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2010.11.30. Electroanalytical methods G. Galbács Electrochemical methods In electrochemical methods of instrumental analysis, one measures voltagge (p(potential)) and/or current siggnals. A varietyy of electrochemical methods have been developed, out of which we are going to discuss the following ones only: potentiometry conductometry coulombmetry electroggyravimetry voltammetry (polarography/amperometry/stripping v.) The discussion of electrochemical methods assumes the knowledge of redox reactions, the Nernst equation, galvanic cells and electrodes… so first we are going to review these notions briefly. 1 2010.11.30. Electrodes Electrodes are interfaces between metallic and electrolyte conductors of electricity. Electrical conduction in metals means the transportation of electrons, whereas cations and anions are the mobile charge carriers in electrolytes. Thus, any instrumental setup, which measures current and/or voltage signals, neccessarily will have an interface between metallic conductors (wires) and electrolytes. Electrochemical (galvanic) cells 2 2010.11.30. Nernst equation, electromotive force R⋅T A R⋅T A Nernst equation: E =E + ⋅ln oxidized = E − ⋅ln reduced 0 n⋅F A 0 n⋅F A reduced oxidized At 298K and in dilute solutions: E =E + 0.0591⋅lgcoxidized 0 n c reduced U=EMF=Ecathode −Eanode How can a galvanic cell be used for analysis? Standard hydrogen electrode (SHE) Ag+ 1 M HCl U=E −E =E +0.0591⋅lgc −0 cathode anode 0,Ag+ 1 Ag+ U−E U−0.799 0,Ag+ 0.0591 lgc = → c =10 Ag+ 0.0591 Ag+ 3 2010.11.30. Potentiometry Reference electrodes Most common types of reference electrodes: standard hydrogen electrode (SHE) Ag/AgCl electrode HgCl (calomel) electrode 2 2 The latter two types are more frequently used in analytical chemistry, because they have a more practical construction. 4
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