Fe(III) is polarographically reduced to Fe(II) to potentials more negative than
ID: 500792 • Letter: F
Question
Fe(III) is polarographically reduced to Fe(II) to potentials more negative than ~+0.4 V and is further reduced to Fe(0) at -1.5 V (all against a saturated calomel reference electrode). Fe(II) is also reduced to the metal at -1.5 V. A polarogram is run (using a dropping mercury electrode) using a solution containing Fe(III) and/or Fe(II). A current is recorded at 0V, and its magnitude is 12.5 microA. A wave is also recorded with an E1/2 of -1.5 V, and its magnitude is 30.0 microA. Identify the iron species (3+ and /or 2+) in solution and calculate the relative concentration of each.
Explanation / Answer
Solution:
The iron species are identified by using the standard electrode potential as below,
Fe2++2e- <--> Fe(s) Eo(V) = -0.44 V
Fe3++e-<--> Fe2+ Eo(V) = 0.77 V
[Fe(CN)6]3-+e- <--> [Fe(CN)6]4- Eo(V) = 0.36 V
The final reduction of potential is -0.44-0.77-0.36 = - 1.55 V
The 3+ ion reduces to 2+ giving 0.77 V an in reverse it gives -0.77 V hence the number of iron species in the solution is +2 and the answer is Fe2+
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