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

A Discussion of selected references
349
[1975GRI]
The crystal structure of tritin(II) dihydroxide oxide sulfate, Sn 3 (OH) 2 OSO 4 , was
determined. The compound crystallises in the orthorhombic system with a =
(13.045 ± 0.002) Å, b = (4.9383 ± 0.0014) Å, c = (12.140 ± 0.002) Å. The space group
is Pca2 1 . There are four formula units in the unit cell.
The positions of the tin, sulfur, and oxygen atoms were obtained from
Patterson and Fourier syntheses. Full matrix least-squares refinement, based on
452 independent reflections, gave a final R-value of 6.9%. The tin and oxygen atoms
form infinite chains connected by distorted sulfate groups. Two tin atoms are fourcoordinated
by oxygen while the third is three-coordinated. The Sn–O bonding
distances vary within the range 2.01 – 2.51 Å with a standard deviation of 0.003 Å. The
shortest tin – tin distances are 3.523, 3.578, and 3.580 Å with a mean standard deviation
of 0.004 Å.
[1975KLI/BAR]
The solubility of amorphous SnO 2 in acidic and alkaline fluoride solutions has been
studied at 25, 50 and 200 °C. The equilibration time was 30 days at 25 °C, 7 days at
200 °C. The dissolved tin(IV) in the equilibrated solutions was determined
colorimetrically using phenylfluorone. In acidic solution the formation of Sn(OH) 3 F(aq)
was suggested at 25 and 50 °C, while Sn(OH) 2 F 2 (aq) was suggested at 200 °C. In
alkaline fluoride solutions the predominant complex was found to be Sn(OH) 4 F – at 25
2
4 2
and 50 °C, and Sn(OH) F − at 200 °C. For the reactions
* ο
10
SnO 2 (s) + n HF + (2 – n) H 2 O(l) Sn(OH) 4–n F n (aq)
log K = − (5.0 ± 0.2) (25 °C, n = 1), − (4.4 ± 0.1) (50 °C, n = 1) and − (3.5 ± 0.2)
(200 °C, n = 2) has been reported.
* ο
In alkaline solutions log10
K = − (5.4 ± 0.1) (25 °C, n = 1), − (5.8 ± 0.2)
(50 °C, n = 1) and − (3.8 ± 0.2) (200 °C, n = 2) has been reported for the reaction
SnO 2 (s) + n F – + 2 H 2 O(l) Sn(OH) 4F n− n .
Although the experimental work seems to have been done carefully, there are
several deficiencies in this work: (i) the accuracy of the analytical method used is rather
limited at [Sn(IV)] tot = 10 –7 – 10 –6 M, (ii) the compositions of the dissolved mixed
complexes are not certain (e.g. the formation of Sn(OH) 2 F + or Sn(OH) 5 F 2– is also
possible), (iii) the formation of the binary hydroxido complexes, e.g. Sn(OH) + 3 at pH =
1 or Sn(OH) −
5 at pH = 10, was not considered (iv) the pH of the solutions was
calculated and not measured. Therefore, the reported constants cannot be used to derive
selected values.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012