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.
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                                                                          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
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                                                                                                                                    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+
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                                                                                                                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.
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