414
P. P. Roma´ nczyk and S. S. Kurek
use an internal standard, of which ferrocene is the most popular. Its oxidation potential obviously depends on the solvent used, and in accurate calculations, it should
be remembered that it may be affected by the type and concentration of the base
electrolyte, which has to be used in excess in voltammetric techniques.
Recently, it has been reported that using microporous glass frits, like Vycor glass,
to separate aqueous and non-aqueous solutions, may be a source of an additional error
exceeding 50 mV in the measured potential [21]. However, it concerns only solutions
of low ionic strength. For base electrolyte concentrations used typically in cyclic
voltammetry (0.1 M) and concentrated aqueous solutions in reference electrodes,
this effect is fortunately close to zero.
2.2 Absolute Potential of Fc + /Fc
Ferrocene has been used as an internal standard in non-aqueous electrochemistry since long, and in 1984 IUPAC recommended [22, 23] to report measured potentials versus Fc
+ /Fc, then assumed to be a solvent-independent redox
system. This was based on a virtually constant difference between E 1/2 of ferrocene
and bis(biphenyl)chromium(I) tetraphenylborate measured in 0.1 M tetraethyl/tetrabutylammonium perchlorate solutions [22]. Following the above IUPAC recommendation and another one defining the E
◦
abs (SHE) equal to (4.44 ± 0.02) V [6] (see
footnote 1 on page 3), and combining it with the only known value of E°(Fc
+ /Fc) in
aqueous solutions measured as 0.400 V versus SHE [24], it was assumed that the universal absolute potential, E
◦
abs
Fc
+
/Fc
, the same for all solvents, is equal to 4.84 V.
In fact, as it will be shown below, the oxidation potential of Fc depends on the solvent
used. Ferrocene undergoes reversible 1e oxidation. Owing to the fact that geometry
differences between neutral and oxidised forms are negligible, inner reorganisation
energy is very low. This results in a perfect 59 mV value of the cyclic voltammetric
anodic-to-cathodic peak separation, as expected for a reversible or Nernstian electrode process. In this case, it means fast ET, the rate of electrode process controlled
by diffusion, and at each point, the concentration ratio [Ox]/[Red] at the electrode
surface is equal to that calculated from the Nernst equation for the electrode potential,
provided ohmic potential drop (iR) is zero).
As mentioned above, the 1984 IUPAC recommendation was based on the assumption of the Fc
+ /Fc absolute redox potential being the same in all solvents, which
soon appeared to be only a very crude approximation. The development of efficient
quantum chemical calculations and accurate solvent models allowed the calculation
of the Fc
+ /Fc potential in various solvents. Thus, Namazian et al. [25] carried out
high-level ab initio calculations to obtain reliable values of the absolute reduction
potentials of Fc
+ /Fc in MeCN, 1,2-dichloroethane (DCE), and dimethylsulphoxide
(DMSO). They combined the G3(MP2)-RAD-Full-TZ gas-phase energetics (on top
of LanL2TZf/6-31G(d) geometries), regarded as “chemically accurate”, with solvation Gibbs energies from two implicit solvent models, SMD and COSMO-RS. The
authors have also established that employing PCM and CPCM models results in
P. P. Roma´ nczyk and S. S. Kurek
use an internal standard, of which ferrocene is the most popular. Its oxidation potential obviously depends on the solvent used, and in accurate calculations, it should
be remembered that it may be affected by the type and concentration of the base
electrolyte, which has to be used in excess in voltammetric techniques.
Recently, it has been reported that using microporous glass frits, like Vycor glass,
to separate aqueous and non-aqueous solutions, may be a source of an additional error
exceeding 50 mV in the measured potential [21]. However, it concerns only solutions
of low ionic strength. For base electrolyte concentrations used typically in cyclic
voltammetry (0.1 M) and concentrated aqueous solutions in reference electrodes,
this effect is fortunately close to zero.
2.2 Absolute Potential of Fc + /Fc
Ferrocene has been used as an internal standard in non-aqueous electrochemistry since long, and in 1984 IUPAC recommended [22, 23] to report measured potentials versus Fc
+ /Fc, then assumed to be a solvent-independent redox
system. This was based on a virtually constant difference between E 1/2 of ferrocene
and bis(biphenyl)chromium(I) tetraphenylborate measured in 0.1 M tetraethyl/tetrabutylammonium perchlorate solutions [22]. Following the above IUPAC recommendation and another one defining the E
◦
abs (SHE) equal to (4.44 ± 0.02) V [6] (see
footnote 1 on page 3), and combining it with the only known value of E°(Fc
+ /Fc) in
aqueous solutions measured as 0.400 V versus SHE [24], it was assumed that the universal absolute potential, E
◦
abs
Fc
+
/Fc
, the same for all solvents, is equal to 4.84 V.
In fact, as it will be shown below, the oxidation potential of Fc depends on the solvent
used. Ferrocene undergoes reversible 1e oxidation. Owing to the fact that geometry
differences between neutral and oxidised forms are negligible, inner reorganisation
energy is very low. This results in a perfect 59 mV value of the cyclic voltammetric
anodic-to-cathodic peak separation, as expected for a reversible or Nernstian electrode process. In this case, it means fast ET, the rate of electrode process controlled
by diffusion, and at each point, the concentration ratio [Ox]/[Red] at the electrode
surface is equal to that calculated from the Nernst equation for the electrode potential,
provided ohmic potential drop (iR) is zero).
As mentioned above, the 1984 IUPAC recommendation was based on the assumption of the Fc
+ /Fc absolute redox potential being the same in all solvents, which
soon appeared to be only a very crude approximation. The development of efficient
quantum chemical calculations and accurate solvent models allowed the calculation
of the Fc
+ /Fc potential in various solvents. Thus, Namazian et al. [25] carried out
high-level ab initio calculations to obtain reliable values of the absolute reduction
potentials of Fc
+ /Fc in MeCN, 1,2-dichloroethane (DCE), and dimethylsulphoxide
(DMSO). They combined the G3(MP2)-RAD-Full-TZ gas-phase energetics (on top
of LanL2TZf/6-31G(d) geometries), regarded as “chemically accurate”, with solvation Gibbs energies from two implicit solvent models, SMD and COSMO-RS. The
authors have also established that employing PCM and CPCM models results in