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

324
A Discussion of selected references
log10 β 2 = 1.71, log10 β 3 = 1.69. Apparently the author took no special precaution to
prevent the oxidation of tin(II). In addition, the ionic strength varied from 0.01 to 0.9 m
during the measurements. Therefore, the reported constants were not considered any
further in this review.
[1969FED/BOL]
Potentiometric measurements using a tin amalgam electrode have been performed to
study the formation of the tin(II)-bromido complexes at 25 °C in NaClO 4 media (I = 1.0,
2.0, 3.0, 4.0, 6.0 and 8.0 M). Up to I = 4 M, the formation of four bromido complexes
2 q
( SnBr
−
q
, q = 1, 2, 3, 4) was assumed to explain the experimental data, while the
authors suggested the formation of six bromido species (q = 1 to 6) at I = 8 M. The
Vasil'ev equation has been applied to extrapolate the log10
β q (q = 1 to 4) values to
I = 0. The background electrolyte (NaClO 4 ) was entirely replaced by NaBr during the
measurements, which resulted in a probably substantial change of the activity
coefficients. Therefore, the reported experimental data were re-evaluated for the
purposes of this review using the data points corresponding to at most 20% replacement
of the background electrolyte ([Br – ] tot ≤ 0.2 I ). Considering the formation of SnBr + ,
SnBr 2 (aq) and SnBr −
3 the experimental data can be well reproduced up to I = 4 M. At
2
higher ionic strength a further species ( SnBr − 4
) should be also taken into account. An
uncertainty of ± 0.3 has been assigned to the recalculated log10
β q values.
[1970BAR/KLI]
The solubility of cassiterite (SnO 2 ) in pure water and in NaOH solutions was studied at
25 °C. Equilibrium between the solid and solution phases was established within a
month. The dissolved tin(IV) was determined colorimetrically using
p-nitrophenylfluorone, as well as by a polarographic method. Before the analysis, the
volume of the aliquots was reduced by a factor 3 (by evaporation in presence of sulfuric
acid), since the solubility of SnO 2 in water is less than the detection limit of the method
applied (0.5 μg·ml –1 tin(IV)). The experimental data were explained by the formation of
the complex Sn(OH) − 5 .
The reported data were re-evaluated for the purposes of this review. The data
are compatible with the formation of only Sn(OH) − 5 :
Sn(OH) 4 (aq) + p H 2 O(l) Sn(OH) p 4+ p + p H+ . (A.39)
The resulting constants are as follows: log10 K s,0
= − (6.52 ± 0.05) and
*
log10 β 1,5 ((A.39), p = 1) = − (11.28 ± 0.09), and Δε((A.39), p = 1, NaOH) =
(0.03 ± 0.05) kg·mol –1 . The solubility of SnO 2 determined in pure water is considerably
higher than that obtained in [1997AMA/CHI] and [1998ODA/AMA]. Probably the
reason for this discrepancy is that despite of the sample treatment applied (volume
reduction by evaporation) the concentration of tin(IV) in the samples was close to the
detection limit. In alkaline solutions, the solubility of SnO 2 was three log units smaller
−
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012