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

A Discussion of selected references
325
than in [1997AMA/CHI] and [1998ODA/AMA]. The reason of this difference is not
clear. In [1970BAR/KLI] no information is given concerning the separation of the
samples from CO 2 during the one-month equilibration. Since the hydroxide ion
concentration was calculated from the initially added NaOH solutions, it is possible that
the actual pH was significantly smaller than the calculated one, due to the dissolution of
CO 2 . Therefore, the above listed re-evaluated constants were not used in the present
review.
[1970BON/TAY]
This paper reports d.c. and a.c. polarographic measurements on aqueous solutions
containing Sn 2+ and F – ions. The results can be used to determine stability constants of
fluorido complexes of tin(II). In addition the standard potential of Sn 2+ /Sn can be
estimated from the polarographic half-wave potential of Sn 2+ + 2 e – Sn(0) vs. the
reference electrode Ag/AgCl.
ο
At 25 °C and I = 1.0 M NaClO 4 the authors found for Reaction (A.40)
Sn 2+ + 2 Ag(s) + 2 Cl – + (sat) Hg(l) Sn(Hg) sat + 2 AgCl(s)
(A.40)
Δ r E ' (A.40) = − 0.374 V. For reaction (A.41) [1967KRA] obtained in the same ionic
medium
o
2 AgCl(s) + H 2 (g) 2 Ag(s) + 2 Cl – + 2 H + (A.41)
Δ r E ' (A.41) = 0.23227 V. This leads to E co ' (Sn 2+ /Sn(cr), 25 °C, I = 1.0 M NaClO 4 ) =
− (0.1407 ± 0.0020) V, when a Sn in a saturated tin amalgam is accounted for
[2003YEN/GRO]. The uncertainty of ± 0.0020 V has been estimated by the reviewers.
With ε(Sn 2+ , ClO − 4
) = (0.19 ± 0.04) kg·mol –1 , Eq. (II.35) and Eq. (VI.5) lead to:
o 2+ o ⎛RT
ln(10) ⎞ ⎡ ⎛mNaClO
⎞
⎤
4
Em(Sn /Sn) = Ec ' + νi
log10 2D ε m −
⎜ ⎟⋅⎢∑
⋅ ⎜ ⎟+ +Δ ⋅
ClO4
⎥
⎝ nF ⎠ ⎣ ⎝ cNaClO4
⎠
⎦
+ − 2+ −
o
where ∑ ν i = 1,
Δ ε = 2 ε(H ,ClO 4 ) − ε(Sn ,ClO 4 ) and E (Sn 2+ /Sn(cr),
298.15 K) = − (0.1252 ± 0.0027) V has been obtained. Although this value is of the
right order of magnitude it was not used for regression, because the results of
[1917NOY/TOA], [1928PRY], [1938HAR/WHI], [1957WAK/SAL] and
[1973VAS/GLA] appear to be better founded experimentally.
A remark with respect to the analysis of polarographic measurements seems to
be in order. The uncertainty with respect to the equilibrium between Sn(Hg) β-Sn +
Hg(l) prevailing in this special case and the expected error of at least ± 1 mV render this
method unsuitable for the determination of accurate and precise data.
Several polarographic (conventional and rapid a.c./d.c.) methods have been
used to study the complex formation between tin(II) and fluoride ion in 1 M NaClO 4
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012