Standard reduction potentials are measured with relativity to hydrogen which has be universally set to have a potential of zero.The cell must be at 298K, 1atm, and all solutions must be at 1M.For the same chemical species the standard reduction potential and standard oxidation potential are opposite in sign. The standard oxidation potential measures the tendency for a given chemical species to be oxidized as opposed to be reduced.Standard reduction potential measures the tendency for a given chemical species to be reduced.Below is a table of standard reduction potentials. Therefore, when a species at the top is coupled with a species at the bottom, the one at the top will become reduced while the one at the bottom will become oxidized. The species at the top have a greater likelihood of being reduced while the ones at the bottom have a greater likelihood of being oxidized. The table shown below is simply a table of standard reduction potentials in decreasing order. This can be done using an activity series. When solving for the standard cell potential, the species oxidized and the species reduced must be identified. Figure (2) - Determining the Standard Reduction Potential of Copper.The following diagrams show how a standard reduction potential is determined. When the standard reduction potential is determined, the unknown chemical species is being reduced while hydrogen is being oxidized, and when the standard oxidation potential is determined, the unknown chemical species is being oxidized while hydrogen is being reduced. Standard reduction and oxidation potentials can both be determined in this fashion. By using a galvanic cell in which one side is a SHE, and the other side is half cell of the unknown chemical species, the potential difference from hydrogen can be determined using a voltmeter. Therefore, when the standard reduction and oxidation potential of chemical species are measured, it is actually the difference in the potential from hydrogen. Universally, hydrogen has been recognized as having reduction and oxidation potentials of zero.
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