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

A Discussion of selected references
333
solutions (V) and solutions (I) to (IV) agree with each other. To check whether the
assumption
log 10 ([Sn 2+ ]/[Sn 4+ ]) = log 10 ([Sn(II)]/[Sn(IV)]) + log 10 [Cl – ] + (2.4 ± 0.2)
(A.50)
is valid in the 4 M HCl containing solutions (I) to (IV) as well, the equilibrium
potentials measured in these solutions were re-evaluated separately, see Figure A-30
and Figure A-31.
ο
The mean value of E (Sn 4+ /Sn 2+ ) has been calculated assuming that Eq. (A.50)
is correct and 4 M HCl is the constant ionic medium in solutions (I) to (IV).
ο
The re-evaluation of the digitalised data resulted in E (Sn 4+ /Sn 2+ , 4 M HCl,
298.15 K) = (225.3 ± 6.1) mV, the calculated uncertainty, see Table A-25, seems to be
realistic in view of Eq. (A.50). It is unlikely that the uncertainty of log 10 m is less than
ο
± 0.2, consequently the uncertainty of E was expected to be ± 6 mV. The assumption
of this paper is certainly questionable that reliable stability constants of Sn(IV) chlorido
complexes in 4 M HCl and 1 M Na 2 SO 4 + 1 M H 2 SO 4 , respectively can be based on
stability constants of Sn(II) chlorido complexes valid at I = 0 [1949RIC/POP]. As the
calculations are internally consistent, Eq. (A.50) might be applicable for the evaluation
ο
of E (Sn 4+ /Sn 2+ ) from other cell-potential data in HCl media, e.g. [1914FOR/BAR] and
[1934HUE/TAR], if m, see Eq. (A.49), can be determined independently.
Figure A-30: Variation of E h as a function of log 10 [Sn 2+ ]. Experimental data of
[1972DES/JOV] at 25 °C (, : [Sn(IV)] = 0.1 M; , : [Sn(IV)] = 0.01 M; solid
lines: linear regression analyses). Only solid symbols have been considered for
regression analysis.
250
200
E
h
/ mV
150
100
exp. data [1972DES/JOV]
4.0 M HCl E°(Sn 4+ /Sn 2+ ) / mV
[Sn(IV)] = 0.1 M, 223.9
[Sn(IV)] = 1.0 M, 222.1
-4 -3 -2 -1 0
log 10
[Sn 2+ ]
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012