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

280
A Discussion of selected references
[1941GAR/HEI]
The experimental data of [1941GAR/HEI] were re-evaluated, taking into account the
formation constants of the tin(II) hydroxido and chlorido complexes selected by this
review. The re-evaluation of the solubility of SnO(cr) in hydrochloric acid solutions, in
pure water under oxygen-free conditions, and in sodium hydroxide solutions led to the
following three solubility constants reported here.
SnO(cr) + 2 H + Sn 2+ *
+ H 2 O(l) log10 K s,0 (SnO, cr, 298.15 K)
*
SnO(cr) + H 2 O(l) Sn(OH) 2 (aq) log10 K s,2 (SnO, cr, 298.15 K)
SnO(cr) + OH – + H 2 O(l) Sn(OH) − *
3 log K (SnO, cr, 298.15 K)
Solubility of SnO(s) in HCl solutions
10 s,3
Garrett and Heiks [1941GAR/HEI] prepared SnO(s) by reacting tin(II) chloride
solutions with sodium hydroxide solutions and thereby precipitating tin(II) hydroxide.
The tin(II) hydroxide was dehydrated in warm conductivity water containing a small
amount of sodium hydroxide. The results of six analyses gave an average 100w(Sn) =
87.6 for the Sn content of the solid phase (theoretically: 100w(Sn) = 88.123).
The solubility of SnO was determined in dilute solutions of HCl ([HCl] ini = 4.4 × 10 –4 –
9.6 × 10 –2 mol·kg –1 ). The total concentration of the dissolved tin(II) was determined by
a polarographic method and by redox titrations using K 2 Cr 2 O 7 solution. An attempt was
made to approach equilibrium from supersaturation and undersaturation by reacting
SnO(s) with HCl solutions at 35 °C and 25 °C for several days, respectively. Then the
sample pairs were both thermostated at 25 °C for a total of seven days. Provided SnO(s)
is more soluble at 35 °C than at 25 °C and the 35 °C samples reached equilibrium
before they were cooled to 25 °C equilibrium was approached from supersaturation in
those experiments indeed. The usefulness of this method has been questioned
[1963SCH].
ο
The re-evaluation (see Figure VII-8) resulted in Δ fGm(SnO, cr, 298.15 K) =
− (253.11 ± 1.38) kJ·mol –1
* o
which is equivalent to log10 K s,0
= (2.00 ± 0.24) for
ο
Reaction (A.20), when the selected Δ fGm(Sn 2+ , 298.15 K) = − (27.39 ± 0.30) kJ·mol –1
ο
and Δ
fGm(H 2 O, l, 298.15 K) = − (237.14 ± 0.04) kJ·mol –1 from the NEA auxiliary data
is taken into account.
SnO(cr) + 2 H + Sn 2+ + H 2 O(l)
(A.20)
Three deficiencies need to be criticised in this work. (1) The observed
quantities are the total molality of Sn(II) and the initial molality of HCl. Information on
the equilibrium pH or molality of free Sn 2+ values would have facilitated the
re-evaluation and improved the precision of the measurement. (2) Dissolution of SnO(s)
in dilute HCl leads to hydroxido and chlorido complex formation, thus at least 4 species
contribute appreciably to the total molality of Sn(II). Consequently the accuracy of the
* o
log K (A.20) determination depends largely on the accuracy of the respective
10 s,0
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012