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

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A Discussion of selected references
4−x
Table A-77: Formation constants β x of SnClx
complexes (units on the molality basis)
determined in HClO 4 background solutions at different ionic strength (the estimated
experimental errors (3σ) are in parentheses) and extrapolated values to zero ionic
strength and to I m = 6.41 mol kg –1 by SIT.
I m (I c ) log 10 β 1 log 10 β 2 log 10 β 4 log 10 β 5 log 10 β 6
5.62 (4.5) 2.91(0.36) 4.96(0.36) 8.01(0.30) 8.91(0.45) 8.40(0.50)
7.42 (5.6) 2.80(0.30) 5.19(0.30) 8.75(0.30) 9.81(0.40) 9.89(0.40)
8.12 (6.0) 3.13(0.12) 5.77(0.10) 9.27(0.12) 10.20(0.15) 10.15(0.18)
10.03 (7.0) 3.52(0.15) 6.57(0.12) 10.39(0.10) 11.38(0.10) 11.69(0.12)
12.18 (8.0) 4.40(0.30) 7.45(0.15) 11.69(0.10) 12.43(0.12) 12.67(0.15)
→ 0 † 3.19 ± 0.50 5.95 ± 0.36 9.57 ± 0.32 10.93 ± 0.41 9.83 ± 0.49
Δε/kg·mol –1 – 0.26 ± 0.06 – 0.45 ± 0.04 – 0.64 ± 0.03 – 0.60 ± 0.04 – 0.67 ± 0.05
→ 6.41 ‡ ← 2.73 ± 0.17 5.08 ± 0.14 8.31 ± 0.14 9.38 ± 0.17 9.28 ± 0.20
†
‡
Extrapolation to zero ionic strength by weighted linear regression and error propagation assuming
Gaussian probability distribution, and 95% confidence limits of parameters [2003BEV/ROB].
Interpolation to average ionic strength I m = 6.41 mol·kg –1 .
For the right side, a Hg-pool electrode has been constructed, which is
connected to the outside environment via a Pt-wire welded into the bottom of the glass
wall of the compartment. This compartment also has a tightly fitted Teflon lid, with
three inlets: in one, high purity (99.996%) Ar purging gas is led to the system, the other
one for admitting the titrant solutions in the cell and the third one for venting. The
quality of Ar was checked with bubbling the gas through alkaline pyrogallol solution;
extensive bubbling, for a couple of hours, did not cause any discoloration in the
solution, which proves, that O 2 content of the Ar-gas can be considered as insignificant.
The lack of O 2 was also checked via adding I – ions to the background solutions;
resulting in no visually detectable I 2 formation (in strongly acidic solutions O 2 readily
oxidises I – ion). This control reaction also proved, that other oxidants (i.e., traces of
ClO − , Cl 2 ) are absent from HClO 4 and HClO 4 -HCl background electrolytes as well.
3
Platinised Pt-electrodes were prepared according to the literature
[1998SIP/HEF]. The electrode consists of high purity (99.999%) Pt-wire (d = 1.5 mm)
and 5 mm × 8 mm high purity (99.999%) Pt-sheets. The platinisation was performed
(after careful cleaning, as described in [1934BRO], by using H 2 PtCl 6 in strongly acidic
HCl ([HCl] TOT = 0.2 M) solution. The two electrodes were connected to a high
impedance multimeter via crocodile clips.
The experimentally observed potential values as a function of
o
lg([Sn(IV) free ]/[Sn(II) free ]) are depicted in Figure A-51. The evaluation of E (Sn 4+ /Sn 2+ )
in chloride containing media requires the knowledge of the stepwise formation
2−x
4 x
constants of the complexes SnClx
and SnCl
−
x
. For the former the values selected in
this review were used, and the pertinent values for the latter are listed in Table A-78.
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012