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

A Discussion of selected references
265
[1917KEL]
Minute brilliant crystals were observed in a cavity inside a metallic mass found in an
Indian cemetery on Hogtown Bayou, Santa Rosa County, Florida. The metal was
recognised as tin. The crystals were dissolved by being boiled with aqueous sodium
carbonate solution. Tin and chlorine were determined gravimetrically as SnO 2 (s) and
AgCl(s), respectively.
The analytical results (see Table A-2) are consistent with the empirical formula
SnCl 2·SnO. The presence of abhurite in these crystals, found on metallic tin in an Indian
mound, seems to be obvious, but without being confirmed by an X-ray investigation it
remains rather speculative.
Table A-2: Composition of tin(II) oxide hydroxide chloride [1917KEL].
Composition Exp. 1 corr. Exp. 2 SnCl 2·SnO Sn 21 Cl 16 (OH) 14 O 6 (cr) abhurite
w(Sn) 71.10% 73.06% 73.20% 73.45%
w(Cl) 21.74% 21.98% 21.68% 16.71%
w(diff.) 7.16% 4.96% 4.93% 9.84%
[1917NOY/TOA]
The equilibrium constant of the following reaction
Sn(s) + Pb 2+ Sn 2+ + Pb(s)
was determined in aqueous HClO 4 -Sn(ClO 4 ) 2 -Pb(ClO 4 ) 2 solutions at 25 °C. Equilibrium
was attained from both sides within 10 to 19 days. The mean values of 11 independent
ο
measurements given were m 2+ Sn / m 2+ Pb = (2.98 ± 0.02), E (Sn 2+ /Sn(0)) −
ο
E (Pb 2+ /Pb(0)) = − (RT ln(10)/2F)·log 10 [ m 2+ / m 2+ ] = − (14.0 ± 0.1) mV.
The re-evaluation of these experimental data neglecting the ionic strength
dependence resulted in
ο
E (Sn 2+ /Sn(0)) −
Sn
Pb
ο
E (Pb 2+ /Pb(0)) = − (14.03 ± 0.29) mV.
When (RT ln (10)/2F)·log 10 ([Sn 2+ ]/[Pb 2+ ]) is plotted vs. m −
ClO
(see Figure A-6)
4
the slope of the linear regression line is − (RT ln (10)/2F)·Δε, where Δε = ε(Sn 2+ , ClO − 4
)
− ε(Pb 2+ , ClO − 4
).
mClO 4
The value of the difference of the standard potentials extrapolated to
− = 0 is
ο
E (Sn 2+ /Sn(0)) −
ο
E (Pb 2+ /Pb(0)) = − (14.46 ± 0.31) mV.
As m −
ClO
is varying from 0.76 to 1.24 mol·kg –1 only, the reliability of Δε thus
4
obtained, should not be overestimated. Consequently the value of ε(Sn 2+ , ClO − 4
) =
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012