Potentiometrische Bestimmung des pH-Wertes einer Sauerstoffhalbzelle

  

Ein galvanisches Element besteht aus einer Standardsilberhalbzelle und einer Sauerstoffhalbzelle. Man misst eine Spannung von 0,3 V, wobei die Silberhalbzelle den positiven Pol bildet.

a) Begründen Sie aus den Messergebnissen,  welche Reaktionen an den Elektroden ablaufen
b) Berechnen Sie den pH-Wert in der Sauerstoffhalbzelle.

 

 Normalpotentiale 

reduzierte Form                                                  oxidierte Form                  

Li                                                                        Li+ +e-                                              -3,03

Ca                                                                       Ca2+ + 2e-                                         -2,76

Na                                                                       Na+ + e-                                            -2,71

Al                                                                        Al3+ +3e-                                          -1,69

 Zn                                                                       Zn2+ + 2 e-                                         -0.76

Fe                                                                        Fe2+ + 2 e-                                        -0,44

 Pb                                                                      Pb2+ + 2 e-                                        -0,13

H2 + 2 H2O                                                          2 H3O+  +  2 e-                                  0,00

Cu                                                                       Cu2+ + 2e-                                         +0,35

4 OH-                                                                  O2 + 2 H2O + 4 e-                             +0,40

Fe2+                                                                     Fe3+ + e-                                           +0,75

Ag                                                                       Ag+ + e-                                            +0,81

 2 Cl -                                                                   Cl2 + 2e-                                           +1,36

Au                                                                       Au3+ + 3e-                                         +1,38

12 H2O + Mn2+                                                    MnO4- + 8H3O+ + 5e-                        +1,50

Pb2+ + 6H2O                                                        PbO2 +4H3O+ + 2e-                          +1,46                                                                           

2 SO42-                                                                S2O82- + 2e-                                       +2,05

2 F-                                                                      F2 + 2e-                                             +2,85

 

ungefähre Überspannungen an Platinelektroden (in V)

Wasserstoff                            - 0,03

Sauerstoff                               +0,4

Chlor                                      +0,008

 

Matthias Rinschen (C) 2006 - 2009, Mail: deinchemielehrer [at] gmx [dot] de, Impressum