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

X.3 Arsenic compounds and complexes 241
However, Duffield et al. considered only the formation of Sn(OH) 3+ 2
and Sn(OH) −
6 ,
disregarding e.g. the complex Sn(OH) − 5 . Therefore, the equilibrium constants reported
in [1991DUF/WIL] cannot be used to calculate selected values.
Table X-5: Experimental formation constants * 4
for the processes p Sn + + q PO 2 7 +
+
4 4
r H Sn H (P2O 7) p − q+
r
p r q obtained from pH-metric measurements by
[1991DUF/WIL] (I = 0.15 M NaCl, T = 298 K).
pqr
log 10 β pqr
111 23.56 ± 0.42
110 22.61 ± 0.27
121 33.36 ± 0.06
120 27.08 ± 0.09
12-1 19.84 ± 0.12
* The protonation constants of pyrophosphate determined by
the authors: log 10 β 11 = (8.14 ± 0.01), log 10 β 12 = (14.01 ± 0.01),
log 10 β 13 = (15.78 ± 0.02), log 10 β 14 = (16.63 ± 0.10); the formation
constant of the hydroxo complexes considered in the data
evaluation: log 10 β 10-1 = – 1.937, log 10 β 10-6 = – 24.108.
4−
X.3 Arsenic compounds and complexes
X.3.1 Tin arsenides
In the binary system Sn-As, two intermediate phases SnAs and Sn 4 As 3 are known.
The thermodynamic data in this system are described by Gladyshev and
Pashkov [1978GLA/PAS], Koshchenko et al. [1980KOS/PAS2] and Fitzner and Kleppa
[1996FIT/KLE].
X.3.1.1 SnAs(cr)
Fitzner and Kleppa [1996FIT/KLE] used direct synthesis drop calorimetry in fused
silica capsules to measure the standard enthalpy of formation of several binary alloy
compositions in the systems Si-As, Ge-As and Sn-As.
The standard enthalpy of formation of SnAs(cr) determined by this method is
f
ο
m
Δ H (SnAs, cr, 298.15 K) = − (9.5 ± 3.2) kJ·mol –1 .
This selected value is in very good agreement with the one recommended by
Gokcen (− 11.3 kJ·mol –1 ) in his recent assessment of the As-Sn system [1990GOK].
CHEMICAL THERMODYNAMICS OF TIN, ISBN 978-92-64-99206-1, © OECD 2012