Practical investigations
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Could you explain to me how metals accelerate the decomposition of ascorbic acid?
Igloo writes ...
When you refer to “metal”, I assume that you are referring to metal ions, since these can be water-soluble and can act as homogeneous catalysts.
Understanding this sort of catalysis involves an knowledge of redox potentials, and I have to assume that you have already studied this topic.
The “decomposition” of ascorbic acid usually refers to its oxidation to dehydroascorbic acid, and this redox process is associated with an electrode potential of +0.06 volt. If the process involves aerial oxidation then the relevant redox half reaction involving oxygen has a redox potential of +0.40 volt.
Now this process can be accelerated by any metal ion/metal ion system with an intermediate redox potential, e.g. Cu2+/Cu+, which has a potential of +0.15 volt. First the ascorbic acid might reduce the Cu2+ to Cu+ whilst the Cu+ ions formed can subsequently reduce the O2 to OH-. If the activation energies of these two steps are lower than that of the direct oxidation of ascorbic acid to dehydroascorbic acid, then you can see how Cu2+ can catalyse the process.
In practice it is well known that copper(II) ions catalyse the process considerably, so I have now given you a much simplified explanation of why this occurs. Other examples of d-block metal ions catalysis can probably be explained via a similar theory.
I am aware that other metal ions in the s-block can also sometimes catalyse the process, but I do not know how this works.
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updated: 21 January 2008
