Here is a reply to my emf query received through the
electrochemistry mailbase. If there are any further responses, may
I ask that they be sent to BOTH
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AND
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May I say how grateful I am for all the replies so far. They have
given much food for thoughts.
Joe Lee, UMIST
Joe,
I
think you've really got all you need in the replies given on the
chem-education list, but I would suggest that you look at the
corrosion literature, as this question is at the core of corrosion
electrochemistry (which is treated surprisingly differently from
chemical electrochemistry). Basically all of the metals will corrode
in aerated sulphuric acid, so all that you are measuring is the
difference in the corrosion potentials of the different metals.
Copper, being the most noble, will only corrode significantly with
oxygen reduction, hence you will get a mixed potential between Cu
-> Cu2+ and O2 -> OH-. All of the other metals will corrode with
hydrogen evolution, so you get a mixed potential between Zn ->
Zn2+ or whatever and H+ -> H2. The reverse reactions are neglible
until the metal ion concentrations reach a significant value (greater
than molar in all cases), hence under normal circumstances the
potential is controlled by the kinetics of the anodic and cathodic
reactions, and is therefore not predicted by the Nernst equation. In
teaching corrosion electrochemistry, we tend to concentrate on
electrochemical kinetics, as thermodynamics rarely gives anything
more than an indication of what is possible.
Bob Cottis
Corrosion and Protection Centre, UMIST
Dr Joe Lee
Department of Chemistry,
UMIST,
Manchester,
M60 1QD,
UK
Phone 0161 200 4513
Fax 0161 236 7677
Email [log in to unmask]
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