Author Topic: The short “use the overvoltage” FAQ  (Read 1256 times)

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Nicodem

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The short “use the overvoltage” FAQ
« on: February 20, 2004, 12:12:00 PM »
I posted this info about the overvoltage phenomenon in an attempt to promote the idea of electrochemistry here at The Hive as I suspect this field has a lot of future for clandestine chemists.



Q: What is this table useful for?
A: If you have an article saying that a certain electroreduction works on some exotic electrode having a H2 overvoltage of -1V you can check the table and see that a Pb electrode might also work since it has a lower overvoltage. Of course, this does not mean it will work as good or work at all but only that it is worth checking.

Q: I don’t feel like using electroreductions for my precious nitrostirenes yet. Is there anything simpler for beginners, like something where I don’t need a membrane? Just for practicing in electrochemistry?
A: Sure, for example if you use Zn dissolving reductions you can simply recycle your zinc powder with an easy to build electrolytic cell and you will have your high quality zinc powder back for no cost. But you have to chose the appropriate current and a cathode having an overvoltage lower than Eo for Zn (-0.763V):
The high overvoltage associated with the formation of hydrogen permits the electrolytic deposition of several metals that require potentials at which hydrogen would otherwise be expected to interfere. For example, it is readily shown from their standard potentials that rapid formation of hydrogen should occur at well below the potential required for the deposition of zinc from a neutral solution. Nevertheless, quantitative deposition of zinc can be attained provided a mercury or copper electrode is used; because of the high overvoltage of hydrogen on these metals, little or no gas is evolved during the electrodeposition.

Q: Is there anything else that can influence this “overvoltage” phenomenon besides the electrode and the current?
A: Many things influence it and here are just a few of the important factors:
Charge-transfer polarization arises when the rate of the oxidation or reduction reaction at one or both electrodes is not sufficiently rapid to yield currents of the size demanded by theory. The overvoltage arising from charge-transfer polarization has the following characteristics:
1. Overvoltages increase with current density (current density is denned as amperes per square centimeter of electrode surface).
2. Overvoltages usually decrease with increases in temperature.
3. Overvoltages vary with the chemical composition of the electrode, often being most pronounced with softer metals such as tin, lead, zinc, and particularly mercury.
4. Overvoltages are most marked for electrode processes that yield gaseous products such as hydrogen or oxygen; they are frequently negligible where a metal is being deposited or where an ion is undergoing a change of oxidation state.
5. The magnitude of overvoltage in any given situation cannot be predicted exactly, because it is determined by a number of uncontrollable variables.


Citations and the table taken from
Skoog and Leary. Principles of Instrumental Analysis. 4th edition.


lugh

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More Information on Overvoltage
« Reply #1 on: February 20, 2004, 04:58:00 PM »
While those assertions are definitely correct, the phenomenon of overvoltage is far more complicated, this article that was originally uploaded in

Post 474907

(lugh: "Reference Articles: Overvoltage + Al couples", Serious Chemistry)
will fill in the gaps:



;)