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

366
A Discussion of selected references
[1979VAS/GLA]
Redox potentials for Sn 4+ /Sn 2+ in perchloric acid solutions containing SnCl 2 and SnCl 4
were determined at 25 °C and ionic strength 2 − 4 M HClO 4 , using the cell:
Pt, H 2 | HClO 4 SnCl 2 , SnCl 4 , HClO 4 | Hg, Pt.
After 3 to 3.5 h electrode potentials were stable shifting less than 0.02 mV per
hour, thus indicating equilibration. Concentration of Sn 2+ was determined
iodometrically after measuring electrode potentials.
Equilibrium concentrations of Sn 2+ and Sn 4+ were calculated considering
hydrolysis (Sn(OH) 3+ 2
, Sn(OH) + 2 , Sn(OH) + 3 , Sn(OH) 4 ) [1971NAZ/ANT] and chlorido
complex formation (SnCl + , SnCl 2 , SnCl − 3 ) using constants from literature including
work of their own group.
The standard potential of the half-cell Sn 2+ , Sn 4+ | Hg, Pt according to reaction
Sn 4+ + H 2 (g) Sn 2+ + 2 H +
(G)
(A.72)
ο
was found to be E (Sn 4+ /Sn 2+ ) = (0.1522 ± 0.0005) V. The calculated Gibbs energy of
formation of Sn 4+ ο
in aqueous solution is Δ G (Sn 4+ ) = (1.63 ± 0.13) kJ·mol –1 .
f
The results of Vasil’ev et al. are flawed by two numerical mistakes.
m
(1) While Eqs. (6), (7) and (8) used in [1979VAS/GLA] for extrapolation to ionic
strength I = 0 are quite correct, – ψ(I) has erroneously been ascribed a negative
value.
(2) In the original Table 2 of [1979VAS/GLA] row 13 and 14 of column 8 as well
as column 9 should be exchanged.
In addition Vasil’ev et al. conclude from earlier experiments [1977VAS/GLA]
that SnCl 4 and (NH 4 ) 2 SnCl 6 dissociate completely in 0.6 to 2.0 M HClO 4 .
In Table A-42 the experimental data of [1979VAS/GLA] are listed. As the SIT
method is based on molality as composition variable these data were transformed by the
following semi-empirical equation:
m = c ⋅ [1 / d + a⋅ c + b⋅ c ]
(A.73)
o 2
B B w − − − −
(ClO4 + Cl ) (ClO4
+ Cl )
o
where at 25 °C c B , d
w
= 0.997045 kg·dm –3 , c − −
(ClO4
+ Cl ) , and a = 0.044672
dm 6·mol –1·kg –1 , b = 0.002291 dm 9·mol –2·kg –1 are the concentration of substance B given
in Table A-42, the density of pure H 2 O, the total anion concentration, and the empirical
fit parameters when Eq. (A.73) is regressed using ξ(HClO 4 ) = mHClO4/
c HClO4
listed in
Table II-5. In the pertinent case c HClO4
>> c −
Cl and thus the ξ(HClO 4 ) values should be
a reasonable approximation to the real ones.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012