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

304
A Discussion of selected references
[1960KLE/VAS]
Results of investigations of the solubility of alpha- and beta-tin acid in nitric acid and
sodium hydroxide solutions in absence and presence of NaF are reported for t = 20 °C.
Alpha-tin acid was prepared by reaction of tin with sulfuric acid and beta-tin acid by
reaction with nitric acid. Precipitates were several times washed with distilled water.
Solubility in solutions of 0.01 – 1.0 M HNO 3 or NaOH and beta-tin acid in
0.05 – 0.5 M NaOH is such low that tin could not be detected by the applied indicator
(hematoxylin, sensitivity ≥ 5 × 10 –7 M). Solubilities of alpha-tin acid in solutions
containing NaF and HNO 3 (pH = 1) at (20 ± 0.1) °C:
NaF/M 0.05 0.1 0.2 0.3 0.4 0.5
Sn(IV)/M 0.0004 0.0010 0.0021 0.0032 0.0045 0.0055
No decrease of the total fluoride content in solution was observed, that means
the solid did not absorb F – .
Since at pH = 1 fluoride is associated as HF(aq) the dissolution reaction below
is assumed.
SnO(OH) 2(s) + n HF(aq) → SnO(OH) 2−n F n (aq) + n H 2 O(l)
(A.34)
For the fluorido complex in solution K = {SnO(OH) 2−n F n (aq)}/HF(aq) n , the
total concentration of tin is approximately equal to the concentration of the fluorido
complex. From a plot log 10 ([Sn(IV)] – log 10 ([NaF]) for the solubility data above n = 1
was obtained. In absence of solid tin acid and excess of fluoride n increases. An
equilibrium constant was derived using a dissociation constant K HF(aq) =1.16 × 10 –3 from
the data above:
K = {SnO(OH)F(aq)}/HF(aq) = (1.05 ± 0.09) × 10 –2 .
In addition a dissociation constant of the fluorido complex according to
Reaction (A.35) of K = (5.3 ± 0.2) × 10 –7 was calculated.
SnO(OH)F(aq) → SnO(OH) + + F –
(A.35)
The chemical form of the fluorohydroxido complex (SnO(OH)F) is arbitrary, it
could also be Sn(OH) 3 F or Sn(OH) 3 F(H 2 O) 2 .
The equilibrium constants are rough estimates, because control of ionic
strength was not considered.
[1961CON/PAU]
Potentiometric measurements, using tin amalgam electrode, have been performed to
study the formation of tin(II)-fluoride complexes in 0.5 and 2.0 M (Na,H)ClO 4 media at
298.15 K under nitrogen atmosphere ([F – ] tot = 0.12 to 0.24 M, [Sn] tot = 0.004 to
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012