Chemical Thermodynamics of Tin - Volume 12 - OECD Nuclear ...

VI.3 Sn 4+ 99
Sn 4+ + H 2 (g) Sn 2+ + 2 H +
(Sn 4+ ο
ο
Δ G ) = − Δ G (VI.24) − Δ G (Sn 2+ ).
f
ο
m
r
m
f
m
(VI.24)
ο
A value of Δ (Sn 2+ fGm
) has been selected in this review, and the critical
assessment of all the investigations of Reaction (VI.24) is reported in Section VI.3.1.
VI.3.1 Redox reaction Sn 4+ /Sn 2+
The redox couple Sn 4+ /Sn 2+ according to Reaction (VI.24) is thermodynamically the
connecting link between aqueous tin(IV) and tin(II) chemistry [2001SEB/POT].
Surprisingly limited reliable thermodynamic data are available in the literature for
aqueous chemistry of tin(IV) [2002HUM/BER]. Until recently values of the standard
ο
electrode potential E (Sn 4+ /Sn 2+ ) were reported in four papers only, see Table VI-6,
and all suffer from major deficiencies as discussed in the respective Appendix A entries.
ο
Table VI-6: E of Sn 4+ /Sn 2+ couple at 298.15 K.
ο
E /mV medium method deficiences authors
recalc.
144 ± 2 1.0 - 5.3 M HCl cell (a)
Sn(IV), Sn(II) chlorido and hydroxido
complexes?
[1914FOR/BAR]
original
154 ± 3 0.10 - 2.02 M HCl cell (b)
Sn(IV), Sn(II) chlorido and hydroxido
complexes?
[1934HUE/TAR]
original
228 ± ?
4 M HCl,
1 M Na 2 SO 4 / H 2 SO 4 cell (c)
Sn(IV) chlorido, sulphato complexes?
Extrapolation I → 0?
[1972DES/JOV]
original
152.2 ± 0.5 2 - 4 M HClO 4 cell (d)
Sn(IV) chlorido complexes neglected!
sign error!
[1979VAS/GLA]
(a) Hg | Hg 2 Cl 2 | KCl (1 M) SnCl 2 , SnCl 4 , HCl | Hg
(b) Pt, H 2 | HCl | SnCl 2 , SnC l 4 , HCl | Hg
(c) Hg | Hg 2 Cl 2 | KCl (1 M) SnCl 2 , SnCl 4 , HCl, Na 2 SO 4 , H 2 SO 4 | Hg
(d) Pt, H 2 | HClO 4 SnCl 2 , SnCl 4 , HClO 4 | Hg
Clearly the mixed chloride and sulfate medium of Despić et al.
[1972DES/JOV] obscures the extrapolation to zero ionic strength. Thus the value of
Vasil’ev et al. [1979VAS/GLA] seemed to confirm the generally accepted result of
Huey and Tartar [1934HUE/TAR]. As the value of Forbes and Bartlett
[1914FOR/BAR] deviates less than 10 mV from the mean value of the former ones,
Latimer‘s [1952LAT] selection E ο (Sn 4+ /Sn 2+ ) = (150 ± 10) mV seemed amply justified
[1974POU], [1982WAG/EVA]. Re-evaluation of the data listed in [1979VAS/GLA],
however, shows that the results are flawed by two numerical mistakes.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012