Use of SnO for the determination of standard gibbs energy of formation of SnO2 by oxide electrolyte e.m.f. measurements

Article abstract of DOI:10.1016/S0040-6031(01)00416-6

Evidence was obtained from DTA for the disproportionation of SnO into Sn(1) and SnO₂ at 815 K below a heating rate of 20°C/min in helium and at 800 K in air at 10°C/min heating rate. The e.m.f. of a galvanic cell with SnO/SnO₂ and Sn(1)/SnO₂ as electrodes using 15 mol% YO_1.5 stabilised ZrO₂ (15 YSZ) as the electrolyte showed SnO disproportionation into Sn and SnO₂ to be complete by about 750 K. The e.m.f. of the galvanic cells with SnO/SnO₂ as the test electrode, N₂ (P_O2 ≈ 10^-3 atm)/Pt or Fe/Fe_xO as the reference electrodes and with 15 YSZ as the electrolyte measured over the ranges 776-1140 and 772-1206 K, respectively, yielded the expression ΔG_f⁰ (SnO₂) ± 2.2 (kJ mol^-1) = -568.9 + 0.2007 T (K) for the standard Gibbs energy of formation of SnO₂. A third-law analysis of the e.m.f. data yielded -578.3 ± 4.0 kJ mol^-1 for the standard enthalpy of formation, ΔH_f,298⁰ of SnO₂(s).

Full text of DOI:10.1016/S0040-6031(01)00416-6

Thermochimica Acta 371 (2001) 95±101
Use of SnO for the determination of standard Gibbs energy of
formation of SnO2 by oxide electrolyte e.m.f. measurements
C. Mallikaa, A.M. Edwin Suresh Rajb, K.S. Nagarajab, O.M. Sreedharana,*
aThermodynamics and Kinetics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102, India
bDepartment of Chemistry, Loyola Institute of Frontier Energy, Loyola College, Chennai, Tamil Nadu 600 034, India
Received 7 September 2000; accepted 12 December 2000
Abstract
Evidence was obtained from DTA for the disproportionation of SnO into Sn(l) and SnO2 at 815 K below a heating rate of
208C/min in helium and at 800 K in air at 108C/min heating rate. The e.m.f. of a galvanic cell with SnO/SnO2 and Sn(l)/SnO2
as electrodes using 15 mol% YO1.5 stabilised ZrO2 (15 YSZ) as the electrolyte showed SnO disproportionation into Sn and
SnO2 to be complete by about 750 K. The e.m.f. of the galvanic cells with SnO/SnO2 as the test electrode, N2
ꢀPO2 ꢁ 10 3 atm†/Pt or Fe/FexO as the reference electrodes and with 15 YSZ as the electrolyte measured over the ranges 776±
1
1140 and 772±1206 K, respectively, yielded the expression DG0f ꢀSnO2† Æ 2:2 ꢀkJ mol † ˆ 568:9 ‡ 0:2007T ꢀK† for the
1
standard Gibbs energy of formation of SnO2. A third-law analysis of the e.m.f. data yielded 578:3 Æ 4:0 kJ mol for the
standard enthalpy of formation, DHf0;298 of SnO2(s). # 2001 Elsevier Science B.V. All rights reserved.
Keywords: SnO disproportionation; Oxide e.m.f. on SnO; SnO2 Gibbs energy
1. Introduction
of formation, DHf0;298 of SnO2. In an attempt to resolve
these discrepancies, this investigation was undertaken.
Further, in all the e.m.f. measurements reported in the
literature, a mixture of Sn(l) and SnO2 constituted the
test electrode and in no study, pure SnO was used as
the starting material. Therefore, in the present inves-
tigation, SnO/SnO2 was used as the test electrode to
assess the temperature of complete dissociation into
Sn and SnO2 appropriately corroborated by thermo-
analytical studies.
Impedance studies have been reported in the litera-
ture on thin ®lms of b-Sn with SnO and SnO2 for
humidity sensing applications [1]. Recently, Edwin
Suresh Raj et al. [2] have reported the humidity
sensing properties of SnO2±SnMoO4 composites.
Extensive thermoanalytical studies on Sn and its
oxides had also been reported [3±8] and although
several solid electrolyte e.m.f. measurements have
so far been reported [9±18] for the evaluation of the
standard Gibbs energy of formation, DG0f of SnO2 at
higher temperatures, there is still considerable scatter
in the Gibbs energy data and indications of tempera-
ture dependent errors in the derived standard enthalpy
2. Experimental procedure
The starting materials were spectrographically pure
Sn granules (DSC standard material, Perkin-Elmer,
Switzerland), iron sponge (purity better than 99.9%,
Cerac, USA), Fe2O3 (purity >99.99%, Johnson
* Corresponding author. Fax: ‡91-4114-40301.
E-mail address: oms@igcar.ernet.in (O.M. Sreedharan).
0040-6031/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 1 ) 0 0 4 1 6 - 6

96
C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
Matthey, UK) and SnO (total metallic impurities
<0.1%, Fluka, Switzerland). Tin(IV) oxide was made
by oxidising SnO in air at 1000 K for 24 h. Powders of
SnO and SnO2 in the mole ratio of 9:1 were mixed and
compacted into cylindrical pellets of 10 mm diameter
and 2±3 mm thickness at a pressure of 100 MPa.
Likewise, electrode pellets of Sn/SnO2 were made
of Sn(s) and SnO2 in the mass ratio of 4:1. The
preparation of Fe/FexO electrode pellets was described
elsewhere [19]. A cylindrical solid electrolyte tube
with one end closed and closed end ¯at conforming to
the dimensions of outer diameter 12.7 mm, inner
diameter 9.8 mm and length 350 mm as well as a
solid electrolyte cup with the dimensions of outer
diameter 10 mm, inner diameter 7 mm and depth
5 mm with a ¯at bottom were made of 15 mass%
YO1.5 stabilised ZrO2 (15 YSZ, Nippon Kagaku
Togyo, Japan). The tube electrolyte was used in the
galvanic cells (II) and (III) and the cup electrolytes in
the cells (I) and (IV).
were studied over the ranges 753±1000, 776±1140,
788±965 and 772±1206 K, respectively. Puri®ed He
was used as the cover gas. The temperature was
measured using a Pt±10 mass% Rh/Pt thermocouple
calibrated at the freezing temperatures of high-purity
metals such as Sn, Zn, Sb and Ag. Other details of the
experiments including voltage measurement, tests for
the absence of asymmetric potentials, details for
puri®cation of helium and temperature control are
the same as reported elsewhere [20,21].
3. Results and discussion
3.1. TG/DTA studies
The TG studies were mainly conducted for a pre-
liminary estimation of the temperature range for the
existence of SnO in air and in a relatively inert atmo-
sphere in order to ®x the operating parameters for the
galvanic cell studies. The results of these TG/DTA
studies on SnO are summarised in Table 1. It was seen
from non-isothermal oxidation studies at a heating rate
of 108C/min that the oxidation of SnO commenced at
612 K and 98% of the total weight gain up to 1263 K
was complete by 1030 K. The ®nal product was found
to be SnO2 with a speck of Sn. Since the weight gain
from 612 K was more or less gradual up to 1030 K
without any signi®cant stepwise change in weight at a
given temperature, one would not expect an exotherm
corresponding to oxidation. However, there was a
sharp exotherm at 800 K which should therefore be
attributable to the disproportionation of SnO into Sn
and SnO2. The shape of the TG curve did not warrant
any inference such as the formation of an intermediate
phase, Sn3O4 although no attempt was made to quench
A horizontal TG/DTA system, Model-Seiko 320
was used for non-isothermal oxidation studies on
SnO up to 1273 K in air at a heating rate of 108C/
min. The TG/DTA of SnO in dynamic helium (PO2 of
the order of 1 Pa) was recorded at 208C/min heating/
cooling rate over 2±3 cycles.
The e.m.f. of the galvanic cells
Pt; C; SnO; SnO2j15 YSZjSnO2; Snꢀl†; C; Pt
(I)
Pt; C; SnO; SnO2j15 YSZjN2 ꢀPO2 ꢁ 10 3 atm†; Pt
(II)
Pt; N2 ꢀPO2 ꢁ 10 3 atm†j15 YSZjair
ꢀPO2 ˆ 0:21 atm†; Pt and
(III)
(IV)
Pt; Fe; FexOj15 YSZjSnO; SnO2; C; Pt
Table 1
TG/DTA study of SnO
S. No. Conditions
TG
DTA
Inference
1
Static air
Heating rate: 108C/min
612±1030 K, weight gain
Exotherm at 800 K
±
Disproportionation;SnO2
with Sn traces
Heating rate: 108C/min
1030±1263 K, less than 2% of total weight gain
Partial oxidation of Sn
2
Helium ꢀPO2 ꢂ 1 Pa†, heating rate: 208C/min
2a
2b
2c
First heating
First cooling
Second heating
540±815 K, weight gain: 2±3%
Exotherm at 815 K
Exotherm at 489 K
2SnO ! Sn ‡ SnO2
930±453 K, weight gain: 1%
334±800 K, weight gain: 0.5%
Freezing of Sn
Endotherm at 500 K Melting of Sn

C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
97
a sample from about 810 K for phase identi®cation.
Survey of literature revealed that several techniques
were employed in investigating the disproportionation
Subsequently, the temperature was raised to 753 K
which was still about 60 K lower than the dispropor-
tionation temperature from DTA (at 20 K/min heating
rate). At this temperature, there was a build up of
e.m.f. up to 6±7 mV which eventually decayed to a
null e.m.f. within 2 h. The temperature was then raised
in steps of 25 up to 1000 K. Throughout this range
753±1000 K, only a null e.m.f. ꢀ0 Æ 1:0 mV† could be
recorded upholding the coexistence of Sn(l) with SnO2
in the anode as well, so as to be identical with the
phases in the cathode side.
Having established from cell (I) that SnO dispro-
portionates into a coexisting mixture of Sn(l) and
SnO2, the remaining cells (II) and (IV) (wherein
SnO constituted the main test electrode material) were
®rst heated to about 1000 K for a few hours to ensure
true biphasic equilibrium in the test electrode prior
to e.m.f. measurements during subsequent thermal
cycles.
È
of SnO including Mossbauer [22] and TEM [3] be-
sides conventional XRD [5,8,12] and DSC [6]. In some
of these studies, an environment depleted with respect
to oxygen to the extent of about 10 8 atm (1 mPa) of
PO2 was used to prevent the oxidation of Sn.
In the second part of the investigation, high purity
helium gas with a PO2 of 10 5 atm (1 Pa) was only
used which would not prevent simultaneous partial
oxidation of SnO. However, some of the investigations
on SnO mentioned earlier pointed out the formation of
an intermediate oxide, Sn3O4 prior to ®nal oxidation to
SnO2. In contrast, a study on the aerial oxidation of
thin ®lms of Sn at 483 K by Dolotov et al. [8] for more
than 20 h revealed the formation of amorphous SnO2
which crystallised on further heating to 723 K as
observed by XRD. This observation showed that given
suf®cient time and surface area, Sn could be aerially
oxidised to SnO2 at lower temperatures without form-
ing Sn3O4.
To identify the disproportionation temperature of
SnO by suppressing extensive oxidation (which is a
side reaction), a TG/DTA run was taken on a sample of
90 mg of SnO at a heating rate of 208C/min in
dynamic helium ꢀPO2 ꢁ 1 Pa† at a ¯ow rate of
10 dm3/h. This run showed the temperature of dis-
proportionation to be 815 K and the formation of Sn
was con®rmed by a freezing exotherm at 489 K of
super cooled tin in the ®rst cooling cycle, which was
further con®rmed by the melting endotherm at 500 K
in the second heating cycle (Table 1), in addition to
veri®cation by XRD up on completion of the DTA run.
From the foregoing discussions it is inferred that if
SnO was to be used to determine the equilibrium PO2
over SnO2/Sn(l), it should be ®rst heated to a tem-
perature beyond 815 K for suf®cient time.
The e.m.f. results from cells (II) and (IV) are listed
in Table 2. To facilitate the conversion of the reference
state of oxygen from N2/Pt electrode bearing about
10 3 atm of oxygen, the e.m.f. of cell (III) was
measured over the range 788±965 K (Table 3) and
the least-squares expression for these values was
derived to be
EꢀIII† Æ 2:6 ꢀmV† ˆ 2:2 ‡ 0:0921T ꢀK†
(1)
For further correction of the standard state of oxygen
in the reference air/Pt electrode, a PO2 of 0.2088 atm
was assumed for dry air. The e.m.f. results on cell (III)
were obtained by making use of the same two com-
partment assembly as in the case of cell (II) in order to
minimise systematic errors. Thus, the e.m.f. readings
on cell (II) given in Table 2 were corrected for the
standard state of oxygen in the reference electrode by
combining with e.m.f. values interpolated from Eq. (1)
followed by correction for the PO2 of air. Likewise, the
e.m.f. readings on cell (IV) (Table 2) were corrected
for the standard state of oxygen in the reference Fe/
FexO electrode by interpolating numerical values of
DG0f (from literature [24]) at the temperatures at which
e.m.f. values were recorded. In this manner, the 60
e.m.f. readings in Table 2 were converted into corre-
sponding values of e.m.f. for the hypothetical cell
3.2. e.m.f. studies
Attempts were made to measure the e.m.f. of cell (I)
at 710 K. The choice of this temperature (710 K) was
based on a recent coulometric study on SnO by Yang
et al. [23], according to whom Sn3O4 coexisted with
Sn(l) over a narrow range of temperature between 696
and 731 K. Unfortunately, the impedance of the
cell was too high to enable such measurements.
Pt; Snꢀl†; SnO2 k O2 ꢀPO2 ˆ 1 atm†; Pt
and are plotted in Fig. 1.
(V)

98
C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
Table 2
Table 2 (Continued )
Measured e.m.f. results for cells (II) and (IV)
Cella
T (K)
E (mV)
Cella
(II)A
T (K)
E (mV)
772.3
1150.6
1193.2
1021.4
997.2
28.87
100.76
109.33
75.88
71.12
57.76
88.66
91.66
82.44
801.2b
954.8
954.45
860.13
911.43
737.31
880.20
921.52
769.10
869.69
985.24
799.53
901.19
944.11
819.61
933.26
874.7
1139.8
923.0
859.3
923.6
1086.1
1104.6
1058.8
1094.3
938.4
758.4b
1048.4
891.0
a Cell (II): N2 ꢀPO ꢁ 10 3 atm†/Pt as reference; cell (IV): Fe/
2
FexO reference electrode.
b Values rejected for the least-squares analysis as a result of
wide scatter exhibited by them in third-law treatment.
825.1
1018.6
841.6
(II)B
816.9b
1079.5
1109.9
883.8
944.56
779.22
758.83
903.05
892.31
809.51
849.76
862.42
Prior to the least-squares analysis of the 60 data
points, it was necessary to eliminate those which were
not consistent with calorimetric data in the literature.
For this purpose, a third-law analysis of the e.m.f.
results was adopted. The values of the standard
enthalpy of formation, DHf0;298 of SnO2 were derived
from such a third-law treatment of the e.m.f. values
making use of the free-energy functions (FEFs) for
SnO2(s) and the elements compiled by Lamoreaux
et al. [25]. These authors listed the expressions of
FEFs for SnO2(s), Sn(l) and O2(g) as ®ve term ®ts
valid up to 1000 K and as other such polynomial ®ts
for temperatures extending far beyond 1000 K. To
facilitate interpolation, DFEF (for the formation reac-
tion of SnO2(s) from Sn(l) and O2(g)) was ®tted into
900.4
1033.5
970.0
947.2
(II)C
808.6
791.6
955.13
966.16
839.62
934.54
872.02
912.86
748.19
890.56
788.71
882.19
975.90
829.59
985.5
833.2b
932.4
867.4b
1124.3
906.4
1064.7
916.4
776.2
1002.8
(IV)A
878.1
1135.7
1109.0
947.7
50.00
97.75
93.33
61.82
111.72
94.91
66.44
53.49
106.42
80.23
71.16
84.54
77.31
36.33
103.74
87.35
Table 3
Experimental results for the cell (III): Pt; N2 ꢀPO2 ꢁ 10 3 atm†
j15 YSZjair ꢀPO2 ˆ 0:21 atm†; Pt
1206.0
1122.0
971.7
Run
1
T (K)
E (mV)
797.2
833.2
853.8
902.9
68.44
72.56
79.64
80.55
900.4
1179.2
1052.2
1013.3
1065.5
1036.8
827.2
2
886.2
964.7
787.8
872.9
820.7
897.8
83.20
86.08
70.78
75.78
75.62
78.30
1166.8
1083.0
(IV)B
853.0
44.58

C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
99
Fig. 1. e.m.f. results of the hypothetical cell (V): Pt; Snꢀl†; SnO2 k O2 ꢀPO2 ˆ 1 atm†; Pt.
two term expressions as follows:
These values so calculated from Eq. (3) using DFEF
values (interpolated at the 60 experimental tempera-
tures) are plotted against experimental temperature in
Fig. 2. The standard deviation about the mean was
calculated to be Æ1.3 kJ mol 1 after omitting ®ve data
points (denoted by ``b'' in Table 2). Thus, a value of
1
DFEF ꢀJ K 1 mol † ˆ 210:8 ‡ 0:0008T ꢀK†;
ꢀ700 1000 K†
(2a)
(2b)
1
DFEF ꢀJ K 1 mol † ˆ 208:4 0:0017T ꢀK†;
1
578.3 kJ mol could be assigned for DHf0;298 of
ꢀ1000 1200 K†
SnO2(s). To estimate the overall uncertainty in
DHf0;298, an algebraic sum of Æ1.3 (from Fig. 2),
Æ2.2 kJ for DG0f ꢀSnO2† (from Eq. (6); see later)
and that associated with DFEF had to be taken into
account. Therefore, an uncertainty of Æ4.0 kJ could be
The values of DHf0;298 were computed using the
equation
DHf0;298 ˆ DGf0 TDFEF
(3)
Fig. 2. DHf0;298ꢀSnO2† determined by third-law computations from the e.m.f. data.

100
C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
1
ascribed to the value of 578.3 kJ mol
for the
carried out over a temperature range overlapping to
a large extent with the present one are compared in this
®gure. It is seen that the present line for DG0f of
SnO2(s) is in close agreement with that found by
Bannister [18]. The other four e.m.f. studies yielded
values of DG0f which are within 2 kJ more negative (at
the mean temperature of measurements) than those
from Bannister and the present investigation. It should
be highlighted that all the 10 different e.m.f. measure-
ments reported in the literature were made with Sn(l)/
SnO2 as the starting material, whereas the present
investigation alone had made use of SnO/SnO2 as the
initial electrode material which upon equilibration at
or above 753 K resulted in the coexistence of Sn(l)/
SnO2.
standard enthalpy of formation of SnO2(s) at 298 K.
The third-law analysis thus provided a means for
eliminating ®ve points out of the 60 listed in Table 2
for the computation of a least-squares expression for
the e.m.f. values of the hypothetical cell (V) which is
given as follows:
EꢀV† Æ 5:8 ꢀmV† ˆ 1474:1 0:5201T ꢀK†
valid over the range 772±1206 K.
(4)
Corresponding to 4 F of electricity the overall
virtual cell reaction for cell (V) could be represented
as
Snꢀl† ‡ O2ꢀg†„SnO2ꢀs†
(5)
The approach of the Sn/SnO2 equilibrium starting
from SnO/SnO2 in contrast to all other e.m.f. measure-
ments which were directly on Sn/SnO2 helped to
assess the accuracy of the data as much as the preci-
sion of the e.m.f. technique. Con®rmation of the Sn/
Sn3O4 equilibrium which was reported to be existing
over a narrow range 696±731 K, as reported from the
coulometric study by Yang et al. [23] could not be
made owing to high impedance of the cell (II) at such
lower temperatures. The log PO2 of Sn/SnO2 derived
from Eq. (6) could be represented as follows:
The Gibbs energy change for reaction (5) (also equal
to DG0f of SnO2(s)) was therefore computed using the
familiar Nernst equation ꢀDG0f ˆ 4FE†:
1
DG0f ꢀSnO2† Æ 2:2 ꢀkJ mol
†
ˆ
568:9 ‡ 0:2007T ꢀK†
(6)
Bannister [18], who employed Pt or ‰Pt ‡ ꢀU0:38
Sc0:62†O2ÆxŠ/N2 ꢀPO2 ꢁ 10 3 atm† as the reference
electrode for determining the oxygen potential of
Sn/SnO2, had also reviewed nine earlier reports on
such e.m.f. measurements. To assess the agreement of
Eq. (6) from the present investigation with those
reported in the literature by employing similar
e.m.f. technique, a plot of DG0f is given in Fig. 3. It
may be noted that only those studies which were
log PO2 Æ 0:12 ꢀatm† ˆ 10:484 29719T ꢀK†
(7)
This equation could be of help in the choice of a buffer
mixture for a lower range of log PO2 in controlled
atmosphere synthesis of advanced materials because
Fig. 3. Standard Gibbs energy of formation of SnO2(s) from e.m.f. measurements.

C. Mallika et al. / Thermochimica Acta 371 (2001) 95±101
101
[4] V.M. Glazov, A.S. Pashinkin, A.S. Malkova, V.V. Zharov, Zh.
Fiz. Khim. 64 (1990) 811.
of the ease of bubbling a cover gas through liquid tin
(melting temperature: 505 K).
[5] V.P. Stets, V.P. Karlov, G.N. Butuzov, A.A. Ryabukha,
Poroshk. Metall. 11 (1989) 10.
This investigation had provided clear evidence
through both DTA and e.m.f. studies for the dispro-
portionation of SnO into Sn(l) and SnO2 at or above
753 K. Further, these studies yielded DG0f values for
SnO2(s) in close agreement with those reported by
Bannister. In addition, approach of Sn/SnO2 equili-
brium starting with SnO/SnO2 facilitated an assess-
ment of the accuracy of the e.m.f. measurements. A
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1
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DHf0;298 of SnO2(s) from these studies in close agree-
[11] G. Petot-Ervas, R. Farhi, C. Petot, J. Chem. Thermodyn. 7
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1
ment with 577:6 Æ 0:2 kJ mol
reported from a
calorimetric study by Lavut et al. [26].
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143.
[13] T.A. Ramanarayanan, A.K. Bar, Metall. Trans. B 9 (1977) 485.
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Acknowledgements
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(1982) 429.
The authors are deeply indebted to Dr. V.S. Raghu-
nathan, Associate Director, Materials Characterisation
Group, Dr. Baldev Raj, Director, Materials, Chemistry
and Reprocessing Group, Dr. P. Rodriguez, Director,
IGCAR and to Rev. Dr. John Pragasam, Director,
LIFE, Loyola College, for their keen interest and
constant encouragement throughout the course of this
work. Support from BRNS grant for this research
work is gratefully acknowledged.
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