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

VI.3 Sn 4+ 101
(NH 4 ) 2 SnCl 6 seemed to dissociate completely in 0.6 – 2.0 M HClO 4 . The conclusion
drawn from these surprising observations clearly contradicts the results of Fatouros et
al. [1978FAT/ROU] who based their study of Sn(IV) chlorido complexes on a
symmetric cell without liquid junction, see the Appendix A entry for [1978FAT/ROU].
Fatouros et al. found the complete series from SnCl 3+
2
to SnCl − 6
in 5 M HClO 4
solutions, where Sn(IV) hydroxido complex formation should essentially be suppressed.
ο
The stability constants of the Sn(IV) chlorido complexes range from 3 < log β < 11.
A rationale of these seemingly contradictory experimental results could be that
hydrolysis, which certainly is effective in 0.6 to 2 M HClO 4 [1977VAS/GLA], and
dissociation cannot be distinguished by measurements with Vasil’ev and Glavina’s
cells.
Thus, the assumed link [2001SEB/POT] between aqueous tin(IV) and tin(II)
chemistry has been in fact missing! This prompted Gajda et al. [2009GAJ/SIP] to
o
perform potentiometric studies to determine E (Sn 4+ /Sn 2+ ) in strongly acidic solution,
in order to suppress as far as possible hydrolytic processes leading to Sn(IV) hydroxido
complexes of unknown stability. However, due to several experimental difficulties (see
Appendix A) only the experiments performed in (I – 1) M HClO 4 + 1 M HCl, (I – 1) =
3, 4, 5 M) provided reliable Sn 4+ /Sn 2+ redox potentials, which necessitated the
4−x
determination of the stepwise formation constants of SnClx
complexes in 4.5 to 8.0 M
HClO 4 solutions, too (see Section VIII.3.2.2).
For the electrochemical measurements involving chloride containing mixed
background electrolyte the electrochemical cell (Z) (with (I – 1) = 3, 4, 5 M) was
employed.
Pt,H 2 |1 M HCl, (I – 1) M HClO 4 x M SnCl 2 , y M SnCl 4 , 1 M HCl,(I – 1) M HClO 4 |Hg (Z)
Under such conditions, the electrode potentials were found to be stable within
± 0.2 mV after 30 − 60 minutes (as the system is relatively “well buffered” against
O 2 -traces). The reproducibility of the parallel runs were found to be reasonable
(± 4 mV, see the Appendix A entry for [2009GAJ/SIP]), the slope of the experimental
plots was found to be close to the theoretical value (29.58 mV/decade). The evaluation
of the experimental data required the knowledge of the formation constants β
x
of tin(II)
and tin(IV) chlorido complexes at a given ionic strength I. Since a mixed background
electrolyte has been used, the true formation constants are not available. In the case of
tin(IV) the constants determined for HClO 4 background were used to extra/interpolate
to the given ionic strength. In the case of tin(II) the data set for NaClO 4 background
electrolyte, selected in this review, has been used. Although, in the case of tin(II), β 1
and β 2 determined for NaClO 4 could be converted to HClO 4 , similar conversions cannot
be made for β 3 (ε(H + , SnCl − 3 ) is unknown) and β 4 (Δε and ε(H + , SnCl 2− 4
) are unknown).
In addition, a mixed background electrolyte was used, therefore it seemed more reliable
to assume similar ionic strength dependence in NaClO 4 and HClO 4 /HCl background
electrolytes.
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CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012