The thermal decomposition of copper(II) oxalate revisited

Article abstract of DOI:10.1016/j.tca.2006.03.008

DSC, TG and TG-FT-IR, and XRPD have been used to examine the effects of supposedly inert atmospheres of argon and nitrogen on the mechanism of the thermal decomposition of copper(II) oxalate. The DSC curves in pure argon at 10 °C min^-1 show a broad endotherm with onset at about 280 °C and maximum at about 295 °C. In mixtures of argon and nitrogen, as the proportion of argon gas is decreased, the endothermic character of the decomposition decreases until, when nitrogen is the main component, the decomposition exhibits a complex broad exothermic character. XRPD studies showed that, regardless of the proportions of nitrogen and argon, the DSC residues consisted of mainly copper metal with small amounts of copper(I) oxide (cuprite) and, under some conditions, traces of copper(II) oxide (tenorite). Various explanations for this behaviour are discussed and a possible answer lies in the disproportionation of CO₂(g) to form small quantities of O₂(g) or monatomic oxygen. The possibility exists that the exothermicity in nitrogen could be explained by reaction of the nitrogen with atomic oxygen to form N₂O(g), but this product could not be detected using TG-FT-IR.

Full text of DOI:10.1016/j.tca.2006.03.008

Thermochimica Acta 446 (2006) 91–100
The thermal decomposition of copper(II)
oxalate revisited
Emmanuel Lamprecht, Gareth M. Watkins, Michael E. Brown∗
Chemistry Department, Rhodes University, Grahamstown 6140, South Africa
Available online 17 March 2006
Dedicated to Professor Wolfgang Hamminger on the occasion of his 65th anniversary.
Abstract
DSC, TG and TG-FT-IR, and XRPD have been used to examine the effects of supposedly inert atmospheres of argon and nitrogen on the
mechanism of the thermal decomposition of copper(II) oxalate. The DSC curves in pure argon at 10 ◦C min−1 show a broad endotherm with onset
at about 280 ◦C and maximum at about 295 ◦C. In mixtures of argon and nitrogen, as the proportion of argon gas is decreased, the endothermic
character of the decomposition decreases until, when nitrogen is the main component, the decomposition exhibits a complex broad exothermic
character. XRPD studies showed that, regardless of the proportions of nitrogen and argon, the DSC residues consisted of mainly copper metal
with small amounts of copper(I) oxide (cuprite) and, under some conditions, traces of copper(II) oxide (tenorite). Various explanations for this
behaviour are discussed and a possible answer lies in the disproportionation of CO2(g) to form small quantities of O2(g) or monatomic oxygen.
The possibility exists that the exothermicity in nitrogen could be explained by reaction of the nitrogen with atomic oxygen to form N2O(g), but
this product could not be detected using TG-FT-IR.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Copper(II) oxalate; Metal carboxylates; Thermal decomposition; Atmosphere effect; Purge gases
1. Introduction
The overall chemical equation is then:
(α + β + γ)CuC2O4(s) → αCu(s) + 2αCO2(g)
+ (1/2)βCu2O(s) + (1/2)βCO(g) + 1(1/2)βCO2(g)
+ γCuO(s) + γCO(g) + γCO2(g)
Some anomalies in the thermal behaviour of other copper(II)
carboxylate complexes [1] indicated a need to re-examine some
aspects of the thermal decomposition of copper(II) oxalate,
which is reported to be an exothermic process under nitrogen
purge [2–4], in a self-generated atmosphere [5], and in various
atmospheresincludinghydrogenandcarbondioxide[6]. Decom-
position may take place by various pathways [7,8], of which the
three main competing reactions are:
(2)
where α + β + γ = 1. We may substitute for α using α = 1 − β − γ
so that Eq. (2) becomes:
CuC2O4(s) → (1 − β − γ)Cu(s) + 2(1 − β − γ)CO2(g)
+ (1/2)βCu2O(s) + (1/2)βCO(g)
CuC2O4(s) : a → Cu(s) + 2CO2(g);
b → (1/2)Cu2O(s) + (1/2)CO(g)
+ 1(1/2)CO2(g);
+ 1(1/2)βCO2(g) + γCuO(s) + γCO(g)
+ γCO2(g)
(3)
This may be simplified to:
c → CuO(s) + CO(g) + CO2(g)
(1)
CuC2O4(s) → (1−β−γ)Cu(s) + (1/2)βCu2O(s) + γCuO(s)
+ ((1/2)β+γ)CO(g) + (2 −(1/2)β−γ)CO2(g)
∗
Corresponding author. Tel.: +27 46 6038254; fax: +27 46 6225109.
E-mail address: m.brown@ru.ac.za (M.E. Brown).
(4)
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.03.008

92
E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
in which β is the fraction of the available copper(II) oxalate that
undergoes decomposition via process b, and γ is the fraction that
undergoes decomposition via process c. Individually, 0 ≤ β ≤ 1
and 0 ≤ γ ≤ 1, and together β + γ ≤ 1.
accompanied by the evolution of CO(g), CO2(g) and (1/2)O2(g)
or O(g). The consequences of such a mechanism are considered
in the discussion of results.
An expression for the standard enthalpy change for reaction
(3) can be written as a function of β and γ:
2. Experimental
2.1. Preparation and characterization
ꢀrH◦(β, γ) = (1/2)βꢀfH◦Cu2O(s) + γꢀfH◦CuO(s)
+ ((1/2)β + γ)∆fH◦CO(g) + (2−(1/2)β−γ)
Copper(II) oxalate was prepared [3] by precipitation from
aqueous solutions of copper(II) sulfate (0.20 mol L−1) and
potassium oxalate (0.13 mol L−1). The product was dried in air
at 120 ◦C for 5 days prior to use. The calculated formulae from
analytical data for the dried and undried material are CuC2O4,
0.02H2O and CuC2O4, 0.1H2O, respectively.
The X-ray powder diffraction patterns, using nickel-filtered
Cu K␣ radiation, for the undried and dried copper(II) oxalate
are shown in Fig. 1. No observable structural changes occurred
as a result of drying.
× ∆fH◦CO2(g)− ∆fH◦CuC2O4(s)
Using standard values for the enthalpies of formation of
Cu2O(s), CuO(s), CO(g) and CO2(g) [9] and a published value
[10] of −751.3 kJ mol−1 for ꢀfH◦CuC2O4(s), this may be writ-
ten numerically as
(5)
ꢀrH◦(β, γ) (kJ mol−1) = 58.13β + 127.72γ − 35.56
(6)
If it is assumed that only path a is followed, i.e. that no oxi-
dation takes place via paths b and c, then ꢀrH◦ (β = 0, γ = 0)
(kJ mol−1) = − 35.56. If it assumed that complete oxidation
takes place via paths b and c, then β + γ = 1, and substitution
of β = 1 − γ in Eq. (6) gives:
The infrared spectra of both the undried and dried copper(II)
oxalate samples in KBr pellets compared well with the spectrum
reported in the literature [3].
2.2. Thermal analysis equipment
ꢀrH◦(γ) (kJ mol−1) = 69.59γ + 22.57
(7)
Samples of copper(II) oxalate were examined using a Perkin-
Elmer 7-Series differential scanning calorimeter (DSC) and
thermogravimetricanalyzer(TG)coupledwithevolvedgasanal-
ysis (EGA) by FT-IR. The DSC was calibrated for temperature
using the melting points of indium metal and zinc metal, and
for heat-flow using the standard enthalpy of fusion of indium
metal. These calibrations were performed under argon purge,
and measurements with the same calibrants under nitrogen were
practically unchanged. The TG was calibrated for temperature
under argon purge using the Curie points of Perkalloy and zinc.
Samples were heated in the DSC, in crimped (but not sealed)
aluminium pans, from 25 to 350 ◦C in argon, nitrogen, mixed
nitrogen–argon, andoxygenatmospheresatvariousheatingrates
(10, 5 and 2 ◦C min−1) and purge flow rates (15–25 mL min−1).
Thus, if complete oxidation takes place via path b only (i.e.
γ = 0), then ꢀrH◦ (γ = 0) (kJ mol−1) = 22.57 and if complete
oxidation takes place via path c only (i.e. γ = 1), then ꢀrH◦
(γ = 1) (kJ mol−1) = 92.16. So the DSC response for this reaction
can be exothermic or endothermic, depending on β and γ. The
most exothermic DSC response possible is −35.56 kJ mol−1 and
the most endothermic response possible is 92.16 kJ mol−1
.
If only processes a and b take place (i.e. γ = 0), then Eq. (4)
becomes:
CuC2O4(s) → (1 − β)Cu(s) + (1/2)βCu2O(s)
+ (1/2)βCO(g) + (2 − (1/2)β)CO2(g)
and by setting γ = 0 in Eq. (6):
(8)
(9)
ꢀrH◦(β) (kJ mol−1) = 58.13β − 35.56
and −35.56 ≤ ꢀrH◦(β) (kJ mol−1) ≤ +22.57.
If only processes a and c take place (i.e. β = 0), then Eq. (4)
becomes:
CuC2O4(s) → (1 − γ)Cu(s) + γCuO(s) + γCO(g)
+ (2 − γ)CO2(g)
(10)
(11)
and Eq. (6) becomes:
ꢀrH◦(γ) (kJ mol−1) = 127.72γ − 35.56
and −35.56 ≤ ꢀrH◦(γ) (kJ mol−1) ≤ +92.16.
The oxalates of nickel, manganese, silver, mercury and lead
have been proposed [11] to decompose by a mechanism of dis-
sociative evaporation, i.e. by the simultaneous gasification of all
reaction products irrespective of their volatility, and subsequent
condensation of low-volatility species [12–15]. The decomposi-
tions of silver(I), nickel(II) and mercury(II) oxalate are said to be
Fig. 1. X-ray powder diffraction patterns, using nickel-filtered Cu K␣ radiation,
for undried and dried copper(II) oxalate.

E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
93
The specifications of the nitrogen used stated maximum impu-
rities in vpm of 3 for oxygen and for water and 2 for CO2 + CO.
The figures for the argon used were slightly lower at 2 for oxy-
gen and for water, 6 for nitrogen and 2 for CO2 + CO. Some
experiments were done under argon and nitrogen using sealed
pans perforated by a single pin-prick.
The mixedatmospheresweregenerated by meansof aneedle-
valve system. The TG and the DSC were purged for several hours
at every change of atmosphere, and purged for at least 15 min
after each sample change before applying the temperature pro-
gramme. The TG furnace tube was kept under a positive pressure
of the atmosphere in use to minimize interference from residual
air. A 15 ppb oxygen/moisture/hydrocarbon trap in tandem with
an indicating trap did not alter the observed behaviour.
To ensure that the DSC furnace was purging correctly, the
reference furnace was blocked with a strip of adhesive tape and
the flow rate through the sample furnace checked at the common
exhaust line. The procedure was then repeated with the sample
furnace blocked and the reference furnace open. The sample and
reference furnaces were both found to be purging normally.
The TG exhaust line was interfaced to an FT-IR gas cell fitted
to a Perkin-Elmer FT-IR-2000 spectrometer. To test whether the
evolution of carbon monoxide could be detected in the purge
gas in the presence of the exposed platinum parts of the furnace,
a sample of nickel squarate [16] was decomposed in argon. A
weak IR absorption was observed in the carbon monoxide region
(doublet-envelope centred at 2143 cm−1 [17]).
beyond 320 ◦C showed a marked mass gain, probably due to
oxidation by traces of residual air in the furnace.
The residues under argon and nitrogen at 315 ◦C account,
respectively, for 43.5% and 43.2% of the original sample masses.
By comparison, the calculated mass percentage of copper in
the dried copper oxalate is 41.84%. It is clear, therefore, that
some oxidation of the copper in the samples has taken place, by
residual air in the TG, by disproportionation of carbon dioxide,
orboth. TheX-raypowderdiffractionpatternsoftheTGresidues
confirmed that some oxidation to Cu (I) oxide had taken place
under both argon and nitrogen atmospheres.
Residual air in TG furnaces can be extremely difficult [18]
to remove effectively because of less than ideal design char-
acteristics of their purging arrangements. This is particularly
true of a Perkin-Elmer TG furnace that has been modified for
interfacing to an FT-IR gas cell. These modifications are men-
tioned in the manufacturer’s on-line catalogue [19] and similar
modifications are described in more detail in the open literature
[20]. Substances that are very sensitive to oxidation, such as
finely divided metal powders, including the residues from the
decomposition of copper(II) oxalate [18] have been suggested
as diagnostic tools for checking purging efficiency.
The formation of copper(I) oxide through the disproportion-
ation of carbon dioxide must result in the evolution of carbon
monoxide. Three-dimensional plots of the evolved gas analy-
sis data for TG runs under argon and nitrogen are shown in
Figs. 3 and 4, respectively. The Y-axes are labelled with the
TG programme temperature as calculated from the run-time.
Under both argon and nitrogen atmospheres, the main decom-
position step (beginning at about 270 ◦C) is accompanied by
the release of carbon dioxide (expected CO2 anti-symmetric
stretch at 2349 cm−1 and CO2 bend at 667 cm−1 [17]) and water
(marked increase in the water symmetric stretch at 3652 cm−1
[17]). In addition, a very slight disturbance in the base-plane
at about (2200 cm−1, 300 ◦C) is observable in the runs under
both argon and nitrogen. Greatly magnified spectrum slices in
this temperature region (not illustrated) exhibit the fundamen-
3. Results
3.1. TG-FT-IR results in inert atmospheres
The TG curves for copper(II) oxalate heated from 25 to
315 ◦C at 10 ◦C min−1 under argon and nitrogen atmospheres
are given in Fig. 2. The curves are practically identical within the
limits of sample-to-sample repeatability, showing a slow mass-
loss of approximately 0.8% between 25 and 264 ◦C, followed by
rapid decomposition between 264 and 310 ◦C. Residues heated
Fig. 3. TG-FT-IR evolved gas analysis results for copper(II) oxalate heated in
argon from 25 to 315 ◦C at 10 ◦C min−1. The interval from 315 to 400 ◦C is
plotted only because of a time-lag between the TG and IR responses. Linear IR
wavenumber scale.
Fig. 2. The TG curves for copper(II) oxalate heated from 25 to 315 ◦C at
10 ◦C min−1 under argon and nitrogen atmospheres.

94
E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
Table 1
The DSC results for copper(II) oxalate heated in oxygen from 25 to 350 ◦C at
10 ◦C min−1
Sample mass, ms (mg)
Residual mass, mr (mg)
100ms/mr
4.913
2.582
52.55
5.449
2.843
52.17
Q (J g−1
ꢀrH (kJ mol−1
)
−1470
−1475
)
−223.3
−224.0
diffraction pattern is very weak, indicating poor crystallinity of
the residue. The residue also readily adsorbed water from the
atmosphere. Table 1 summarizes the DSC results in oxygen.
Assuming the oxidation reaction:
CuC2O4(s) + 0.5O2(g) → CuO(s) + 2CO2(g)
the enthalpy of formation of copper(II) oxalate is given by:
ꢀfH◦CuC2O4(s) = −ꢀrH◦ + ꢀfH◦CuO(s)
+ 2ꢀfH◦CO2(g)
Taking the integrated exothermic DSC response as ꢀrH◦ and
substituting standard values for ꢀfH◦CuO(s) and ꢀfH◦CO2(g)
[9] gives ꢀfH◦CuC2O4(s) = −718.3 kJ mol−1. This is approxi-
mately 5% below Le Van’s published value of −751 kJ mol−1
[10].
Fig. 4. TG-FT-IR evolved gas analysis results for copper(II) oxalate heated in
nitrogen from 25 to 315 ◦C at 10 ◦C min−1. The interval from 315 to 400 ◦C is
plotted only because of a time-lag between the TG and IR responses. Linear IR
wavenumber scale.
tal carbon monoxide absorption (doublet-envelope centred at
2143 cm−1 [17]).
A considerable dead volume exists between the TG furnace
and the FT-IR gas-cell, so a substantial broadening of the FT-IR
absorbance signals along the time-axis is to be expected, and a
time-delay of approximately 2 min exists between the appear-
ance of a thermal event in the TG signal and the event being
observed at the spectrometer. The large platinum surfaces in the
TG furnace may also promote catalytic conversion of carbon
monoxide to carbon dioxide in the presence of residual air.
3.3. DSC results in inert atmospheres
The DSC curves for copper oxalate heated in argon, nitro-
gen and mixed argon–nitrogen atmospheres are shown in Fig. 6.
In pure argon there is a broad endotherm with onset at about
280 ◦C and a shoulder at about 293 ◦C reaching a maximum
at about 295 ◦C. As the proportion of argon in the purge
gas is decreased, the endothermic character of the decompo-
sition decreases until, when nitrogen is the main component,
the decomposition exhibits a broad exothermic character. The
derivative DSC curves in Fig. 7, illustrate the complexity of the
3.2. DSC results in oxygen
A sample of dried copper(II) oxalate heated in the DSC under
oxygen from 25 to 350 ◦C at 10 ◦C min−1 (see Fig. 5) gave a
strong exotherm in two incompletely resolved stages with onset
at about 250 ◦C and maximum at about 307 ◦C. The residue was
identified by X-ray powder diffraction as CuO (tenorite) with no
discernable trace of Cu2O (cuprite) or copper metal. The X-ray
Fig. 6. DSC curves of copper(II) oxalate, heated in crimped aluminium pans, in
Fig. 5. DSC and derivative DSC curves of copper(II) oxalate heated in oxygen
from 25 to 350 ◦C at 10 ◦C min−1. The derivative curve was smoothed by means
of a simple 11-point boxcar algorithm.
various mixed argon–nitrogen atmospheres from 25 to 350 ◦C at 10 ◦C min−1
.
Samples were of similar masses (4 mg). Baseline correction was performed by
simple linear subtraction.

E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
95
Table 3
DSC results for copper(II) oxalate heated in crimped aluminium pans under
mixed argon-nitrogen atmospheres
Argon (26 mL min−1
and nitrogen
)
Argon (5 mL min−1
and nitrogen
(33 mL min−1
)
)
(7 mL min−1
)
Average residual mass (%) 41.92 0.20
41.75 0.20
Average Q (J g−1
Average ꢀrH (kJ mol−1
)
110
3
−257 20
)
16.8
0.5
−39
3
solid residues to suggest that different reactions are taking place
under the four different inert atmospheres.
3.4. Decomposition in a self-generated atmosphere
Fig. 7. Derivative DSC curves of copper(II) oxalate (see Fig. 6), heated in
crimped aluminium pans, in various mixed argon–nitrogen atmospheres from 25
to 350 ◦C at 10 ◦C min−1. Samples were of similar masses (4 mg). The deriva-
tives were smoothed using a simple 11-point boxcar algorithm.
Some samples were heated in sealed cold-welded pans with
the lids perforated by a single pin-prick to allow the evolved
gases to escape slowly under pressure, thus surrounding the
decomposing samples with their own self-generated atmo-
sphere. Theresultsweresimilartothosewithcrimped(unsealed)
pans but not as self-consistent. Under argon purge, an endother-
mic decomposition was observed, whereas under nitrogen an
exotherm was obtained. The residue mass percentages still cor-
responded well to the calculated mass-percentage of copper,
41.84%. The average thermochemical data for the runs under
nitrogen are very close to those obtained using crimped pans,
but the average enthalpy of decomposition under argon is about
1.6 times greater.
endothermic decompositions under pure argon and mainly argon
atmospheres. At least four poorly resolved stages are apparent.
The exotherms are also complex, showing at least two steps.
The complexity of the endotherms under argon suggests some
contribution from an exothermic process that is dominant under
mainly nitrogen and pure nitrogen atmospheres.
XRPD studies showed that, regardless of the proportions of
nitrogen and argon, the DSC residues consisted of mainly copper
metal with small amounts of copper(I) oxide (cuprite) and no
discernable traces of copper(II) oxide (tenorite). The possibility
that some oxidation of the residues took place during handling
could not be excluded.
3.5. DSC results at various heating rates
Fig. 8 shows the DSC curves for copper(II) oxalate heated
under argon (25 mL min−1) from 25 to 360 ◦C at 10, 5 and
2 ◦C min−1. In addition to the expected shift towards lower tem-
peratures in the onset, peak and end temperatures resulting from
the different heating rates, the DSC curves appear more com-
plex as the heating rate is decreased. At 10 ◦C min−1 (curve a),
the left-hand lobe of the derivative DSC curve shows only one
step, whereas the right-hand lobe shows two overlapping steps.
In curve b (5 ◦C min−1), there is a shoulder on the left-hand
lobe, and clearer resolution of the two steps in the right-hand
lobe. Curve c (2 ◦C min−1) shows two incompletely resolved
steps and the right-hand lobe shows further separation of the
two steps observed in A and B.
Tables 2 and 3 summarize the mass and thermochemical data
obtained from these DSC runs. The average percentages of the
original sample masses (ms) that the masses of the residues (mr)
constitute do not vary significantly from one inert atmosphere
to the next. The calculated mass percentage of copper in the
dried copper(II) oxalate is 41.84%, which is very close to the
experimental values of 100mr/ms.
From the XRPD and mass data, there is no significant differ-
ence in the extent of oxidation to Cu2O of the samples from one
inert atmosphere to the next. There is thus no evidence from the
Table 2
DSC results for copper(II) oxalate heated (a) in crimped aluminium pans, and (b)
in sealed but pierced pans, under 100% argon and 100% nitrogen atmospheres
at flow rates of 32 1 mL min−1
The X-ray powder diffraction patterns of the DSC residues
showthattheresidueofthe10 ◦C min−1 runconsistsofamixture
of copper metal and copper(I) oxide (cuprite), with no detectable
copper(II) oxide (tenorite). At the slower heating rates, some
copper(II) oxide formation is observed.
100% Argon
100% Nitrogen
(a) Crimped pans
Average residual mass (%)
41.9 0.2
187 10
28.4 1.5
41.8 0.2
−398 10
−60.4 1.5
Fig. 9 shows DSC curves for copper(II) oxalate heated
under nitrogen (24 mL min−1) from 25 to 360 ◦C at 10, 5 and
2 ◦C min−1. The left-hand lobes of the derivative DSC curves
are all clearly single, but as under argon purge, the right-hand
lobes show two steps becoming more clearly separated at lower
heating rates. The X-ray powder diffraction patterns of the DSC
residues under nitrogen follow the same trend as those under
Average Q (J g−1
)
Average ꢀrH (kJ mol−1
)
(b) Sealed but pierced pans
Average residual mass (%)
41.6 0.3
307 50
41.6 0.1
Average Q (J g−1
)
−415 13
Average ꢀrH (kJ mol−1
)
47
8
−63
2

96
E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
Fig. 8. DSC curves for copper(II) oxalate heated under argon (25 mL min−1
from 25 to 360 ◦C at: (a) 10 ◦C min−1; (b) 5 ◦C min−1; (c) 2 ◦C min−1
)
Fig. 9. DSC curves for copper(II) oxalate heated under nitrogen (25 mL min−1
from 25 to 360 ◦C at: (a) 10 ◦C min−1; (b) 5 ◦C min−1; (c) 2 ◦C min−1
)
.
.
argon. The residue at 10 ◦C min−1 consists of a mixture of
copper metal and copper(I) oxide (cuprite) with no detectable
copper(II) oxide (tenorite), but the magnitude of the tenorite
peaks increases as the DSC heating rate decreases.
Table 4 is a summary of the DSC results at different heating
rates under argon and nitrogen. It is clear that slower heating
rates give rise to greater oxidation of the residues under both
argon and nitrogen.
at best and the proportions of each cannot be estimated with any
accuracy from the X-ray powder diffraction and mass data.
The greater relative masses of the residues (i.e. 100mr/ms)
at 2 ◦C min−1 under both argon and nitrogen cannot reasonably
be attributed to oxidation by residual air in the DSC furnace.
Preliminary purging was extensive and the longer runtimes asso-
ciated with slower heating rates would further decrease the
effects of any such contamination. Even under the unreason-
able assumption that all the oxygen impurity in the purge gas
(2–3 vpm) reacts with the sample during the total run time,
Although the data appear to suggest that more of both cop-
per(I) oxide (cuprite) and copper(II) oxide (tenorite) are formed
at 2 than at 5 ◦C min−1, these conclusions are semi-quantitative
Table 4
DSC results for copper(II) oxalate heated at various heating rates in crimped aluminium pans under argon and under nitrogen atmospheres (24 1 mL min−1
)
Heating rate (◦C min−1
)
100% Argon
10
100% Nitrogen
10
5
2
5
2
Residual mass (%)
42.38
263
40.0
42.68
254
38.5
44.08
224
34.0
42.20
−435
42.62
44.05
Q (J g−1
ꢀrH (kJ mol−1
)
−385
−331
)
−66.1
−58.5
−50.2

E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
97
Fig. 11. DSC curves for copper(II) oxalate heated under nitrogen from 25 to
360 ◦C at 10 ◦C min−1 at various flow rates: (a) 30 mL min−1; (b) 24 mL min−1
(c) 15 mL min−1
;
.
Fig. 10. DSC curves for copper(II) oxalate heated under argon from 25 to 360 ◦C
at 10 ◦C min−1 at various flow rates: (a) 33 mL min−1; (b) 25 mL min−1; (c)
3.6. DSC results at various purge gas flow-rates
17 mL min−1
.
Figs. 10 and 11 show DSC curves of copper(II) oxalate heated
10 ◦C min−1 under argon and nitrogen, respectively, at various
flow rates. Table 5 summarizes the DSC results.
Under both argon and nitrogen, decreasing the flow-rate of
the purge gas results in peak broadening and loss of resolution.
Very small increases in relative residue mass are observed under
both argon and nitrogen as the purge-rates are decreased, which
are also inconsistent with oxidation by oxygen contamination in
it can be calculated that this is not enough to account for
the observed mass increase. This mass increase is thus prob-
ably due to greater oxidation of the sample material during
decomposition by reaction with carbon dioxide. Mohamed et
al., report similar relative residue masses of around 44% under
nitrogen and carbon-dioxide atmospheres [6] at various heating
rates.
Table 5
DSC results for copper(II) oxalate heated at 10 ◦C min−1 at various flow rates in crimped aluminium pans under argon and under nitrogen atmospheres
Flow rate (mL min−1
)
100% Argon
33
100% Nitrogen
30
25
42.38
263
40.0
17
42.47
489
74.3
24
42.20
15
42.41
Residual mass (%)
41.84
187
28.4
41.87
−397
Q (J g−1
ꢀrH (kJ mol−1
)
−435
−749
)
−60.4
−66.1
−114

98
E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
the purge gases. The absolute values of ꢀrH◦ under both argon
and nitrogen, as estimated by integrating the DSC curves, appear
to depend very much on the purge gas flow rate. ꢀrH◦ approxi-
mately doubles as the flow-rates are approximately halved. This
behaviour does not agree with what is observed with the standard
calibrant (indium metal).
The X-ray diffraction patterns under argon and nitrogen
show that all of the DSC residues consist of mixtures of cop-
per metal and copper(I) oxide (cuprite), with no copper(II)
oxide (tenorite) observed. In agreement with the mass data, the
X-ray diffraction patterns suggest that, under both argon and
nitrogen atmospheres, the relative amount of copper(I) oxide
in the DSC residues increases as the purge gas flow-rate is
decreased.
be described by Eq. (9):
ꢀrH◦(β) (kJ mol−1) = 58.13β − 35.56
For the overall DSC response to be endothermic would require
(9)
β > 0.6117 and Eq. (8) becomes:
CuC2O4(s) → 0.3883Cu(s) + 0.3059Cu2O(s)
+ 0.3059CO(g) + 1.6942CO2(g)
andthecalculatedresidualmassshouldbe45.06%oftheoriginal
sample mass. This is significantly different from the experimen-
tal value of approximately 42% obtained for the various DSC
runs at 10 ◦C min−1
.
The reasoning above may be reinforced by removing any
dependence in the thermochemical calculations on the value
of ꢀfH◦CuC2O4(s) by defining βendo as the value of β at
the experimental endothermic response of 46.68 kJ mol−1 and
βexo, similarly, at the experimental exothermic response of
−63.00 kJ mol−1, then:
4. Discussion
The fact the thermal decomposition of copper(II) oxalate
is exothermic in atmospheres often regarded as inert, such as
nitrogen and carbon dioxide, and in hydrogen, a reducing atmo-
sphere, but appears to be endothermic in argon, suggests that
a different reaction is taking place under argon. However, all
other observations made in this study suggest that the reactions
under nitrogen and under argon are the same. X-ray diffrac-
tion studies of residues from DSC experiments under argon and
nitrogen show that these are very similar, and no significant dif-
ference could be observed by mass measurements of the DSC
residues or by thermogravimetry. TG-FT-IR evidence suggests
that practically the same gaseous products are released during
decomposition under argon and nitrogen.
In nitrogen, the possible formation of copper nitride has to
be considered. Cu3N exists [9] and decomposition tempera-
tures reported in the literature vary widely. The most reliable
reports, which show DSC curves in specified atmospheres,
record exothermic decomposition at temperatures greater than
327 ◦C under nitrogen [21], and at approximately 350 ◦C under
argon [22]. The X-ray diffraction pattern [23] of copper nitride
is distinguishable from those of copper metal and the copper
oxides. However, there was no evidence from XRPD patterns of
the DSC residues, obtained in this study, of the presence of cop-
per nitride, so any participation by copper nitride in the thermal
decomposition of copper(II) oxalate in nitrogen would have to be
temporary, and thermochemical contributions to the formation
and subsequent decomposition of copper nitride as an interme-
diate would cancel out. Exothermic decomposition is also not
exclusive to nitrogen, but occurs under CO2 and H2 atmospheres
as well.
βendo − βexo = 1.883
which is not possible, because 0 ≤ β ≤ 1. The mechanism is
therefore not consistent with the observed experimental results,
and cannot satisfactorily explain the endothermic DSC response
under argon.
The DSC results at 5 and 2 ◦C min−1 are more difficult to
interpret because of the additional formation of copper(II) oxide
(tenorite) under both argon and nitrogen atmospheres at these
heating rates. Everything else being unchanged, the effect of
copper(II) oxide formation, at the expense of carbon dioxide
formation, would be to render the DSC response more endother-
mic. The mass data and the X-ray powder diffraction patterns of
the residues at different heating rates give no indication that the
DSC residues under argon are different to those under nitrogen,
and yet very different DSC responses are observed.
The DSC runs at different purge flow rates show that the DSC
responses are strongly dependent on the purge rate. Under both
argon and nitrogen purge, small increases in mass percentages of
the residue are observed as the purge rate is decreased. XRPD of
the residues show that only copper metal and copper(I) oxide are
present, so oxidation of the solid residue at the expense of carbon
dioxide production cannot satisfactorily explain the observed
thermal behaviour.
A mismatch in thermal conductivity between the evolved
gaseous products and the DSC purge gas can lead to spurious
results. Hallbrucker and Mayer [24] used helium (50 mL min−1
)
as a purge gas in a study of the thermal decomposition of large
samples of copper(II) sulfate pentahydrate and found that spuri-
ous exotherms accompanied the evolution of water vapour. This
was because of the large difference between the thermal con-
ductivities of water vapour and of helium. They also reported
similar behaviour under helium purge with samples known to
evolve nitrogen gas. It would appear that the evolution of a gas
with a significantly lower thermal conductivity than the instru-
ment purge gas has the effect of blanketing the sample, thus
reducing the heat flow from the sample furnace to the atmo-
sphere compared to that of the reference furnace. The thermal
Oxidation by impurities in the purge gases of the DSC is
unlikely because of the similarity of the impurity specifications
for argon and nitrogen and because preliminary DSC studies
with a 15 ppb oxygen trap showed the same behaviour as without
the trap.
For all the DSC runs in which the sample was heated at
10 ◦C min−1, the residues were identified by X-ray powder
diffraction as mainly copper metal with a small amount of cop-
per(I) oxide (cuprite). Carbon monoxide gas was detected in the
gas evolved during TG-experiments, so the DSC response could

E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
99
conductivity of argon is a slightly closer match to that of car-
bon dioxide, the expected major evolved gas component in this
study, than is the thermal conductivity of nitrogen, but the dif-
ferences are small compared with the thermal conductivities of
helium and hydrogen.
As mentioned in Section 1, kinetic arguments have been
used to propose that the decompositions of silver(I), nickel(II)
and mercury(II) oxalates are accompanied by the evolution of
CO(g), CO2(g) and (1/2)O2(g) (or O), and that the oxalates
of lead(II) and manganese(II) decompose to give CO(g) and
(1/2)O2(g) [11]. They are said to decompose by a mecha-
nism of dissociative evaporation, i.e. by the congruent gasifi-
cation of all reaction products irrespective of their saturated
vapour pressure, with subsequent condensation of low-volatility
species. Considering only the fraction of the available carbon
dioxide, ω, which is released as CO(g) and either (1/2)O2(g)
or O(g):
Fig. 12. The dependence of ꢀrH◦(β, ω) on β and ω for the proposed mechanisms
involving the loss of (1/2)O2(g) (surface a) and O(g) (surface b), showing the
marked sensitivity to ω, the fraction of available CO2 that is given off as CO
and (1/2)O2(g) or O (g).
CuC2O4(s) → (1−β − γ)Cu(s) + (1/2)βCu2O(s)+ γCuO(s)
+ [(1/2)β + γ + ω(2 − (1/2)β − γ)]CO(g)
+ (1 − ω)(2 − (1/2)β − γ)CO2(g)
or
ꢀrH◦(β, ω) (kJ mol−1) = 58.13β + 1064.14ω
− 266.04βω − 35.56
+ [ω(2 − (1/2)β − γ)/2]O2(g)
or
The sensitivity of these equations to the parameter, ω, is much
greater than their sensitivity to β, so if a relatively small propor-
tion of the available carbon dioxide were released as CO(g) and
O2(g), or as CO(g) and O(g), it would have a great effect on the
observed heat flow, causing it to be more endothermic.
CuC2O4(s) → (1−β − γ)Cu(s) + (1/2)βCu2O(s) + γCuO(s)
+ [(1/2)β + γ + ω(2 − (1/2)β − γ)]CO(g)
+ (1 − ω)(2 − (1/2)β − γ)CO2(g)
Fig. 12 illustrates the dependence of ꢀrH◦(β, ω) on β and ω
for these mechanisms. Changes in ω have a much greater effect
than changes in β on whether the reaction will be endothermic or
exothermic. From the equations it can be seen that the amounts
(and hence the masses) of the solid residues are not functions
of ω. These mechanisms could thus explain the endothermic
DSC responses, but many of the exothermic DSC responses
in this study yield negative values considerably greater than
−35.56 kJ mol−1 (the minimum value of the functions above).
The exothermic DSC responses under nitrogen reported in the
literature in recent years are by no means uniform as shown in
+ ω(2 − (1/2)β − γ)O(g)
if the oxygen is assumed to be monatomic. To simplify the prob-
lem, only mechanisms which do not involve the formation of
copper(II) oxide will be considered (i.e. with γ = 0). Substitut-
ing the standard enthalpies of formation gives the numerical
expressions:
ꢀrH◦(β, ω) (kJ mol−1) = 58.13β + 565.83ω
− 141.46βω − 35.56
Table 6
Published DSC results for the thermal decomposition of copper(II) oxalate in different atmospheres and at various heating rates compared with the results obtained
in this study
Atmosphere
Sample mass (mg)
Heating rate (◦C min−1
)
ꢀrH (kJ mol−1
)
Reference
Argon (sealed ampoules)
28–32
4
4–10
10–15
4
4–10
10–15
1–5
2
10
5
5
10
5
5 and 10
20
−29
28
2
[5]
Argon (33 mL min−1
)
This
[27]
[6]
This
[27]
[6]
[3]
This
[6]
Nitrogen (30 mL min−1
Nitrogen (40 mL min−1
Nitrogen (30 mL min−1
)
)
)
−9
2
−11.9
−60
Air (30 mL min−1
Air (40 mL min−1
Oxygen
)
)
−134
−18.0
−138
−224
−43.5
−21.1
5
4
Oxygen (36 mL min−1
)
4
10
5
5 and 10
Carbon dioxide (40 mL min−1
Hydrogen (40 mL min−1
)
10–15
10–15
)
[6]

100
E. Lamprecht et al. / Thermochimica Acta 446 (2006) 91–100
Table 6. In addition, the ranges of the functions above are heavily
dependent on the accuracy of the value for ꢀfH◦CuC2O4(s).
Published DSC/DTA results for the decomposition of cop-
per(II) oxalate at different heating rates and under various atmo-
spheres are compared in Table 6 with the results obtained in
this study. The largest value of ꢀrH◦ under nitrogen (i.e. that
obtained in this study) is nearly six times the smallest. There
is also little agreement between the DSC results under oxy-
gen. The calculated enthalpy of the oxidation, using standard
enthalpies of formation for CuO(s) and CO2(g) [9] and a pub-
lished value [10] of −751.3 kJ mol−1 for ꢀfH◦CuC2O4(s) is
−190.6 kJ mol−1. An experimental value close to this calculated
value was obtained in this study.
The decompositions of copper(II) formate, acetate, malonate,
fumarate and maleate take place [4] by complex mechanisms
involving the formation of volatile and unstable copper(I) inter-
mediates that cause copper to be deposited on the container
walls. There is also evidence for the decomposition of copper(II)
oxalate under vacuum proceeding via a copper(I) intermediate
[25]. In this study, although the complexity of the DSC responses
is consistent with a multi-step process, no evidence of copper
deposition on the sample containers was observed.
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