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

332
A Discussion of selected references
[Sn(II)] = [Sn 2+ ]·{1 + [Cl – ]·(β 1(2) + β 2(2)·[Cl – ] + β 3(2)·[Cl – ] 2 + β 4(2)·[Cl – ] 3 )}
[Sn(IV)] = [Sn 4+ ] + [SnCl 3+ 2
] + [ SnCl + 2
] + [ SnCl + 3
] + [SnCl 4 ] + [ SnCl − 5
]
[Sn(IV)] = [Sn 4+ ]·{1 + [Cl – ]·( β 1(4) + β 2(4)·[Cl – ] + β 3(4)·[Cl – ] 2 + β 4(4)·[Cl – ] 3 + β 5(4) [Cl – ] 4 )}.
In the range [Cl – ] ≥ 0.32 M SnCl 2 , SnCl − 2
3
, SnCl − 4
and SnCl + 3
, SnCl 4 , SnCl − 5
,
respectively are the dominant species. Moreover β 3(4) /β 2(2) = β 4(4) /β 3(2) = β 5(4) /β (2) =
const. In this case Eq. (A.48) transforms into
E h =
ο
E
V
(Sn 4+ /Sn 2+ ) − (k/2)·{log 10 ([Sn(II)]/[Sn(IV)]) + log 10 [Cl – ] + log 10 m}
where (RT/2F) ln (10) has been abbreviated to (k/2).
(A.49)
A plot of E h vs. log 10 [Cl – ] at log 10 ([Sn(II)]/[Sn(IV)]) = 0 becomes indeed linear
for log 10 [Cl – ] ≥ 0.38 and coincides reasonably with the theoretical slope = − 29.59 mV, see
Figure A-29.
Figure A-29: Variation of E h as a function of log 10 [Cl – ]. Experimental data of
[1972DES/JOV] at 25 °C (, : [Sn 2+ ] = [Sn 4+ ] = 0.01 M, solid line: linear
regression).
220
200
E
h
/ mV
180
160
140
E°(Sn 4+ /Sn 2+ ) / mV = 155.83 (
) + 29.59 log 10
m
m = 250 [1972DES/JOV], E°( Sn 4+ /Sn 2+ ) = 226.8 mV (228 mV)
-1.5 -1.0 -0.5 0.0 0.5
log 10
[Cl – ]
When the value (m = 250, log 10 m = 2.4) given by Despić et al. is used
ο
E
V
(Sn 4+ /Sn 2+ ο
) = 226.8 mV agrees nicely with E (Sn 4+ /Sn 2+ ) = 228 mV according to
[1972DES/JOV]. In this paper it has not been unambiguously demonstrated, however,
ο
that E (Sn 4+ /Sn 2+ ) and m have been determined independently of one another. The
index V of E ο indicates that neither the inert electrolyte nor the ionic strength of
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012