THERMOCHEMISTRY OF GLYCERYL TRINITRATE

E. A. Miroshnichenko, L. Vo P. Shelaputina, S. A. and Yu. A. Lebedev

I. Korchatova, Zyuz'kevich,

UDC 541.11:536.722:662.232.1:547.232

Experiments involving the determination of the enthalpy of formation of glyceryl tri- nitrate (GTN) can be divided into two groups. Prior to the introduction of thermochemical standards AHf ~ (GTN) values from -334.7 to -414.2 kJ/mole were obtained. In modern hand- books AHf ~ (GTN) values of -370.7 • 2.9 kJ/mole [i] and -372.4 kJ/mole [2], which are based on the data in [3] and earlier investigations, are recommended. In [3] calculations were made from the enthalpies of explosion of weighed ~4-7 g-samples of GTN in a calorimeter. The final composition of the products of explosion varies from experiment to experiment, and the calculation of the heat effect in each experiment was made on the basis of chemical analysis of the reaction products; this caused a scatter of • in the values [4].

The enthalpy of vaporization of GTN [5-7] was determined from the dependence of the vapor pressure on the temperature. In [6, 7] the GTN samples were not characterized with respect to their purity. In [5] the enthalpy of vaporization (108.8 • 11.3 kJ/mole) was measured using a high-purity sample (I00.0 mole % with respect to the fusion curve) by the gas-saturation method, which is not sufficiently reliable, just like the effusion method as applied to liquids (enthalpy of vaporization 100.2 kJ/mole [7]). There are approximate val- ues of the heat of vaporization of GTN at the boiling point (77.4 kJ/mole, ~250~

In the present research we made a thermochemical study of GTN using samples that were characterized quantitatively with respect to their purity [8]. The energy of combustion was measured with a precision semimicrocalorimeter [9], the enthalpy of vaporization was measured by means of the standard microcalorimetric method [i0], the enthalpy of solution was measured with a Calvet microcalorimeter, and the enthalpies of dilution were measured with a differential flow microcalorimeter.

The GTN used for the determination of the thermochemical properties was obtained by ni- tration of analytical-grade glycerol (GOST 6259-75) with H2SO4-HNO 3 (50:50), washed successively with water, 3% sodium carbonate solution, and three times with water, and recrystallized from 80% HNO 3 prepared from distilled HNO~. For the two samples of GTN that we obtained we deter- mined the freezing point and obtained the IR spectra with a Specord 75-IR spectrometer (KBr or CaF 2 prisms with a 0.02-mm spacer foil). The spectra contain absorption bands at 3685 and 3600 cm -I, which are due to stretching vibrations of OH groups. The first absorption band is due to residual moisture, while the second is due to admixed glyceryl dinitrate (GDN); the amount of admixed GDN in the GTN was determined. The precise amount of the principal substance in samples additionally dried under high vacuum was established from the fusion curve by the method in [8]. It can be seen from Table i that the amount of the principal substance in the samples was in good agreement with the amount of GDN in them. This made it possible to regard GDN as the principal impurity in the samples.

The energies of combustion of the GTN samples were determined with a modified model of a semimicrocalorimeter designed by the laboratory of thermochemistry of the Institute of Chemical Physics of the Academy of Sciences of the USSR [ii] and intended for the combustion of polynitrogen compounds [9]. The energy equivalent of the calorimeter with respect to the combustion of K-I brand benzoic acid was 864.70 • 0.12 J/~ from six experiments. In order to avoid explosion during ignition, the combustion of the weighed samples of GTN was carried out in a mixture with dimethyl phthalate, the energy of combustion of which under the bomb conditions was -24072.7 • 3.8 J/g, or at a reduced rate of temperature rise during the ex- periments.

Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. D. I. Mendeleev Moscow Chemical-Engineering Institute. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1988-1992, September, 1988. Original article submitted April 23, 1987.

1778 0568-5230/88/3709-1778512.50 �9 1989 Plenum Publishing Corporation

TABLE i. (GTN) Samples

Characteristics of the Glyceryl Trinitrate

Freezing point, Sample o C

GTN-83 12.7 GTN-85 t3.0

Amt. of GDN, " '1 Amt. of GTN~-- mass % (from I mole % (from spectral data) fusion curves)

1.2 { 98.$=0.2 0.2 I 99,72=0.05

TABLE 2. Energy of Combustion of Giyceryl Trinitrate (GTN)

Mass of the GTN sample, g

0,362589 ,0,321008 0,3338~0 @,391410 0,367681 ~,345600

N o t e .

Temp. rise w/correc. i for heat [ exchange,

2,89166 5,28387 2,67089 3,12525 2.91/~79 2,74733

Total heat Correc. fol evolved in the forma- the expt., tion of J HNO=, J

2525,38 2~, 52 46i5,~1 ~ ~32,51 23.09 2729,29 27, 15 2546,38 20,75 2411,32 2~,52

Average 6784.4 (o = 1.7).

forll Energy of Correc. combust, of I ignition the ignit. ]of the filament, J:[ .... sample ~ J

26,61 13,1& 13,85 13, t4 30,42 I3,14 32,97 13,o.2o 20,79 t3,t4 29,71 13,1~-

Speciflc energy of combust, of GTN, J/g

6787,62 6786,36 6787,33 6785,82 6776,78 6782,30

The energy of combustion of the GTN-83 sample in the calorimetric bomb was -6799.8 • 1.2 J/g (from five experiments); this leads to AHf ~ = -366.1 • 0.4 kJ/mole. The experimen- tal data for high-purity GTN-85 are presented in Table 2. The energy of combustion of GTN was related to the equation

C3H~N~09 ( s = 3CO2(g ) ~ 2 .5H20( s ) ~ t . 5 N 2 ~ ) + 0 . 2 5 0 2 ( ~ ( 1 )

The correction for the change in the volume of the gas during combustion was 11.7 kJ/mole, and the correction for reduction to 1 atm was 3.2 kJ/mole. With the introduction of these corrections the enthalpy of combustion of GTN was AHc~163 = -1525.9 i 0.8 kJ/mole. The de- gree of combustion of GTN in the experiments was monitored by means of analysis for the amount of CO 2, which averaged 100.02 • 0.03% relative to the theoretical value.

Proceeding from Eq. (I) and taking the heats of formation of CO= and H20 as being equal to -393.51 • 0.046 kJ/mole and -285.83 • 0.042 kJ/mole we can calculate the standard enthalpy of formation of GTN: AHf~163 = -369.4 • 0.8 kJ/mole. If one takes into account the amount of GDN impurity (0.28 mole %) and the possible error in its determination (0.05%), &Hf~163 = -371ol i 1.7 kJ/mole.

The enthalpy of vaporization was measured with a Calvet microcalorimeter by the method in [i0] at 40-120~ The temperature dependence of the enthalpy of vaporization is ex- pressed by the equation

A H ~ . = 9 8 . 3 6 - - 0 . 2 t 7 t ( 2 )

where 5H v is the enthalpy of vaporization of GTN in kilojoules per mole, and t is the temp- erature in degrees centigrade. From this dependence the enthalpy of vaporization of GTN at 25~ is 92.0 • 2.1 kJ/mole, and at the normal boilin~ point (250~ [6]) it is 44.1 kJ/mole. The error in the measurements was ~1%. The enthalpy of formation in the gas phase can be ob- tained from data on theenthalpy of formation and the enthalpy of vaporization: 5Hf~ = -279.1 • 2.7 kJ/mole.

Above we noted the overstatement of the enthalpy of vaporization of GTN obtained in [5] because of the errors in the gas-saturation method. At the same time it should be noted that the ratio of the enthalpies of vaporization in series of substances measured by the same method should correspond to the true changes in the enthalpy of vaporization in the inves- tigated series of compounds. The enthalpies of vaporization of 1,2,4-trinitroxybutane and 1,2,5-trinitroxypentane presented in [5] are significantly (by 45-65 kJ/mole) lower than the enthalpy of vaporization of GTN. An analysis of the data on the enthalpies of vaporization of liquid substances [12] shows that they are calculated satisfactorily starting from addi- tive group contributions. In our case this additivity is disrupted significantly -- the enthal- pies of vaporization of 1,2,4-trinitroxybutane and 1,2,5-trinitroxypentane should be 5-10 kJ/mole higher than for GTN. This makes it possible to assume that GTN, in contrast to the indicated trinitrates, has a significant specific intermolecular interaction.

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TABLE 3. Thermodynamic Properties of GTN

kJ/mole

1551.6 371.1 92,0 =1.7 =1.7 •

I 279.i I 49.8 -'217 I

Kdis,

liter/mole

1.7.10 -5

ASdis,

J/mole

- I17 ,6

I Gdis, k J/mole

-i i ,8

10

5

2 0

-Z

-5

-Ig

-1#

-I8

0,, olz i

0,5 g(• 0,5 V~ I ,u L ,*0--'

-o-- % --o---

Fig. i. Dependences of the enthalpies of solu- tion AH r and dilution #L of GTN in nitromethane on the molality of the solution ~m.

The study of the solvation of compounds is an effective method for the investigation of specific intermolecular interactions [13]. The expression for the enthalpy of solvation AH s has the form

AHs = AHv~ -- AHv (3)

where AHr~ is the enthalpy of solution at infinite dilution, and AH v is the enthalpy of vaporization of GTN. A comparison of the enthalpy of solvation obtained from experimental data with the value calculated from the molecular refraction gives the enthalpy of specific interaction of the molecules of the investigated substance if solvents that do not form molecular associates with the investigated compound are used. In the case of GTN nitro- methane (NM) is such a solvent.

The enthalpies of dilution of NM solutions of GTN at concentrations from 1.2 to 2-10 -5 mole/liter were determined in the research. The measurements were made with a DPMK-I differ- ential flow microcalorimeter [ 14]. The dependences of the enthalpies of solution and dilution of GTN in NM on the composition of the solution are shown in Fig. I. The curves have three clearly distinct regions (high, transition, and low concentrations). It may be assumed that GTN is associated in concentrated solutions; the associates begin to break down upon dilu- tion, and extrapolation to infinite dilution gives the enthalpy of solution of monomeric GTN in NM.

The expressions for the integral heat of dilution of GTN in NM have the form

f])L = --93.3 r -~ 212.517~ -- 68.6D1 ~2 (IR = i.2 -- 0.04 mole/liter)

(1)L = - - 7 t . 4 5 " t 0 a ~ m --~ 7 0 . 9 i . 1 0 5 m - - i 9 . 8 i - l O = m 'j: (m - - 0 . 0 2 - - 2 -

�9 10 b mole/liter)

The data on the dilution of GTN in NM over the range from 1.2 to 0.04 mole/liter extra- polated to infinite dilution gave an enthalpy of solution of -2.9 k J/mole. Extrapolation of the data on dilution to the region of low concentrations gives the value of the enthalpy of solution up to the infinitely dilute state, which is equal to 56.1 k J/mole.

According to Eq. (3), the enthalpies of solvation of associated and monomeric GTN in NM were -94.98 and =-35.98 kJ/mole, respectively. The calculated enthalpy of solvation of GTN taking into account only the dispersion interactions was evaluated from the dependence

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of the enthalpies of solvation (organic compounds) on the molecular refraction for solutions in nitromethane

AH~ = 0.624--0.253MR

where M R is the molecular refraction of GTN, which is equal to 40 cm3/mole. The energy of specific interaction for associated GTN was 49.8 kJ/mole; this may be due to the self-asso- ciation of GTN, the enthalpy of which is then -49.8 kJ/mole.

The enthalpy of specific interaction with NM for monomeric GTN was -9.2 kJ/moie; this indicates rather strong interaction, which, however, is considerably weaker than the speci- fic interaction of GTN molecules in self-associates, and the self-associates of GTN there- fore break down in NM only in the case of high dilution. The other thermodynamic parameters of the self-association process Kdis, &G, and AS were therefore determined for the region of high dilutions under the assumption of the existence of a dimeric form of the associates [15].

The thermochemical and thermodynamic characteristics of a high-purity sample of GTN are presented in Table 3. The thermodynamic parameters of the dissociation process obtained show that GTN is a strongly associated liquid and that the dissociation of GTN begins to manifest itself only in the region of high dilutions on the order of V~m < 0.2.

CONCLUSIONS

i. Thermochemical investigations of glyceryl trinitrate were carried out to obtain the enthalpy of formation in the standard state, the enthalpy of vaporization, and the enthalpy of formation in the gas phase.

2. The thermodynamic parameters of the intermolecular interactions in liquid glyceryl trinitrate and in solution were evaluated on the basis of solvation effects in nitromethane.

LITERATURE CITED

I. L. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Aca- demic Press, London-New York (1970).

2. D. Stull, A. Vestram, and H. Zincke, Chemical Thermodynamics of Organic Compounds, Wiley, New York (1969).

3. J. Taylor and G. Hall, J. Phys. Chem., 51, 593 (1947). 4. Yu. A. Lebedev, E. A. Miroshnichenko, and Yu. K. Knobel', The Thermochemistry of Nitro

Compounds [in Russian], Nauka, Moscow (1970). 5. M. Kemp, S. Goldhagen, and F. Zihiman, J. Phys. Chem., 61, 240 (1957). 6. A. F. Belyaev, Zh. Fiz. Khim., 22, 91 (1948). 7. J. Vasek and I. Stanek, Chem. Prum., ~, 286 (1959). 8. E. A. Miroshnichenko, L. I. Korchatova, and Yu. A. Lebedev, 4th All-Union Conference

on the Thermodynamics of Organic Compounds [in Russian], Kuibyshev (1985), p. 41. 9. Yu. I. Aleksandrov, T. R. Osipova, Yu. A. Lebedev, et al., 9th All-Union Conference on

Calorimetry and Chemical Thermodynamics [in Russian], Tbilisi (1982), p. 441. I0. Yu. A. Lebedev, E. A. Miroshnichenko, V. P. Vorob'eva, et al., "Microcalorimetric deter-

mination of enthalpies of vaporization and sublimation," Gosstandart, Moscow (All- Union Scientific-Research Institute of Technical Information, Classification, and Coding, Deposited Paper No. 120-83).

ii. Yu. A. Lebedev, E. A. Miroshnichenko, V. P. Lebedev, et al., Zh. Fiz. Khim., !9, 1928 (1975).

12. Yu. A. Lebedev and E. A. Miroshnichenko, The Thermochemistry of Vaporization of Organic Substances [in Russian], Nauka, Moscow (1981).

13. B. N. Solomonov, V. B. Novikov, V. V. Gorbachuk, and A. I. Konovalov, Dokl. Akad. Nauk SSSR, 265, 1441 (1982).

14. A. A. Vichutinskii and A. G. Golikov, 7th All-Union Conference on Calorimetry [in Rus- sian], Moscow (1977), p. 453.

15. F. Rossoti and H. Rossoti, Determination of Stability Constants and Other Equilibri~ Constants [Russian translation], Mir, Moscow (1965).

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