Experimental determination of the enthalpy of formation of dibenzo- p-dioxin

Article abstract of DOI:10.1006/jcht.2001.0951

The standard molar enthalpy of combustion of dibenzo-p-dioxin (DD) was measured at T = 298.15 K by static-bomb combustion calorimetry: Δ_cH_m^o(cr) = -(5723.8 ± 1.4) kJ·mol^-1. The standard molar enthalpy of sublimation of this compound was g measured by microcalorimetry: Δ_cr^g H_m^o = (91.45 ± 0.76) kJ·mol^-1. These values were used to derive the standard molar enthalpies of formation of DD in the crystalline and gaseous states: Δ_fH_m^o(cr) = -(141.6 ± 2.1) kJ·mol^-1 and Δ_fH_m^o(g) = -(50.1 ± 2.2) kJ·mol^-1. The specially obtained sample of DD of high purity {mole fraction x = (0.9999±0.0001)} was used in all measurements.

Full text of DOI:10.1006/jcht.2001.0951

J. Chem. Thermodynamics 2002, 34, 385–390
doi:10.1006/jcht.2001.0951
Available online at http://www.idealibrary.com on
Experimental determination of the enthalpy of
formation of dibenzo- p-dioxin
a
S. M. Pimenova, S. V. Melkhanova, V. P. Kolesov, P. I. Demyanov,
A. N. Fedotov,
Chemistry Department, Moscow State University, Moscow 119899, Russia
and V. P. Vorobieva
Institute of Chemical Physics RAN, Kosygina 4, Moscow 117977, Russia
The standard molar enthalpy of combustion of dibenzo-p-dioxin (DD) was measured
o
at T = 298.15 K by static-bomb combustion calorimetry: 1c H (cr) = −(5723.8 ±
m
−
1
1
.4) kJ · mol . The standard molar enthalpy of sublimation of this compound was
g
cr
o
−1
measured by microcalorimetry: 1 H = (91.45 ± 0.76) kJ · mol . These values were
m
used to derive the standard molar enthalpies of formation of DD in the crystalline and
o
−1
o
gaseous states: 1 H (cr) = −(141.6 ± 2.1) kJ · mol
and 1 H (g) = −(50.1 ±
f
m
f
m
−1
2
.2) kJ · mol . The specially obtained sample of DD of high purity {mole fraction
x = (0.9999±0.0001)} was used in all measurements. ꢀc 2002 Published by Elsevier Science Ltd.
KEYWORDS: thermochemistry; calorimetry; enthalpies of combustion; enthalpies of
formation; dibenzodioxins; environmental protection
1. Introduction
Several attempts have been made to construct a complete set of standard enthalpies of
formation of polychlorinated dibenzo-p-dioxins (PCDDs), which is needed for thermody-
(
1–5)
namic modelling of their formation processes.
All of them include the standard molar
o
enthalpy of formation of unsubstituted dibenzo-p-dioxin 1f H (DD, g) as a base for fur-
m
ther calculations. Therefore, it is necessary to have available the most reliable and precise
o
value of 1f H (DD, g) in deriving such a set.
m
o
Meanwhile, numerous estimates of 1f H (DD, g) result in values which vary from
m
−1
−1 (1–4, 6, 7)
−
48 kJ · mol to −94.1 kJ · mol
(see table 1). The only experimental value
o
−1
was obtained in our Thermochemistry
1
f H (DD, g) = −(59.2 ± 4.4) kJ · mol
m
(8)
Laboratory in 1997.
a
To whom correspondence should be addressed (E-mail: kolesov@thermo.chem.msu.ru).
ꢀc 2002 Published by Elsevier Science Ltd.
0
021–9614/02

3
86
S. M. Pimenova et al.
TABLE 1. Estimated values of the stan-
o
dard molar enthalpy of formation 1 H
of dibenzo-p-dioxin at T = 298.15 K
f
m
o
o
1
H (cr)
1 H (g)
f
m
f
m
References
−
1
−1
kJ · mol
kJ · mol
−
−
−
−
62.8
1
2
3
4
6
7
62.9
55
70
139 ± 25a
−48 ± 25
94.1
b
−
−
a
Calculated in reference 6 as an average of
g
b
o
five estimates;
value obtained in the present work.
calculated from the 1 H
cr
m
The aim of this work was to determine the enthalpy of combustion and the standard
molar enthalpy of formation of DD by using a specially obtained sample of high purity.
There are several reasons for the redetermination of these values: the recovery of CO2 in
the combustion products in reference 8 was slightly less than was expected {the average
amount of CO2 recovered as a fraction of theoretical was equal to (0.9991 ± 0.0005) in
a series of 8 runs}; and the sample of DD used in reference 8 was rather small, so that it
was necessary to add auxiliary substance in all combustion experiments to insure complete
combustion and a suitable temperature rise.
o
o
Since a value of 1f H (DD, g) is of extreme importance for the estimation of 1f H
m
m
values of other PCDDs, the slightest doubt about this value should be excluded. On that
account, special attention was given in this work to the preparation, purification, and
identification of the DD sample.
2. Experimental
Dibenzo-p-dioxin was prepared by reacting 2-chlorophenol with finely powdered potas-
sium hydroxide in a nitrogen atmosphere at T = (458 ± 5) K for 4 h in the presence of
freshly prepared copper powder. The course of synthesis, isolation, and purification of DD
was checked by capillary gas chromatography (g.c.) and chromatomass-spectrometry g.c.-
m.s. (HP 5989 × MS apparatus equipped with 5890 Series II Plus GC, HP-5MS capillary
column, 30 m long, 0.32 mm i.d., 0.24 µm film thickness). Details of all procedures which
allowed very pure DD to be obtained will be published elsewhere. After isolation and pu-
rification, the sample of DD was twice re-crystallized from hexane. Long white needles
were dried at a pressure p = 0.1 Pa. The sample of DD was shown to be chromatographi-
cally pure, and its mass spectra were very close to those in the Wiley-275 MS Library. The
purity of the sample, mole fraction x = (0.9999 ± 0.0001), was determined by the triple-
point depression method by means of a Mettler differential scanning calorimeter DSC-30.

Enthalpy of formation of dibenzo-p-dioxin
387
This result was obtained from 5 measurements of the triple-point temperature. The uncer-
tainty of the purity determination was calculated as ±t ·s, where s is the standard deviation
of the mean value, and t is a Student’s coefficient at the 0.05 significance level. During
d.s.c. measurements, the freezing temperature of DD was found to be (393.4 ± 0.1) K;
it was within the temperature interval given in the literature (between T = 393 K and
(
9)
−1
T = 395 K). The molar mass of DD, M = 184.194132 g · mol , was calculated from
(10)
−3
the IUPAC Table of Standard Atomic Weights.
taken from reference 11.
The density of DD, 1.36 g · cm , was
The energy of combustion was determined by means of a static bomb isoperi-
bolic macrocalorimeter. The temperature rise was measured with a copper resistance
(
12)
thermometer and a bridge circuit.
The uncertainty of the temperature measurements
−5
was about 5 · 10 K. The energy equivalent ε of the calorimeter was determined by
combustion of thermochemical standard benzoic acid (sample K-1, supplied by the
−1
Russian Research Institute of Metrology) and was equal to (54308.0 ± 7.0) J · 
−1
or (10318.5 ± 1.3) J · K . The massic combustion energy of benzoic acid was
−1
−
(26434.0 ± 2.2) J · g under certified conditions at T = 298.15 K. The correction
(13)
for small deviations from these conditions was calculated according to Coops et al.
The uncertainty of the mean ε value (as well as that of the standard massic energy of
g
cr
o
o
combustion 1cu and the standard enthalpy of sublimation 1 H ) was calculated as
m
±
t · s. In the final overall uncertainty of ε given above, the uncertainty in the energy of
combustion of benzoic acid was also taken into account.
Before combustion, DD (about 0.31 g to 0.35 g) was sealed in a weighed Terylene-film
bag which was then placed in a small thin-walled quartz crucible tripod that stood in a
small platinum cup. The gap between the bottom of the quartz crucible and the platinum
was about 8 mm. Preliminary experiments showed that this location of the sample ensured
complete combustion without the use of any auxiliary materials.
3
The bomb, with 1 cm of water added, was charged with purified oxygen to a pressure
of 3.04 MPa. The initial temperature did not differ from 298.15 K by more than 0.02 K.
Provision for ignition was made by means of a platinum wire heated by the discharge of a
capacitor. After each run, the combustion products were analysed for carbon dioxide by the
Rossini method.( No soot was observed after any combustion experiments. Qualitative
tests for CO with indicator tubes were negative within the limits of their sensitivity
14)
−
6
{
x(CO) < 1.10 }.
g
o
The enthalpy of sublimation 1 H was measured in a Calvet microcalorimeter
according to a standard procedure.
cr
15)
m
(
The samples were placed in glass ampoules and
evacuated to remove volatile impurities. The ampoules were sealed, weighed, and placed
in two calorimetric cells that could be evacuated continuously. The enthalpy change
corresponding to the complete sublimation of the sample was determined after breaking
one of these ampoules; then the same procedure was carried out in the second cell. The
energy of ampoule breaking was measured in separate experiments; it did not exceed
0
.2 per cent of the total energy measured in the sublimation experiments. The Calvet
(15)
microcalorimeter was calibrated according to the Joule effect.
The uncertainty of the
determination of the energy equivalent of the microcalorimeter was calculated as ±t · s,
and was equal to 0.3 per cent.

3
88
S. M. Pimenova et al.
TABLE 2. Combustion experiments of dibenzo-p-dioxin at T = 298.15 K. The symbols
in this table have the following meanings: m, mass of sample; m(f), mass of Terylene film;
0
ε , energy equivalent of the calorimeter corrected for the heat capacity of the final bomb
0
contents; 1Rc, increase in the thermometer resistance corrected for heat exchange; ε · 1Rc,
total energy change of the isothermal bomb process; 1U(f), correction for the combustion
energy of Terylene film; 1U(HNO ), correction for the energy of formation of aqueous nitric
3
o
−1
acid from nitrogen, oxygen, and water {based on 1r H (HNO , aq) = −59.8 kJ · mol for
m
3
−3
the concentration 0.1 mol · dm }; 1U(std), standard-state correction, calculated using the
(20)
o
approach proposed by Hubbard et al.;
1cu , standard massic energy of combustion of DD;
V = m(CO , expt)/m(CO , theor), being the ratio of the mass of CO recovered after the
2
2
2
experiment to the mass of CO expected on the basis of the mass of the compound
2
0
o
m
m(f)
ε · 1Rc
1U(f)
1U(HNO )
1U(std)
−1cu
1
3
N
V
−
g
g
J
J
J
J
J · g
1
2
3
4
5
6
7
8
0.322344
0.358354
0.319997
0.315714
0.310812
0.326614
0.326795
0.321318
0.019384
0.016557
0.020006
0.016183
0.014150
0.014157
0.015013
0.015104
10466.3
11521.0
10402.5
10185.7
9992.7
444.4
379.6
458.7
371.0
324.4
324.6
344.2
346.3
0.6
0.6
0.6
0.6
4.8
4.8
4.8
7.2
7.9
7.1
7.5
7.0
7.2
7.4
7.3
31066.5
31066.8
31050.6
31061.7
31068.6
31054.4
31059.2
31061.1
0.9996
0.9998
1.0000
0.9996
0.9997
0.9998
0.9999
1.0001
10479.4
10506.4
10338.9
4.8
o
a,b
−
h1cu i = (31061.1 ± 7.5)
b
hV i = (0.9998 ± 0.0002)
a
o
The uncertainty of the mean 1cu value was calculated as ±t · s, where s is the standard deviation of
the mean value, and t is a Student’s coefficient at the 0.5 significance level, and included the uncertainty of
the calibration; mean value.
b
3. Results
The results of the combustion experiments are summarized in table 2. The standard
o
−1
massic energy of combustion of Terylene film 1cu = −(22927.9 ± 6.3) J · g and
the resulting ratio of the mass of carbon dioxide to the predicted value (2.2897 ± 0.0006)
were measured previously.( The value (∂u/∂p)T for the crystalline sample was assumed
16)
o
to be negligible. The uncertainty of the mean 1cu value was calculated in the same way
as for ε.
The individual result of the measurements of the enthalpy of sublimation for DD are
given in table 3. The enthalpy of sublimation was measured in eight runs at T = 322 K
−1
and was equal to (90.79 ± 0.76) kJ · mol . This value was adjusted to T = 298.15 K
on the basis of Cp,m values of crystalline and gaseous DD from references 17 and 18,
−1
respectively, and was derived as (91.45 ± 0.76) kJ · mol at T = 298.15 K.
The results of our measurements and derived quantities are shown in table 4, where the
o
o
molar values 1cU and 1c H refer to the idealized combustion reaction:
m
m
C12H8O2(cr) + 13O2(g) = 12CO2(g) + 4H2O(l).
(1)

Enthalpy of formation of dibenzo-p-dioxin
389
TABLE 3. Individual results for the
standard molar enthalpies of sublima-
tion of dibenzo-p-dioxin at T = 322 K
g
cr
o
−1
)
1
H /(kJ · mol
m
9
9
0.51
92.33 89.90
91.91 89.69
90.57
90.63
0.79
g
o
−1a
h1 H i = (90.79 ± 0.76) kJ · mol
cr
m
a
Mean value and uncertainty.
TABLE 4. Results and derived quantities for dibenzo-p-dioxin at T =
98.15 K from the present study
2
g
c
1cU (cr)
m
o
o
o
o
−
−1c H (cr)
1 H (cr)
1
H
1 H (g)
f
m
m
f
m
cr
m
kJ · mol 1
−
kJ · mol
5723.8 ± 1.4
−1
kJ · mol
−141.6 ± 2.1
−1
kJ · mol
91.45 ± 0.76
−1
kJ · mol
−50.1 ± 2.2
−1
5
721.3 ± 1.4
o
o
The overall random uncertainties of the 1cU and 1c H values also include the uncer-
m
m
tainty of calibration. Other contributions, such as the uncertainty of the combustion energy
of Terylene film, were found to be negligible. The standard molar enthalpies of formation
(
19)
−1
of CO2(g) and H2O(l) recommended by CODATA,
−(393.51 ± 0.13) kJ · mol
−1
and −(285.830 ± 0.40) kJ · mol , respectively, were employed in the calculation of the
o
1
f H values.
m
4. Discussion
o
The comparison of the 1cu value of DD obtained in this work (table 2) with that
previously published( shows that the results of these two determinations differ noticeably:
8)
−
1
−1
−
(31061.1 ± 7.5) J · g in this work, and −(31022.8 ± 19.2) J · g in reference 8. The
reliability of the value obtained in this work (tables 2–4) is supported by the high purity
of the sample of DD {x = (0.9999 ± 0.0001)} and the practically complete recovery of
CO2 in the combustion products {the average amount of CO2 recovered as a fraction of
theoretical was equal to (0.9998 ± 0.0002) as shown in table 2}. The purity of the sample
used in reference 8 was noticeably lower {x = (0.9990±0.0002)}, and the recovery of CO2
was unsatisfactory {the average amount of CO2 recovered as a fraction of theoretical was
equal to (0.9991 ± 0.0005)}. It is important to point out also that the sample of DD used
in reference 8 was rather small. Therefore, a large quantity of auxiliary energy was needed
to produce a suitable temperature rise and to ensure complete combustion of the sample.
Indeed, the sum of the combustion energies of benzoic acid and Terylene film comprised
(8)
about (73 to 77) per cent of the total energy measured in the combustion experiments.
About the same fraction of the total amount of carbon dioxide was produced in the

3
90
S. M. Pimenova et al.
combustion of auxiliary materials. In this work, no auxiliary materials were used (except a
small amount of Terylene film), and that was favourable for the precision of the results.
The cause of the difference in the two experimental values of the massic energy
of combustion of DD is attributed to insufficient purity of the sample of DD used in
reference 8, such as impurities of noncombustible and combustible substances (moisture or
others). This assumption can explain also the low recovery of CO2 in combustion products
of the previous sample. All the factors mentioned above, in our opinion, provide convincing
proof of the reliability of the results obtained in the present investigation, and the value of
o
−1
,
the standard molar energy of combustion of DD, 1cU (cr) = −(5721.3±1.4) kJ · mol
m
can be recommended for further thermodynamic calculations.
The authors gratefully acknowledge the financial support received for this work from the
Promotion Society for Calorimetry and Thermal Analysis, Japan (Grant No. 1–2) and from
the Russian Foundation for Basic Research (Grant 00-15-97346).
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