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

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VIII Group 17 (halogen) compounds and complexes
10 J·K –1·mol –1 . Also in relation to the formation of other salt hydrates from the
respective anhydrous salts the estimated entropy change of − 31 J·K –1·mol –1 per mole of
water in SnCl 2·2H 2 O is very negative taking into account the layer structure of the
hydrate.
VIII.1.2.3 Basic tin(II) chloride
VIII.1.2.3.1
Composition of basic tin(II) chloride
When concentrated aqueous tin(II) chloride solutions are diluted or alkalised, white or
colourless solid phases precipitate. These precipitates have been repeatedly investigated
to determine 1) the chemical composition [1882DIT], [1882DIT2], [1919CAR],
[1933HAY], 2) the crystal structure [1963DON/MOS], [1981SCH/NES],
[1984ICH/TAK] and 3) the thermodynamic parameters [1930RAN/MUR],
[1992EDW/GIL].
Keller [1917KEL] was probably the first to report a natural occurrence of a
basic tin(II) chloride in the cavity of a metallic mass found in an Indian cemetery.
Corrosion of tin in saline environments results, among others, in chloride-containing
tin(II) phases. Matzko et al. [1985MAT/EVA] describe a secondary tin mineral,
abhurite, found as a corrosion product on ingots of tin from the cargo of a sunken ship.
Dunkle et al. [2003DUN/CRA] report on abhurite and other secondary tin(II) minerals
formed during the corrosion of pewter artefacts [2004DUN/CRA] from another
shipwreck. These authors contend that abhurite as well as romarchite, SnO, and
hydroromarchite, Sn 6 O 4 (OH) 4 , form universally during tin corrosion in seawater
regardless of the composition of the original pewter artefact. Thus the solubilities of
basic tin(II) chloride, tin(II) oxide and tin(II) hydroxide oxide are of considerable
interest for the tin corrosion in general and the mobility of tin(II) in particular.
There is, however, a conspicuous discrepancy concerning the formulae
ascribed to basic tin(II) chlorides. Keller’s [1917KEL] analysis resulted in SnCl 2·SnO.
Carson [1919CAR] reported that two distinct basic salt phases were obtained, viz.
3SnCl 2·5SnO·3H 2 O, and 2SnCl 2·7Sn(OH) 2 . Britton [1925BRI2] stated that empirical
formulae of basic tin(II) chlorides vary from SnCl 1.33 (OH) 0.67 at pH ≈ 1.9 to
Sn 0.14 (OH) 1.86 at pH ≈ 7. Randall and Murakami [1930RAN/MUR] found the
composition of their basic chloride to be Sn(OH)Cl·H 2 O, whereas Hayek [1933HAY]
formulated the compound which he obtained as Sn(OH) 2·SnCl 2 . Donaldson et al.
[1963DON/MOS] were able to identify only one definite crystalline basic chloride
phase, namely Sn 4 (OH) 6 Cl 2 . Ichiba and Takeshita [1984ICH/TAK] suggested that the
true composition was 2SnO·SnCl 2·H 2 O and this became, for some time, the
stoichiometry which had been attributed to abhurite [1985MAT/EVA]. It is obviously
difficult to derive the stoichiometry of basic tin(II) chloride by chemical analysis alone.
Thus an amazing variety of stoichiometries for basic tin(II) chloride, based on chemical
analyses, have been proposed and are listed in Table VIII-3.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012