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

396
A Discussion of selected references
considered in the present assessment, but the reviewers assigned higher uncertainty to
this constant than was reported by the authors.
[1991DUF/WIL]
The chemical speciation of tin(II)- and tin(IV)-pyrophosphate systems were investigated
by a pH-metric method in 0.15 M NaCl solution at 25 °C under nitrogen atmosphere.
The authors also determined the protonation constants of pyrophosphate under identical
conditions. The hydroxido complexes of tin(II) and tin(IV) were taken into account
during the calculations (the formation constants were taken from [1978KRA]), but only
the species Sn(OH) −
3
was formed in appreciable amount (above pH 8). The pH-metric
curves were obtained at different pyrophosphate-tin(II) ratios and for different
pyrophosphate (10 to 50 mM) and tin (3 to 25 mM) concentrations. The authors
reported the formation of a white precipitate below pH 2.6 in the tin(II)-pyrophosphate
system, which was identified as Sn 2 P 2 O 7 (s). Similar observations were not reported in
related publications [1980ORE/AND2], [1986TUR/KRA], [1986TUR/KRA2],
[1987TUR/KRA], which is probably due to the higher concentration of tin(II) applied in
[1991DUF/WIL]. The authors suggested the formation of seven Sn(II) complexes:
2
SnP2PO − 6
7
, Sn(P2O 7) − 5
2
, SnH(P2O 7) − 4
2
, SnH
2(P2O 7) − 3
2
, SnH
3(P2O 7) − 10
2
, Sn(P2O 7) −
3
3
and Sn(OH)(P2O 7) − . Although several common species can be found, the speciation
based on the data of Tur'yan et al. [1986TUR/KRA], [1986TUR/KRA2],
[1987TUR/KRA] and that suggested by Duffield et al. are rather different
(Figure A-46). This is due to the neglected Na + 4
- PO −
2 7 interaction and to the different
experimental conditions used (Tur'yan at al. applied 1 M NaClO 4 as background
electrolyte and a notably wider range of tin(II)/pyrophosphate ratios).
In addition, the authors determined the solubility of Sn 2 P 2 O 7 (s). The solid
Sn(II) pyrophosphate (obtained from Unilever Research, U.K.) was not characterised.
The equilibrium between the solid and the solution (pure water and 0.15 M (Na,H)Cl
with different amount of HCl) phases was found to be reached within 7 hours. The
dissolved tin was determined by atomic absorption spectroscopy. The formation of
pyrophosphate complexes were taken into account during the calculation of solubility
constants. The reported log10 K s,0
values showed rather important pH dependence (see
Table A-59).
The authors also reported the formation of very stable tin(IV)-pyrophosphate
+
3
complexes ( SnHP2O 7 , SnHP 2 O 7 (aq), SnH(P2O 7) − 4
2 , Sn(P2O 7) − 5
2 , SnH 1(P2O 7) −
− 2 ),
which prevents the precipitation of hydrated SnO 2 (s) ion in the whole pH-range studied
(pH = 2 to 10). Among the water soluble hydroxido complexes Sn(OH) −
5 and
2
Sn(OH) − *
6 , only the latter was considered in the calculations ( log10 β 6,1 = – 24.1, taken
from [1978KRA]), and it was found to be dominant above pH 8.3 (however, at this pH
the complex Sn(OH) − 5 should be the dominant species in solution, based on the selected
values for the alkaline hydrolysis of tin(IV), see chapter VII.1.2).
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012