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

238
X Group 15 compounds and complexes
Table X-3 (continued)
Method Ionic media t / °C log10
K
reported
log K
10
ο
estimated (a)
Reference
Sn 2+ + HPO − 2 4
+ HPO 2− 4
Sn(H2PO 4)(HPO 4) Ise-Sn, gl 3 M NaClO 4 25 8.43 ± 0.04 b 10.3 ± 0.2 [2000CIA/IUL2]
Sn 2+ 2
+ HPO −
4
SnHPO 4 (aq)
Ise-Sn, gl 3 M NaClO 4 25 7.55 ± 0.01 b 9.5 ± 0.2 [2000CIA/IUL2]
sol 0.2 M NaClO 4 25 7.83 ± 0.11 [1968CIL]
Sn 2+ 2
+ 2HPO − 4
Sn(HPO 2
4) 2
Ise-Sn, gl 3 M NaClO 4 25 13.88 ± 0.03 b 13.4 ± 0.2 [2000CIA/IUL2]
Sn 2+ 2
+ 3HPO − 4
Sn(HPO 4
4) 3
Ise-Sn, gl 3 M NaClO 4 25 12.68 ± 0.01 b 12.9 ± 0.2 [2000CIA/IUL2]
sol 0.2 M NaClO 4 25 10.04 ± 0.14 [1968CIL]
Sn 2+ 3
+ PO − 4
SnPO −
4
Ise-Sn, gl 3 M NaClO 4 25 14.63 ± 0.01 b 18.0 ± 0.2 [2000CIA/IUL2]
(a) Extrapolated to I = 0 by Ciavatta and Iuliano using the SIT approach and estimated ion interaction
coefficients, see Appendix A.
(b) Calculated from the reported data using pK 1 = 1.86, pK 2 = 6.26 and pK 3 = 10.78 for the dissociation
constants of phosphoric acid [1969BAL/SIL], [1971PET].
X.2.2.2
Aqueous tin(II) pyrophosphato complexes
As in the earlier volumes in this serie, the only polyphosphate(V) species considered is
the pyrophosphate (diphosphate). Other polyphosphoric acid species have negligible
equilibrium concentrations at total phosphate concentrations < 0.045 mol·dm –3 and at
temperatures below 200 °C [1974MES/BAE]. The pyrophosphato complexes of tin(II)
have been studied by potentiometric [1986TUR/KRA], [1986TUR/KRA2],
[1991DUF/WIL], polarographic [1980ORE/AND], [1987TUR/KRA] and
spectrophotometric [1980ORE/AND2] methods, as well as by solubility measurements
[1966MES/IRA]. Only qualitative information is reported in [1966MES/IRA] and
[1980ORE/AND]. For reasons mentioned in Appendix A, the equilibrium constants
reported in [1980ORE/AND2] were not considered in this review. Although the highly
charged pyrophosphate forms very stable complexes with tin(II), several factors
complicate the interpretation of the experimental data. The alkali metal ions are known
to form relatively stable complexes with pyrophosphate, especially above pH 8 where
4
the species PO −
2 7
is dominant in the solution [1994STE/FOT]. Therefore, the
association constants of the pyrophosphate-alkali metal ion complexes (ion-pairs)
should be considered, unless a sufficiently low concentration of the background
electrolyte is used. This association is neglected in all publications dealing with the
tin(II)-pyrophosphate interaction, and only one of them used relatively (but not
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012