something strange. i made an electrochemistry experiment in which i have measured the potential of cell: Ag, AgCl | (1M)KCl ║ [Fe(CN)6] 3-, [Fe(CN)6] 4- | Pt

with different concentrations of Fe+2/Fe+3 . from the results i have found the faraday constant: 96181 Coulon /mol
very close to the published value (99.7% accuracy).

the intercept of the graph of E (v) against ln(Fe+2/Fe+3) is:

intercept:    0.198985714  V    

(the intercept is also the standard potential for the cell)

yet, when i tried to find the standard potential of

Fe3+ +e- -> Fe2+

i got the result of +0.423 V which is almost half of the published value (+0.77 V)

E_cathode =E^° + E_anode  

E_cathode=0.198985+0.224= +0.423 V 

how can it be that the faraday constant is almost accurate and in the same time the result of the standard potential is far from the real one.? i'm sure i don't have any calculation mistakes, if it's necessary i can describe the method of work here.



1 Answer 1


What you are measuring is not the potential $Fe^{2+}/Fe^{3+}$. It is the potentiel of the couple $[Fe(CN)_6]^{4-}/[Fe(CN)_6]^{3-}$ which is equal to +0.36 V in the table.

  • 1
    $\begingroup$ which table are you using? $\endgroup$
    – Ynk
    Commented Jan 12, 2020 at 13:10
  • 1
    $\begingroup$ This is more suited as a comment Maurice $\endgroup$ Commented Jan 12, 2020 at 13:16
  • $\begingroup$ My table is "Chemistry Data Book" , by J G Stark and H G Wallace, p. 70, edited by John Murray, 50 Albermale St, London W1X 4BD, 1994, ISBN 0-7195-3951-X. It has been reprinted every two years since. $\endgroup$
    – Maurice
    Commented Jan 12, 2020 at 13:53
  • $\begingroup$ thanks! but still the faraday constant is more accurate by 14% than the standard potential, even though i used the standard potential for obtaining it. $\endgroup$
    – Ynk
    Commented Jan 12, 2020 at 15:57

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