Oxygen-bomb Combustion Calorimeter Research Articles

Low-temperature heat capacities and thermodynamic properties of ethyl 4-dimethylaminobenzoate (C11H15NO2)

Molar heat capacities of ethyl 4-dimethylaminobenzoate were measured by a high precision automated adiabatic calorimeter over the temperature range from 79 to 397 K. The melting temperature, the molar enthalpy and entropy of fusion were determined to be (336.818 ± 0.017) K, (20.761 ± 0.345) kJ mol−1 and (61.64 ± 1.03) J K−1 mol−1. It has a great potential in the use of the substance as solid–liquid phase change material based on large enthalpy of solid–liquid phase transition and moderate melting temperature. The thermodynamic functions (HT−H298.15 K) and (ST−S298.15 K) were calculated in the temperature range from 80 to 400 K with the interval of 5 K. The mass fraction purity of the sample was determined to be (1−N) = 0.99514 (mol fraction) by the fractional melting method. The constant-volume energy and standard molar enthalpy of combustion were determined, ΔcU (C11H15NO2, s) = −(6,161.73 ± 5.19) kJ mol−1 and Δc\( H_{\text{m}}^{0} \) (C11H15NO2, s) = −(6,167.31 ± 5.19) kJ mol−1, by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. At last, the standard molar enthalpy of formation was derived, Δf\( H_{\text{m}}^{0} \) (C11H15NO2, s) = −(305.03 ± 6.92) kJ mol−1.

Calorimetric determination of the standard molar enthalpies of formation of o-vanillin and trimethoprim

o-Vanillin and trimethoprim are two main raw materials for the synthesis of Valen Schiff bases which have been proved to possess established biological activities and may be used as the antibacterial agent candidates. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of a substance are some of the most fundamental thermodynamic properties, and closely related to other physical, biological, physiological, and chemical properties. However, up to now, no reports about those thermodynamic properties of the title compounds were found in literatures. In this paper, the constant-volume energies of combustion of o-vanillin and trimethoprim at T = 298.15 K and p = 3.01 MPa were measured by a precision oxygen-bomb combustion calorimeter to be ΔcU [o-vanillin(s), 298.15 K] = −(24,971.89 ± 22.02) J g−1 and ΔcU [trimethoprim(s), 298.15 K] = −(26,366.77 ± 21.96) J g−1, respectively. According to the definition of combustion enthalpy, the standard (pΘ = 0.1 MPa) molar enthalpies of combustion of o-vanillin and trimethoprim were determined to be \( \Delta_{\text{c}} H_{\text{m}}^{\varTheta } \) [o-vanillin(s), 298.15 K] = −(3,800.55 ± 3.35) kJ mol−1 and \( \Delta_{\text{c}} H_{\text{m}}^{\varTheta } \) [trimethoprim(s), 298.15 K] = −(7,657.05 ± 6.38) kJ mol−1, respectively. Finally, the standard (pΘ = 0.1 MPa) molar enthalpies of formation of o-vanillin and trimethoprim were calculated to be \( \Delta_{\text{f}} H_{\text{m}}^{\varTheta } \) [o-vanillin(s), 298.15 K] = −(490.85 ± 3.51) kJ mol−1 and \( \Delta_{\text{f}} H_{\text{m}}^{\varTheta } \) [trimethoprim(s), 298.15 K] = −(424.56 ± 6.64) kJ mol−1 from a combination of the experimental values of enthalpies of combustion and some other auxiliary thermodynamic data through a designed thermochemical cycle based on a supposed chemical reaction.

Molar heat capacities and standard molar enthalpy of formation of pyrimethanil butanedioic salt

A noval anilino-pyrimidine fungicide, pyrimethanil butanedioic salt (C28H32N6O4), was synthesized by a chemical reaction of pyrimethanil and butanedioic acid. The low-temperature heat capacities of the compound were measured with an adiabatic calorimeter from 80 to 380 K. The thermodynamic function data relative to 298.15 K were calculated based on the heat capacity fitted curve. The thermal stability of the compound was investigated by TG and DSC. The TG curve shows that pyrimethanil butanedioic salt starts to sublimate at 455.1 K and totally changes into vapor when the temperature reaches 542.5 K with the maximal speed of weight loss at 536.8 K. The melting point, the molar enthalpy (Δfus H m), and entropy (Δfus S m) of fusion were determined from its DSC curves. The constant-volume energy of combustion (Δc U m) of pyrimethanil butanedioic salt was measured by an isoperibol oxygen-bomb combustion calorimeter at T = (298.15 ± 0.001) K. From the Hess thermochemical cycle, the standard molar enthalpy of formation was derived and determined to be Δf H m o (pyrimethanil butanedioic salt)=−285.4 ± 5.5 kJ mol−1.

Heat Capacities and Standard Molar Enthalpy of Formation of Butyl 4-Hydroxybenzoate (C<sub>11</sub>H<sub>14</sub>O<sub>3</sub>)

Low-temperature heat capacities of butyl 4-hydroxybenzoate have been measured by a high precision automated adiabatic calorimeter over the temperature range from 79 to 399 K. The melting temperature, the molar enthalpy and entropy of the phase transition were determined to be (342.227 ± 0.054) K, (26.122 ± 0.192) kJmol-1and (76.33 ± 0.55) JK-1mol-1.The thermodynamic functions (HT-H298.15K) and (ST-S298.15K) were calculated in the range from 80 to 400 K with the interval of 5 K. The constant-volume energy and standard molar enthalpy of combustion have been determined,cU (C11H14O3,s) =-(5760.30 ± 2.70) kJmol-1andcHm0(C11H14O3, s) =-(5765.26 ± 2.70) kJmol-1, by means of a precision oxygen-bomb combustion calorimeter atT= 298.15 K. The standard molar enthalpy of formation has been derived,fHm0(C11H14O3, s) =-( 564.16 ± 4.41) kJmol-1, from the standard molar enthalpy of combustion in combination with other auxiliary thermodynamic quantities through a Hess thermochemical cycle.

Standard molar enthalpy of combustion and formation of quaternary ammonium tetrachlorozincate [n-CnH2n+1 N(CH3)3]2 ZnCl4

The standard molar enthalpy of combustion (ΔcHom) and formation (ΔfHom) of quaternary ammonium tetrachlorozincate [n-CnH2n+1N(CH3)3]2ZnCl4 have been determined for the hydrocarbon chain length from even number 8 to 18 of carbon atoms (n) by an oxygen-bomb combustion calorimeter. The results indicated that the values of ΔcHom increased and ΔfHom decreased with increasing chain length and showed a linear dependence on the number of carbon atoms, which were caused by that the order and rigidity of the hydrocarbon chain decreased with increasing the carbon atoms. The linear regression equations are -ΔcHom =1440.50n +3730.67 and -ΔfHom = −85.32n + 1688.22.

Heat capacities and standard molar enthalpy of formation of pyrimethanil maleic salt and pyrimethanil fumaric salt

Novel anilino-pyrimidine fungicides, pyrimethanil maleic salt, and pyrimethanil fumaric salt (C28H30N6O4) were synthesized by a chemical reaction of pyrimethanil with maleic acid/fumaric acid. The low-temperature heat capacities of the two compounds were measured with an adiabatic calorimeter from 80 to 350 K. The heat capacities of pyrimethanil fumaric salt are bigger than that of pyrimethanil maleic salt in the measurement temperature range. The thermodynamic function data relative to 298.15 K were calculated based on the heat capacity-fitted curves. The melting points, the molar enthalpies (ΔfusHm), and entropies (ΔfusSm) of fusion of pyrimethanil maleic salt and pyrimethanil fumaric salt were determined from their DSC curves. The values indicate that pyrimethanil fumaric salt was more thermostable than pyrimethanil maleic salt. The constant-volume energies of combustion (ΔcUmo) of pyrimethanil maleic salt and pyrimethanil fumaric salt were measured using an isoperibol oxygen bomb combustion calorimeter at T = (298.15 ± 0.001) K. From the Hess thermochemical cycle, the standard molar enthalpies of formation of the two compounds were derived and determined to be ΔfHmo (pyrimethanil maleic salt) = −459.3 ± 4.9 kJ mol−1 and ΔfHmo (pyrimethanil fumaric salt) = −557.2 ± 4.8 kJ mol−1, respectively. The results suggest that pyrimethanil fumaric salt is more chemically stable than pyrimethanil maleic salt.

Low‐Temperature Heat Capacities and Standard Molar Enthalpy of Formation of Gramine (C11H14N2)

AbstractLow‐temperature heat capacities of gramine (C11H14N2) were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 401 K. A polynomial equation of heat capacities as a function of temperature was fitted by least squares method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at 5 K intervals. The constant‐volume energy of combustion of the compound at T=298.15 K was measured by a precision oxygen‐bomb combustion calorimeter as ΔcU=−(35336.7±13.9) J·g−1. The standard molar enthalpy of combustion of the compound was determined to be ΔcHm0=−(6163.2±2.4) kJ·mol−1, according to the definition of combustion enthalpy. Finally, the standard molar enthalpy of formation of the compound was calculated to be Δ;cHm0=−(166.2±2.8) kJ·mol−1 in accordance with Hess law.

Low-Temperature Heat Capacities and Thermodynamic Properties of N-Benzyloxycarbonyl-l-3-phenylalanine (C17H17NO4)

The heat capacities of N-benzyloxycarbonyl-L-3-phenylahmine were measured by means of a fully automated adiabatic calorimeter over the temperature range from (79 to 395) K. A solid liquid phase transition corresponding to the melting process was observed. The temperature, the molar enthalpy, and entropy of the phase transition were determined to be 358.799 K. 31.787 kJ.mol(-1), and 88.594 J.K-1.mol(-1), respectively. The mole fraction purity or the sample used in the adiabatic calorimetric study was determined to be 0.9830 according to the Van't Hoff equation. Two equations of temperature dependence for heat capacities in the solid and liquid state are presented. The thermodynamic functions. (H-T - H-298.15K) and (S-T - S-298.15K). were calculated based on the heat capacity data in the temperature range from (80 to 400) K with an interval of 5 K. The standard molar enthalpies of combustion have been determined to be Delta H-c(m)0(C17H17NO4, s) = -(8308.52 +/- 3.22) kJ.mol(-1), with a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The standard molar enthalpy of formation of the compound has been derived from the enthalpy of combustion, Delta H-f(m)0(C17H17NO4, s) = -(815.05 +/- 0.67) kJ.mol(-1). The thermal stability of Me compound was further studied using TG, and DSC, and a possible mechanism for thermal decomposition was suggested.

Thermodynamic investigation of a solid–solid phase change material: 2-Amino-2-methyl-1,3-propanediol by calorimetric methods

An important solid–solid phase change heat storage material (PCM) 2-amino-2-methyl-1,3-propanediol was investigated by calorimetric methods. The molar heat capacities of the compound were measured in the temperature range from 78 to 405 K by means of a small sample automated adiabatic calorimeter. A solid–solid and solid–liquid phase transitions were found at T = 351.598 K and 380.633 K, respectively, from the experimental C p – T curve. The molar enthalpies and entropies of these transitions were determined to be 24.33 ± 0.02 kJ mol −1, 69.20 ± 0.07 J K −1 mol −1 for the solid–solid phase transition and 4.01 ± 0.01 kJ mol −1, 10.52 ± 0.01 J K −1 mol −1 for the solid–liquid phase transition, respectively. The thermodynamic functions ( H T − H 298.15) and ( S T − S 298.15), were derived from the heat capacity data in the temperature range of 78–405 K with an interval of 5 K. A precise oxygen-bomb combustion calorimeter was used to obtain the standard molar enthalpy of combustion and the standard molar enthalpy of formation, Δ c H m 0 (C 5H 11NO 2, cr) = (−3326.25 ± 0.52) kJ mol −1 and Δ f H m 0 (C 5H 11NO 2, cr) = (−213.37 ± 0.86) kJ mol −1. DSC and TG tests were performed to investigate the solid–solid and solid–liquid phase transition behaviors and its thermostability. The results were in agreement with those obtained from heat capacity measurements, which indicated that 2-amino-2-methyl-1,3-propanediol possesses typical solid–solid phase change material characteristics, such as, suitable transition temperature, high transition enthalpy and good thermal stability.

Thermodynamic investigation of several natural polyols (IV): Heat capacities and thermodynamic properties of adonitol

Adonitol was investigated by adiabatic calorimetry (AC), differential scanning calorimetry (DSC), thermogravity analysis (TGA) and oxygen-bomb combustion calorimetry. The heat capacity was precisely measured in the temperature range from 78 to 400 K by means of a small sample automated adiabatic calorimeter. The melting temperature, the molar enthalpy and entropy of this transition were determined to be T = 369.08 ± 0.45 K, 36.42 ± 0.18 kJ mol −1 and 98.68 ± 0.49 J K −1 mol −1 respectively from the experimental C p– T curve. The thermodynamic functions [ H T 0 − H 298.15 0 ] and [ S T 0 − S 298.15 0 ], were derived from the heat capacity data with an interval of 5 K. The standard molar enthalpy of combustion and formation of the compound have been determined to be Δ c H m 0 ( C 5 H 12 O 5 , cr ) = ( − 2562.4 ± 1.3 ) kJ mo l − 1 and Δ f H m 0 ( C 5 H 12 O 5 , cr ) = ( − 1120.1 ± 1.5 ) kJ mo l − 1 respectively by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The thermostability of the compound was further studied by DSC and TGA measurements.

Calorimetric Study and Thermal Analysis of 4‐(Aminomethyl) Benzoic Acid

Abstract Molar heat capacities of 4‐(aminomethyl)benzoic acid were precisely measured with a small sample precision automated adiabatic calorimeter over the temperature range from 80 to 400 K, and the heat capacity data were fitted to a polynomial equation: Cp,m=162.01+91.771X+6.4372X2+1.7517X3−4.991X4+8.909X5+1.585X6, X= (T−235)/155. The constant‐volume energy of combustion of the compound was measured by means of a precision oxygen bomb combustion calorimeter. The standard enthalpy values of combustion and formation were determined to be: ΔcHm⊖= − (4300±4) kJ·mol−1 and ΔfHm⊖= −(420±5) kJ·mol−1.

Thermodynamic investigation of several natural polyols

The low-temperature heat capacity Cp,m of erythritol (C4H10O4, CAS 149-32-6) was precisely measured in the temperature range from 80 to 410 K by means of a small sample automated adiabatic calorimeter. A solid-liquid phase transition was found at T=390.254 K from the experimental Cp-T curve. The molar enthalpy and entropy of this transition were determined to be 37.92±0.19 kJ mol−1 and 97.17±0.49 J K−1 mol−1, respectively. The thermodynamic functions [HT-H298.15] and [ST-S298.15], were derived from the heat capacity data in the temperature range of 80 to 410 K with an interval of 5 K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined: ΔcHm0(C4H10O4, cr)= −2102.90±1.56 kJ mol−1 and ΔfHm0(C4H10O4, cr)= − 900.29±0.84 kJ mol−1, by means of a precision oxygen-bomb combustion calorimeter at T=298.15 K. DSC and TG measurements were performed to study the thermostability of the compound. The results were in agreement with those obtained from heat capacity measurements.

Molar heat capacities and standard molar enthalpy of formation of 2-amino-5-methylpyridine

The heat capacities (Cp,m) of 2-amino-5-methylpyridine (AMP) were measured by a precision automated adiabatic calorimeter over the temperature range from 80 to 398 K. A solid-liquid phase transition was found in the range from 336 to 351 K with the peak heat capacity at 350.426 K. The melting temperature (Tm), the molar enthalpy (ΔfusHm0), and the molar entropy (ΔfusSm0) of fusion were determined to be 350.431±0.018 K, 18.108 kJ mol−1 and 51.676 J K−1 mol−1, respectively. The mole fraction purity of the sample used was determined to be 0.99734 through the Van’t Hoff equation. The thermodynamic functions (HT-H298.15 and ST-S298.15) were calculated. The molar energy of combustion and the standard molar enthalpy of combustion were determined, ΔUc(C6H8N2,cr)= −3500.15±1.51 kJ mol−1 and ΔcHm0 (C6H8N2,cr)= −3502.64±1.51 kJ mol−1, by means of a precision oxygen-bomb combustion calorimeter at T=298.15 K. The standard molar enthalpy of formation of the crystalline compound was derived, ΔrHm0 (C6H8N2,cr)= −1.74±0.57 kJ mol−1.

Low-temperature heat capacities and standard molar enthalpy of formation of 4-(2-aminoethyl)-phenol (C8H11NO)

This paper reports that low-temperature heat capacities of 4-(2-aminoethyl)-phenol (C8H11NO) are measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 400 K. A polynomial equation of heat capacities as a function of the temperature was fitted by the least square method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at the interval of 5 K. The energy equivalent, εcalor, of the oxygen-bomb combustion calorimeter has been determined from 0.68 g of NIST 39i benzoic acid to be εcalor = (14674.69±17.49)J.K-1. The constant-volume energy of combustion of the compound at T = 298.15 K was measured by a precision oxygen-bomb combustion calorimeter to be ΔcU = -(32374.25±12.93)J.g-1. The standard molar enthalpy of combustion for the compound was calculated to be ΔcHm⊖ = -(4445.47 ± 1.77) k. J · mol-1 according to the definition of enthalpy of combustion and other thermodynamic principles. Finally, the standard molar enthalpy of formation of the compound was derived to be ΔfHm⊖(C8H11NO, s) = -(274.68 ± 2.06) kJ · mol-1, in accordance with Hess law.

Low-temperature heat capacities and standard molar enthalpy of formation of N-methylnorephedrine C11H17NO(s)

This paper reports that low-temperature heat capacities of N-methylnorephedrine C11H17NO(s) have been measured by a precision automated adiabatic calorimeter over the temperature range from T = 78 K to T = 400K. A solid to liquid phase transition of the compound was found in the heat capacity curve in the temperature range of T = 342–364 K. The peak temperature, molar enthalpy and entropy of fusion of the substance were determined. The experimental values of the molar heat capacities in the temperature regions of T = 78–342 K and T = 364–400 K were fitted to two polynomial equations of heat capacities with the reduced temperatures by least squares method. The smoothed molar heat capacities and thermodynamic functions of N-methylnorephedrine C11H17NO(s) relative to the standard reference temperature 298.15 K were calculated based on the fitted polynomials and tabulated with an interval of 5 K. The constant-volume energy of combustion of the compound at T = 298.15K was measured by means of an isoperibol precision oxygen-bomb combustion calorimeter. The standard molar enthalpy of combustion of the sample was calculated. The standard molar enthalpy of formation of the compound was determined from the combustion enthalpy and other auxiliary thermodynamic data through a Hess thermochemical cycle.

Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of l-3-(3,4-Dihydroxyphenyl) Alanine (C9H11NO4)

Low-temperature heat capacities of l-3-(3,4-dihydroxyphenyl) alanine (C9H11NO4) were measured by a precision automated adiabatic calorimeter over the temperature range from (78 to 400) K. A polynomial equation of heat capacities as a function of temperature was fitted by the least-squares method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at 5 K intervals. The constant-volume energy of combustion of the compound at T = 298.15 K was measured by a precision oxygen-bomb combustion calorimeter to be ΔcU = −(21183.5 ± 35.0) J·g−1. The standard molar enthalpy of combustion of the compound was determined to be ΔcH°m = −(4177.8 ± 6.9) kJ·mol−1, according to the definition of combustion enthalpy. Finally, the standard molar enthalpy of formation of the compound was calculated to be ΔfH°m = −(935.9 ± 7.0) kJ·mol−1 in accordance with Hess law.

Thermodynamic investigation of several natural polyols (II)

The low-temperature heat capacity C p,m of erythritol (C 4 H 10 O 4 , CAS 149-32-6) was precisely measured in the temperature range from 80 to 410 K by means of a small sample automated adiabatic calorimeter. A solid-liquid phase transition was found at T=390.254 K from the experimental Cp-T curve. The molar enthalpy and entropy of this transition were determined to be 37.92±0.19 kJ mol -1 and 97.17±0.49 J K -1 mol -1 , respectively. The thermodynamic functions [H T -H 298.15 ] and [S T -S 298.15 ], were derived from the heat capacity data in the temperature range of 80 to 410 K with an interval of 5 K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined: Δ c H 0 m(C 4 H 10 O 4 , cr)=-2102.90±1.56 kJ mol -1 and Δ f H 0 m [(C 4 H 10 O 4 , cr)=- 900.29±0.84 kJ mol -1 , by means of a precision oxygen-bomb combustion calorimeter at T=298.15 K. DSC and TG measurements were performed to study the thermostability of the compound. The results were in agreement with those obtained from heat capacity measurements.

Thermodynamic investigation of room temperature ionic liquid

The molar heat capacities of the room temperature ionic liquid 1-butylpyridinium tetrafluoroborate (BPBF4) were measured by an adiabatic calorimeter in temperature range from 80 to 390 K. The dependence of the molar heat capacity on temperature is given as a function of the reduced temperature X by polynomial equations, C-p,(m) [J K-1 mol(-1)]= 181.43+ 51.297X -4.7816X(2)-1.9734X(3)+ 8.1048X(4)+11.108X(5) [X=(T-135)/55] for the solid phase (80-190 K), C-p,C-m [J K-1 mol(-1)]=349.96+25.106X+9.1320X(2)+19.368X(3)+2.23X(4)-8.8201X(5) [X=( T-225)/27] for the glass state (198-252 K), and C-p,C-m [J K-1 mol(-1)]= 402.40+21.982X-3.0304X(2)+3.6514X(3)+3.4585X(4) [X=(T-338)/52] for the liquid phase (286-390 K), respectively. According to the polynomial equations and thermodynamic relationship, the values of thermodynamic function of the BPBF4 relative to 298.15 K were calculated in temperature range from 80 to 390 K with an interval of 5 K. The glass transition of BPBF4 was observed at 194.09 K, the enthalpy and entropy of the glass transition were determined to be Delta H-g=2.157 kJ mol(-1) and Delta S-g= 11.12 J K-1 mol(-1), respectively. The result showed that the melting point of the BPBF4 is 279.79 K, the enthalpy and entropy of phase transition were calculated to be Delta H-m=8.453 kJ mol(-1) and Delta S-m=30.21 J K-1 mol(-1). Using oxygen-bomb combustion calorimeter, the molar enthalpy of combustion of BPBF4 was determined to be Delta H-c(m)0 =-5451 +/- 3 kJ mol(-1). The standard molar enthalpy of formation of BPBF4 was evaluated to be Delta H-f(m)0 =-1356.3 +/- 0.8 kJ mol(-1) at T=298.150 +/- 0.001 K.

Thermodynamic studies of crystalline 2-amino-5-nitropyridine (C 5H 5N 3O 2)

Low-temperature heat capacity C p,m of 2-amino-5-nitropyridine (C 5H 5N 3O 2; CAS 4214-76-0) was measured in the temperature range from 80 to 396 K with a high precision automated adiabatic calorimeter. No phase transition or thermal anomaly was observed in this range. The thermodynamic functions [ H T − H 298.15] and [ S T − S 298.15] were calculated in the range from 80 to 395 K based on the heat capacity data. The standard molar energy and standard molar enthalpy of combustion have been determined, Δ c U (C 5H 5N 3O 2, s) = −(2676.26 ± 0.24) kJ mol −1 and Δ c H m ° (C 5H 5N 3O 2, s) = −(2673.16 ± 0.24) kJ mol −1, by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 ± 0.001 K. The standard molar enthalpy of formation has been derived, Δ f H m ° (C 5H 5N 3O 2, s) = −(8.97 ± 0.99) kJ mol −1, from the standard molar enthalpy of combustion in combination with other auxiliary thermodynamic quantities through a Hess thermochemical cycle. The thermodynamic properties were further investigated through differential scanning calorimeter (DSC) and the thermogravimetric (TG) analysis.

Thermodynamic investigation of several natural polyols (I): Heat capacities and thermodynamic properties of xylitol

The low-temperature heat capacity C p , m 0 of xylitol was precisely measured in the temperature range from 80 to 390 K by means of a small sample automated adiabatic calorimeter. A solid–liquid phase transition was found from the experimental C p – T curve in the temperature range 360–375 K with the peak heat capacity at 369.04 K. The dependence of heat capacity on the temperature was fitted to the following polynomial equations with least square method. In the temperature range of 80–360 K, C p , m 0 ( J K − 1 mo l − 1 ) = 165.87 + 105.19 x + 1.8011 x 2 − 41.445 x 3 − 41.851 x 4 + 65.152 x 5 + 66.744 x 6 , x = [ T ( K ) − 220 ] / 140 . In the temperature range of 370–390 K, C p , m 0 ( J K − 1 mo l − 1 ) = 426.19 + 5.6366 x , x = [ T ( K ) − 380 ] / 10 . The molar enthalpy and entropy of this transition were determined to be 33.26 ± 0.17 kJ mol −1 and 90.12 ± 0.45 J K −1 mol −1, respectively. The standard thermodynamic functions ( H T 0 − H 298.15 0 ) and ( S T 0 − S 298.15 0 ) , were derived from the heat capacity data in the temperature range of 80 to 390 K with an interval of 5 K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, Δ c H m 0 (C 5H 12O 5, cr) = (−2463.2 ± 1.2) kJ mol −1and Δ f H m 0 (C 5H 12O 5, cr) = (−1219.3 ± 0.3) kJ mol −1, by means of a precision oxygen bomb combustion calorimeter at T = 298.15 K. DSC and TG measurements were performed to study the thermal stability of the compound. The results were in agreement with those obtained from heat capacity measurements.