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

406
A Discussion of selected references
[1995DJU/JEL]
The hydrolysis of tin(II) ion has been studied in 3.0 M (Na)Cl medium at 298 K, by
potentiometric titrations using a glass electrode. In the concentration range
2
1.0 ≤ c(Sn + + ο
) /mM ≤ 10.0 and 0.50 ≤ − log 10 [ c(H ) / c ] ≤ 1.30, the experimental data
were explained by the formation of the following complexes and their respective
2
stability constants, ( log10 β pq , ± σ): Sn 3(OH) + 4 , − (2.70 ± 0.01) and SnOH + ,
− (2.18 ± 0.02). These constants are much higher than those reported in 3 M NaClO 4
(see comment on [1958TOB]), indicating that the hydrolytic processes strongly shifted
2−q
toward the acidic region. Since the formation of SnClq
complexes should retard the
hydrolysis, the authors tried to explain this inconsistency by the highly asymmetric
distribution of the electron density of the hybridised 5s orbital of tin(II), and by the role
of chloride ion which “may aid in oxygen bonding to the tin by their electrostatic
influence on H–OH bonds”. Although these speculations may be true (moreover the
formation of the mixed hydroxido complex Sn(OH)Cl(aq) may result in a similar pH
2−q
shift), the formation of SnClq
complexes were not taken into account during the
evaluation of experimental data, therefore the reported formation constants are not
considered further in this review. From the experimental point of view the correct
determination of hydrogen ion concentration between pH = 0.5 and 1.3 is doubtful using
a glass electrode, and the experimental errors generated in this way may also explain the
above mentioned inconsistency.
In addition, the solubility product of Sn(OH) 2 (s) was determined from the
+
hydrolytic curves, Z (− log 10 c(H )) and found to be, pK s = (27.9 ± 0.1). The hydrolytic
precipitate was examined by elemental analysis, thermogravimetry, IR spectroscopy, X-
ray powder diffraction and scanning electron microscopy. It was presumed that the main
constituent of the precipitate can be formulated as SnO·H 2 O. This paper is particularly
interesting, because the hydrolytic precipitate, presumably SnO·H 2 O, was characterised
structurally. Comparison of the X-ray powder patterns, reported by the authors, with
PDF files 01-084-2157 and 01-077-0452 of JCPDS indicates that the main constituent
of the hydrolytic precipitate was Sn 6 (OH) 4 O 4 , hydroromarchite, but probably a minor
amount of SnO 2 , cassiterite, was also present, see Table A-68.
A recalculation shows that the solubility data of [1995DJU/JEL] neither agree
with those predicted for abhurite, Sn 21 Cl 16 (OH) 14 O 6 , nor with those predicted for SnO,
romarchite, see Figure A-47. By massive parallel displacement the experimental data
coincide rather with the SnO solubility curve than with the abhurite one. This confirms
the authors’ implication that the hydrolytic precipitate contains no chloride. The low
solubility of the hydrolytic precipitate may be related to its SnO 2 , cassiterite, content,
but no thermodynamic data can be derived.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012