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

100 VI Simple tin aqua ions
(1) While the equations used for extrapolation to ionic strength I = 0 are quite
correct and similar to those of the SIT approach, – ψ(I) has erroneously been
ascribed a negative value. This sign error was detected by Hummel et al.
[2002HUM/BER].
Sn 4+ + 2 H 2 (g) Sn 2+ + 2 H +
2
RT ⎛
+ + Δz A I ⎞
= Ec
− +
ο 4 2 o ln(10)
E (Sn /Sn ) '
2 F ⎜
⎝1 + 1.6
Δz 2 = − 10
2
RT ln(10) ⎛ Δz A I ⎞
ψ ( I)
= 2 F ⎜
1+
1.6 I ⎟
⎝ ⎠
I
bI ⎟
⎠
Eq. (2), [1979VAS/GLA]
Eq. (6), [1979VAS/GLA]
Eq. (7), [1979VAS/GLA]
E − ψ ( I)
ο '
c
ο 4+ 2+
RT ln(10)
= E (Sn /Sn ) + bI Eq. (8), [1979VAS/GLA]
2 F
(2) Table 2 of [1979VAS/GLA] contains the experimental values, but some data
must be exchanged, see Appendix A for details.
In order to carry out a SIT analysis based on molality as composition variable
the corrected data were transformed accordingly, see the Appendix A entry for
[1979VAS/GLA]. Correctly calculated Vasil’ev et al.’s data result in a standard
electrode potential of the Sn 4+ /Sn 2+ ο
couple E (Sn 4+ /Sn 2+ , 298.15 K) = (274.0 ± 10) mV,
which differs by more than 120 mV from the value generally accepted.
Now the question arises how Vasil’ev et al. dealt with Sn(II) and Sn(IV)
hydroxido and chlorido complexes in order to find out the concentrations of free Sn 2+
and Sn 4+ ions. Sn(II) hydroxido complexes have to be considered in 2 to 4 M HClO 4
solutions, but only the single experimental study of Nazarenko et al. [1971NAZ/ANT]
is available on the hydrolysis of tin(IV) under acidic conditions. Because several
experimental details are debatable the results of this investigation can be regarded as
approximate estimates only. Using different correlations with Zr 4+ the presence of 10 to
50% hydrolyzed species may be predicted in 4.5 M HClO 4 solution [2009GAJ/SIP]. On
the other hand, the UV spectra of main group metal ions show considerable changes
during hydrolysis [1997SIP/CAP], [2001PER/HEF]. The UV spectra of tin(IV) in 3 to
8 M HClO 4 are nearly identical, indicating only small (if any) changes in the speciation
[2009GAJ/SIP]. This observation renders the above mentioned correlations unreliable
and supports an approach, which neglects the formation of hydroxido species at c HClO4 ≥
4.5 M.
In addition Vasil’ev et al. [1979VAS/GLA] assume that no Sn(IV) chlorido
complexes form in the pertinent medium. This has been concluded from experiments of
Vasil’ev and Glavina [1976VAS/GLA] and [1977VAS/GLA] where SnCl 4 and
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012