation of charge Different ion mobility results in separation in charge
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This note was uploaded on 05/09/2010 for the course CHEM 221 taught by Professor Dr.robertpowers during the Fall '07 term at San Diego.

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221-chapter-15 - Electrodes and Potentiometry Introduction 1 Potentiometry Use of Electrodes to Measure Voltages that Provide Chemical Information

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