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

380
A Discussion of selected references
as a very rough approximation. Further complication is raised from the fact that frozen
solutions have to be used in Mössbauer spectroscopy. The time needed to cool the
samples to liquid nitrogen temperature is 1 to 10 s. Therefore the temperature at which
the equilibrium ‘was frozen’ is unknown.
[1981CHE2]
4−x
Mössbauer spectroscopy was used to determine the formation constants of SnClx
complexes in 0 to 12 M HCl solutions (background electrolyte was not used). The
reported isomer shift, the average value of the species present in the solution, showed
continuous change up to 12 M HCl, which would indicate rather low stability of the
tin(IV)-chlorido complexes. From their data the authors calculated log 10 β 1 = 1.3,
ο
log 10 β 2 = 2.1, log 10 β 3 = 2.3, log 10 β 4 = 2.1, log 10 β 5 = 1.4, log10 β
6
= 0.4, and by activity
ο
correction log10 β
6
= 1.6. These data suggest surprisingly weak complex formation for
tin(IV), similar to tin(II). The reason of the 8 to 10 orders of magnitude difference from
the other reports ([1978FAT/ROU], [2009GAJ/SIP]) is unknown, but as mentioned in
the Appendix A entry of [1981CHE], the used method provide only a very rough
approximation of complex formation constants.
[1981CHE/HSU]
2−
Mössbauer spectroscopy was used to determine the formation constants of SnF
x
x
(x =
1, 2, 3) complexes by adding aqueous HF to a solution containing 0.53 M Sn(ClO 4 ) 2 and
0.59 M HClO 4 (the ionic strength was not kept constant). Both the observed isomer shift
and quadrupole splitting showed characteristic changes by complex formation. The
formation constants determined by the authors (see Table VIII.7) are relatively close to
the other values in the table, nevertheless these constants were not used to derive
selected data (see also the Appendix A entry of [1981CHE]).
[1981FAT/CHE]
The redox reaction of the Sn(II)/Sn(IV) system was studied in aqueous HCl solutions by
steady-state and transient methods under potentiostatic conditions. It was shown that the
rate of the electrochemical reaction is inhibited with increasing reactant concentration.
This behaviour was quantitatively interpreted using a model in which a homogeneous
chemical reaction between the product and the reactant is coupled to charge transfer at
the electrode. The kinetic parameters were calculated. from this model by using a
computer simulation technique. Excellent agreement with all the experimental results
was obtained.
This paper deals essentially with kinetics and mechanism of the redox reaction
in the Sn(II)/Sn(IV) system, but it contains a cursory information on the standard
electrode potential of Sn 4+ /Sn 2+ ο
. The authors claim to have determined E
app
(Sn 4+ /Sn 2+ )
in 6 M HCl with a mercury indicator electrode and presumably the saturated calomel
reference electrode. The value obtained was − 145.4 mV which leads to
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012