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Enthalpies of formation of solid cytosine, L-histidine, and uracil
M-9740 .l. Chem. Thermodynamics 1979,11,911-912
Enthalpies and uracil
of formation
of solid cytosine,
L-histidine,
S. R. WILSON, I. D. WATSON, and G. N. MALCOLM Department of Chemistry, Biochemistry, and Biophysics, Massey University, Palmerston North, New Zealand (Received 16 October 3978; in revised form 19 February 1979)
The apparatus used was a Gallenkamp Bomb Calorimeter (CB-110) with a Hewlett Packard quartz thermometer (HP 2801, resolution 10m4 K). The internal volume of the bomb was 296.1 cm3. The bomb was filled with oxygen to a pressure of 3 MPa. Water (1 cm3) was placed in the bomb and cotton thread (energy of combustion 17 489 J g-l)(l) was used as a fuse. The sample pellets were weighed to an accuracy of 0.02 mg and corrections for buoyancy were applied. The calorimeter was calibrated using benzoic acid (N.B.S. sample 39i, energy of combustion -(26434 _+ 3) J g-l under certification conditions). The apparent energy equivalent of the calorimeter determined as the mean of five calibration runs was (10730 5 2) J K-l, where the uncertainty is expressed as the standard deviation of the mean. The calorimeter was tested using succinic acid as the test substance.@) The average value of the energy of combustion of succinic acid from six determinations which compared satisfactorily with the was AU,“(c, 298.15K) = -(1493 -t l)kJmol-1 selected mean value given by Vanderzee, Mansson, and Sunner@) of - (1492.42 5 0.19) kJ mol-I. Fluka “puriss” L-histidine was recrystallized twice from distilled water and was dried without heating. Fluka “puriss” uracil and anhydrous cytosine were used without further purification. Pellets of the dried substances were formed, broken down, and reformed with successive drying in a desiccator as recommended in reference 2. The results of the combustion experiments for cytosine, L-histidine and uracil are given in table 1. The values of AU,0 refer to the idealized combustion reactions at 298.15 K represented by the equations : WWWs) + W/W,(g> = 4CQdg) + CWW3(~) + WW,(g), 0) GH,O,W) + W/W,(g) = 6CWg) + WW,O(1) + WWg), (2) C&O,Ws) + 40,(g) = 4CWg) + 2WX1) +Nzk). (3) The corrected temperature difference AT was obtained graphically from plots of temperature against time by the “mid-time” methodr3’ and all other calculations were carried out following the procedure of Hubbard, Scott, and Waddington(3) with additional data from reference 4. The values of the physical properties of the three substances, required in the calculations, were estimated as p = 1.2 g cmw3, cp = 1.2 J K-l g-l, and (&&$)r = -0.03 J MPa-1 g-l, all at T = 298.15 K. Values of the energies of combustion, the enthalpies of combustion, and the enthalpies of formation of the substances are listed in table 2. These were obtained 0021-9614/79/090911+02
$01.00/O
0 1979 Academic Press Inc. (London)
Ltd.
912
NOTES TABLE
1. Summary of combustion experiments for cytosine, L-histidine,
m(subs.) g
lO%(fuse) g
lO%&!fNO~) mol
AT z-
cytosine 0.85257 0.91998 1.41458 0.84604 0.83272
3.87 4.02 3.68 3.10 3.51
1.27 1.38 2.07 1.29 1.25
1.4925 1.6100 2.4716 1.4797 1.4572
16036 17298 26557 15898 15657
143
L-histidine 1.74607 1.18197 1.14488 1.60172
4.53 4.11 4.16 3.61
1.86 1.41 1.30 1.90
3.3502 2.2714 2.2023 3.0736
36000 24404 23663 33027
137 100 93 136
uracil 1.65754 1.57658 1.74850 1.60152
3.15 3.76 4.39 3.64
2.3877 2.2708 2.5186 2.3123
25656 24400 27063 24846
113 10.5 121 109
14.4 13.3 15.5 13.9
-A&,, -- J
-W, J
;: ii
and uracil
a -W, --T-
5
-Ai Jg-1
68 70 64 54 61 Mean :
18627 18623 18627 18623 18625 18625 f 1 b
79 72
20494 20501 20524 20495 20504 f 7 *
:: Mean:
55 15377 66 15368 77 15365 64 15406 Mean : 15379 f 9b
a WI denotes corrections to standard state and for nitric acid formation; Wa denotes energy of combustion of fuse. b Standard deviation of the mean.
from table 1 by use of the 1975 atomic weights”) and the values of the enthalpies of formation of carbon dioxide and water given in reference 6. The uncertainties in this table are twice the final overall standard deviations of the means. For AU: these were calculated from the uncertainties in table 1 and the uncertainty in the energy equivalent of the calorimeter. For AH: the uncertainties in the enthalpies of formation of carbon dioxide and water were included also. The very small value of the standard deviation of the mean of Au,” for cytosine in table 1 is not typical of the precision obtainable with our apparatus, so that an arbitrary uncertainty was assigned to AU,0 for cytosine in table 2. TABLE
2. Thermodynamic properties of solid substances at 298.15 K derived from combustion experiments. See text for comment on the uncertainties Substance cytosine Ghistidine uracil
-AUz/kJ
mol-l
2069.2 * 2 3181.2 ZII2.3 1723.8 f 2.2
-Uz/kJ
mol-l
2067.3 zt 2 3180.6 f. 2.3 1721.3 f 2.2
-A&/kJ
mol-1
221.3 f 2.3 466.7 + 2.8 424.4 rfr 2.5
REFERENCES 1. Washburn, E. W. J. Res. Nat. Bur. Stand. (U.S.) 1933, 10, 525. 2. Vanderzee, C. ; Minsson, M.; Sunner, S. J. Chem. Thermodynamics 1972, 4, 533. 3. Hubbard, W. N. ; Scott, P. W.; Waddington, G. Experimental Thermochemistry, Volume 1. Rossini, F. D.: editor. Interscience: New York. 1956, chapter 5. 4. Hu, A. T.; Sinke, G. C.; Minitz, M. J. J. Chem. Thermadjmamics 1972, 4, 283. 5. Atomic weights of the elements 1975. Pure and Applied Chem. 1976,47, 75. 6. ICSU-CODATA Task Group, Report on key values for thermodynamics. J. Chem. Thermodynamics 1976, 8, 603.
Enthalpies and uracil
of formation
of solid cytosine,
L-histidine,
S. R. WILSON, I. D. WATSON, and G. N. MALCOLM Department of Chemistry, Biochemistry, and Biophysics, Massey University, Palmerston North, New Zealand (Received 16 October 3978; in revised form 19 February 1979)
The apparatus used was a Gallenkamp Bomb Calorimeter (CB-110) with a Hewlett Packard quartz thermometer (HP 2801, resolution 10m4 K). The internal volume of the bomb was 296.1 cm3. The bomb was filled with oxygen to a pressure of 3 MPa. Water (1 cm3) was placed in the bomb and cotton thread (energy of combustion 17 489 J g-l)(l) was used as a fuse. The sample pellets were weighed to an accuracy of 0.02 mg and corrections for buoyancy were applied. The calorimeter was calibrated using benzoic acid (N.B.S. sample 39i, energy of combustion -(26434 _+ 3) J g-l under certification conditions). The apparent energy equivalent of the calorimeter determined as the mean of five calibration runs was (10730 5 2) J K-l, where the uncertainty is expressed as the standard deviation of the mean. The calorimeter was tested using succinic acid as the test substance.@) The average value of the energy of combustion of succinic acid from six determinations which compared satisfactorily with the was AU,“(c, 298.15K) = -(1493 -t l)kJmol-1 selected mean value given by Vanderzee, Mansson, and Sunner@) of - (1492.42 5 0.19) kJ mol-I. Fluka “puriss” L-histidine was recrystallized twice from distilled water and was dried without heating. Fluka “puriss” uracil and anhydrous cytosine were used without further purification. Pellets of the dried substances were formed, broken down, and reformed with successive drying in a desiccator as recommended in reference 2. The results of the combustion experiments for cytosine, L-histidine and uracil are given in table 1. The values of AU,0 refer to the idealized combustion reactions at 298.15 K represented by the equations : WWWs) + W/W,(g> = 4CQdg) + CWW3(~) + WW,(g), 0) GH,O,W) + W/W,(g) = 6CWg) + WW,O(1) + WWg), (2) C&O,Ws) + 40,(g) = 4CWg) + 2WX1) +Nzk). (3) The corrected temperature difference AT was obtained graphically from plots of temperature against time by the “mid-time” methodr3’ and all other calculations were carried out following the procedure of Hubbard, Scott, and Waddington(3) with additional data from reference 4. The values of the physical properties of the three substances, required in the calculations, were estimated as p = 1.2 g cmw3, cp = 1.2 J K-l g-l, and (&&$)r = -0.03 J MPa-1 g-l, all at T = 298.15 K. Values of the energies of combustion, the enthalpies of combustion, and the enthalpies of formation of the substances are listed in table 2. These were obtained 0021-9614/79/090911+02
$01.00/O
0 1979 Academic Press Inc. (London)
Ltd.
912
NOTES TABLE
1. Summary of combustion experiments for cytosine, L-histidine,
m(subs.) g
lO%(fuse) g
lO%&!fNO~) mol
AT z-
cytosine 0.85257 0.91998 1.41458 0.84604 0.83272
3.87 4.02 3.68 3.10 3.51
1.27 1.38 2.07 1.29 1.25
1.4925 1.6100 2.4716 1.4797 1.4572
16036 17298 26557 15898 15657
143
L-histidine 1.74607 1.18197 1.14488 1.60172
4.53 4.11 4.16 3.61
1.86 1.41 1.30 1.90
3.3502 2.2714 2.2023 3.0736
36000 24404 23663 33027
137 100 93 136
uracil 1.65754 1.57658 1.74850 1.60152
3.15 3.76 4.39 3.64
2.3877 2.2708 2.5186 2.3123
25656 24400 27063 24846
113 10.5 121 109
14.4 13.3 15.5 13.9
-A&,, -- J
-W, J
;: ii
and uracil
a -W, --T-
5
-Ai Jg-1
68 70 64 54 61 Mean :
18627 18623 18627 18623 18625 18625 f 1 b
79 72
20494 20501 20524 20495 20504 f 7 *
:: Mean:
55 15377 66 15368 77 15365 64 15406 Mean : 15379 f 9b
a WI denotes corrections to standard state and for nitric acid formation; Wa denotes energy of combustion of fuse. b Standard deviation of the mean.
from table 1 by use of the 1975 atomic weights”) and the values of the enthalpies of formation of carbon dioxide and water given in reference 6. The uncertainties in this table are twice the final overall standard deviations of the means. For AU: these were calculated from the uncertainties in table 1 and the uncertainty in the energy equivalent of the calorimeter. For AH: the uncertainties in the enthalpies of formation of carbon dioxide and water were included also. The very small value of the standard deviation of the mean of Au,” for cytosine in table 1 is not typical of the precision obtainable with our apparatus, so that an arbitrary uncertainty was assigned to AU,0 for cytosine in table 2. TABLE
2. Thermodynamic properties of solid substances at 298.15 K derived from combustion experiments. See text for comment on the uncertainties Substance cytosine Ghistidine uracil
-AUz/kJ
mol-l
2069.2 * 2 3181.2 ZII2.3 1723.8 f 2.2
-Uz/kJ
mol-l
2067.3 zt 2 3180.6 f. 2.3 1721.3 f 2.2
-A&/kJ
mol-1
221.3 f 2.3 466.7 + 2.8 424.4 rfr 2.5
REFERENCES 1. Washburn, E. W. J. Res. Nat. Bur. Stand. (U.S.) 1933, 10, 525. 2. Vanderzee, C. ; Minsson, M.; Sunner, S. J. Chem. Thermodynamics 1972, 4, 533. 3. Hubbard, W. N. ; Scott, P. W.; Waddington, G. Experimental Thermochemistry, Volume 1. Rossini, F. D.: editor. Interscience: New York. 1956, chapter 5. 4. Hu, A. T.; Sinke, G. C.; Minitz, M. J. J. Chem. Thermadjmamics 1972, 4, 283. 5. Atomic weights of the elements 1975. Pure and Applied Chem. 1976,47, 75. 6. ICSU-CODATA Task Group, Report on key values for thermodynamics. J. Chem. Thermodynamics 1976, 8, 603.