The Products of Electrolysis
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because the electrode potential ( — 2.71 volts) for Na (an alkali metal) is
more negative than the H-electrode potential, including the H 2 overvoltage.
Electrolysis of Aqueous Solutions (The Positive Electrode)
The electrolytic changes that occur at the positive electrode fall into three
categories.
1. At an inert electrode (Pt, Au, stainless steel, C), the OH" ion will
decompose more easily than any other oxygen-containing negative ion
(even at the low concentration that exists in water) to give
4OH- -» O 2 + 2H 2 O + 4e~
Oxygen-containing ions such as SO|", NOs, or C1O~ remain unchanged.
2. At aninert electrode (Pt, Au, stainless steel, C), the Cl~, Br~, and I~
ions will be converted to the corresponding halogens. For example,
2C1~ -> C1 2 + 2e3. At an active electrode (almost any metal other than Pt, Au, or stainless
steel), the electrode metal goes into solution, and the negative ions in
solution remain unchanged. Thus at a positive Cu electrode, a solution
of NaCl or NaNO 3 will yield neither C1 2 nor O 2 , but a solution of Cu
2+
ions:
Cu -» Cu
2+ + 2eAs a matter of fact, this is the basis for the electrorefining of metals.
The impure metal electrode (say, Cu) is used as the positive electrode,
and a piece of pure Cu metal is used as the negative electrode; the
solution is CuSO 4 . The composition of the solution is so designed, and
the applied voltage so chosen, that the impurities stay in solution or
precipitate out, and only copper plates out at the negative electrode:
at the positive electrode,
Cu -» Cu
2+ + 2eat the negative electrode,
Cu
2+ + 2e~ -» Cu