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The vapour pressure of p-nitrotoluene
M-1095 J. Chem. Thermodvnamics
1980, 12, 559-561
The vapour
pressure
of p-nitrotoluene
D. AMBROSE and H. A. GUNDRY Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex TWII OL W, U.K. (Received 23 July 1979) The vapour pressure of p-nitrotoluene was measured in the range 416 to 513 K and the results were fitted by equations. One of the equations was extrapolated to the triple point; combination of the slope at that point with the enthalpy of fusion allowed calculation of an equation to represent the vapour pressure of solid p-nitrotoluene and hence calculation of its vapour pressure at ambient temperature.
1. Introduction p-Nitrotoluene was purified in this laboratory for possible use as a reference material with a certified triple-point temperature, and we took the opportunity to use part of the purified sample for measurement of its vapour pressure. There are few exact vapour pressures for compounds containing a nitro group, and the values now reported will be useful for reference purposes in any future work on the correlation and estimation of vapour pressures of such compounds.
2. Experimental and results The compound was purified by zone melting, and the final batch was examined by low-temperature adiabatic calorimetry ; the mole fraction of impurity was assessed as 0.00011, the temperature of the triple point was found to be 324.79 K, and the enthalpy of fusion was measured as (16.81 kO.012) kJ mol- l.(r) The vapour-pressure measurements were made with the bubble-cap ebulliometer’2*3’ using automatic equipment for logging the bridge readings from which the temperatures Te8 (treated as thermodynamic temperatures T) were calculated. The measured vapour pressures were fitted by the three different equations: log,,(p/kPa)
= 6.27217-
(TIKWglo(plkPa) where X = {2T-(T&+
1862.295/{(T/K)-75.321},
(1)
= 42 + i
(2)
s=1
@Ax),
Tmin)}/(Tmax- Tmin), Tmin = 415 K,
E,(x) is the Chebyshev polynomial aZ = -2.296, a3 = 0.111, and
‘Lax = 500 K,
in x of degree s,(~) a, = 1275.815, a, = 307.757,
D. AMBROSE
560 ln(p/kPa)
AND
H. A. GUNDRY
= In(p,lkPa)+(T,/T)(b,z+b,r1.5+b,r3+h,zh).
(3)
In equation (3), which is of the form proposed by Wagner,“’ 7 = 1 - T/T,, the critical temperature T, = 743 K, the critical pressure pc = 3207 kPa, ln(p,/kPa) = 8.07318, b, = -7.90170, b2 = 1.05721, b, = -3.38494, and b4 = -3.27741. The observed temperatures and pressures and their residuals from equations (1) to (3) appear in table 1; the last three points were omitted when the equations were fitted since they prevented a good fit being obtained with any of the equations tried, and it seems probable that the sample had begun to decompose as the temperature approached that of normal boiling. TABLE
1. Observed
TIK
vapour
pressures
(1) 6.549 7.142 9.254 10.956 12.999 15.403 18.066 22.110 26.135
’ These points
were omitted
pcalc has been obtained
AplPa
@Pa
416.648 421.260 426.309 431.231 436.355 441.588 446.644 453.253 458.907
: Ap = pobs - p+ where (3)
(2)
1 0 1 -1 -I -1 -1 1 1 in fitting
from equations
PikPa
0 0 1 0 0 0 0 0 0
AplPa (1)
(3) 465.377 471.714 478.393 -485.718 492.098 498.872 506.208 511.835 512.419
31.460 37.505 44.864 54.256 63.659 75.033 89.121 a 101.268” 102.565”
(1) to
1 2 -2
(2)
(3)
-1 -1 -4
-1 -5
8 2 -10 -38 -95 -137
0
7 3 -5 -29 -85 -128
7 3 -4 -25 -73 -114
the equations.
3. Discussion Equation (3) was obtained by use of the constrained fitting procedure described by Ambrose, Counsell, and Hicks@’ with different critical temperatures until the resultant values of T, and pc conformed fairly well with incremental schemes used for correlation of these properties. (‘-*’ In these schemes the increments for the nitro group depend solely on the data for nitromethane, and in this study the criticalpressure increment was adjusted so as to be in accord with the value of the critical pressure of nitromethane suggested’@ as being more consonant with the vapour pressures at lower temperatures than the one currently to be found in reference books. Equation (1) should not be applied outside the range 416 to 513 K but equations (2) and (3) are applicable from the triple point to the normal boiling temperature ; in the experimental range the values of p and dp/dT calculated from them differ negligibly, but below 416 K small differences will be found. Equation (3) may also be used at higher temperatures but it is doubtful whether the compound would have more than a transitory existence in these conditions. The critical values proposed allow calculation of a value for the acentric factor w = 0.422 and application of correlations based on the principle of corresponding states. According to this work the normal boiling temperature of p-nitrotoluene is (511.825+0.003) K, where the uncertainty given allows for the difference between
THE
VAPOUR
PRESSURE
OF p-NITROTOLUENE
561
equation (2) and equation (3), and dp/dT = 2.28 kPa K-’ at that temperature. Hence the enthalpy of vaporization AvapH = 46.6 kJ mol-’ and the entropy of vaporization A”,,H/T = 91.0 J K-’ mol- ‘. For calculation of AvapH, the second virial coefficient was estimated”’ as B = - 1890 cm3 mol- ’ and the volume of the liquid was taken as V. = 150 cm3 mol-’ (an uncertainty of 100cm3 mol-’ in the estimated value of (B- V,) gives rise to an error of about 0.1 kJ mol -I in A,,,H). One of the constraints in the procedure for fitting equation (3) is designed to improve its reliability when extrapolated downwards, and for such extrapolation the equation is insensitive to the exact position of the critical point. Similar equations obtained with the critical temperature assigned values of 730 K and 750 K showed negligible differences from equation (3) in the range from room temperature to 550 K (p = 200 kPa). Rounding the critical pressure, however, caused unnecessary differences to appear so the apparently over-precise 3207 kPa yielded by the fitting program was retained. Extrapolation of equation (3) to the triple point gives p = 67.27 Pa and dp/dT = 4.52 Pa K-r at 324.79 K. Combination of these values with the enthalpy of fusion allows calculation of an equation for the vapour pressure of the solid: ln(p/Pa)
= 32.2514-9108/(T/K).
(4)
This calculation has recently been discussed in detail”“’ with respect to the vapour pressure of solid acetic acid-here the vapour is assumed to be essentially monomeric and the effect of the estimated non-ideality of the vapour is negligible. From equation (4) we get p = 5.5 Pa at 298.15 K. Use of equation (2) for the extrapolation gives a value about 5 per cent higher, and the vapour pressure of the supercooled liquid obtained by direct extrapolation of equations (2) or (3) is 9 to 10 Pa. A recent paper from this laboratory (rl) shows that small quantities of p-nitrotoluene in a differential scanning calorimeter may exist in a metastable form, and account may need to be taken of this fact in the use of equation (4). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Andon, R. J. L.; Connett, J. E.: Martin, J. F. To be published. Ambrose, D. ; Ellender, J. H.; Sprake, C. H. S.; Townsend, R. J. Chem. Thermo&uzmics 1976,8. Ambrose, D. J. Phys. E. 1968, I, 41. Ambrose, D.; Counsell, J. F.: Davenport, A. J. J. Chem. Thermodynamics 1970, 2, 283. Wagner, W. Ctyogenics 1973, 8,470. Ambrose, D.; Counsell, J. F.; Hicks, C. P. J. Chem. Thermodynamics 1978, 10. 771. Ambrose, D. Correlation and estimation ~f’vapour-liquid critical properties. 1. Critical temperatures organic compounds, NPL Report Chem. 92, 1978. Ambrose, D. Correlation and estimation qf vapour-liquid critical properties. II. Critical pressure.v critical volumes qf organic compounds, NPL Report Chem. 98, 1979. Pitzer, K. S.: Curl, R. F. J. Am. Chem. Sot. 1957, 79, 2369. Ambrose, D. J. Chem. Thermodynamics 1979, 11, 183. Richardson, M. J. : Savill, N. G. Thermochim. Arta 1979, 30, 327.
165.
of and
1980, 12, 559-561
The vapour
pressure
of p-nitrotoluene
D. AMBROSE and H. A. GUNDRY Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex TWII OL W, U.K. (Received 23 July 1979) The vapour pressure of p-nitrotoluene was measured in the range 416 to 513 K and the results were fitted by equations. One of the equations was extrapolated to the triple point; combination of the slope at that point with the enthalpy of fusion allowed calculation of an equation to represent the vapour pressure of solid p-nitrotoluene and hence calculation of its vapour pressure at ambient temperature.
1. Introduction p-Nitrotoluene was purified in this laboratory for possible use as a reference material with a certified triple-point temperature, and we took the opportunity to use part of the purified sample for measurement of its vapour pressure. There are few exact vapour pressures for compounds containing a nitro group, and the values now reported will be useful for reference purposes in any future work on the correlation and estimation of vapour pressures of such compounds.
2. Experimental and results The compound was purified by zone melting, and the final batch was examined by low-temperature adiabatic calorimetry ; the mole fraction of impurity was assessed as 0.00011, the temperature of the triple point was found to be 324.79 K, and the enthalpy of fusion was measured as (16.81 kO.012) kJ mol- l.(r) The vapour-pressure measurements were made with the bubble-cap ebulliometer’2*3’ using automatic equipment for logging the bridge readings from which the temperatures Te8 (treated as thermodynamic temperatures T) were calculated. The measured vapour pressures were fitted by the three different equations: log,,(p/kPa)
= 6.27217-
(TIKWglo(plkPa) where X = {2T-(T&+
1862.295/{(T/K)-75.321},
(1)
= 42 + i
(2)
s=1
@Ax),
Tmin)}/(Tmax- Tmin), Tmin = 415 K,
E,(x) is the Chebyshev polynomial aZ = -2.296, a3 = 0.111, and
‘Lax = 500 K,
in x of degree s,(~) a, = 1275.815, a, = 307.757,
D. AMBROSE
560 ln(p/kPa)
AND
H. A. GUNDRY
= In(p,lkPa)+(T,/T)(b,z+b,r1.5+b,r3+h,zh).
(3)
In equation (3), which is of the form proposed by Wagner,“’ 7 = 1 - T/T,, the critical temperature T, = 743 K, the critical pressure pc = 3207 kPa, ln(p,/kPa) = 8.07318, b, = -7.90170, b2 = 1.05721, b, = -3.38494, and b4 = -3.27741. The observed temperatures and pressures and their residuals from equations (1) to (3) appear in table 1; the last three points were omitted when the equations were fitted since they prevented a good fit being obtained with any of the equations tried, and it seems probable that the sample had begun to decompose as the temperature approached that of normal boiling. TABLE
1. Observed
TIK
vapour
pressures
(1) 6.549 7.142 9.254 10.956 12.999 15.403 18.066 22.110 26.135
’ These points
were omitted
pcalc has been obtained
AplPa
@Pa
416.648 421.260 426.309 431.231 436.355 441.588 446.644 453.253 458.907
: Ap = pobs - p+ where (3)
(2)
1 0 1 -1 -I -1 -1 1 1 in fitting
from equations
PikPa
0 0 1 0 0 0 0 0 0
AplPa (1)
(3) 465.377 471.714 478.393 -485.718 492.098 498.872 506.208 511.835 512.419
31.460 37.505 44.864 54.256 63.659 75.033 89.121 a 101.268” 102.565”
(1) to
1 2 -2
(2)
(3)
-1 -1 -4
-1 -5
8 2 -10 -38 -95 -137
0
7 3 -5 -29 -85 -128
7 3 -4 -25 -73 -114
the equations.
3. Discussion Equation (3) was obtained by use of the constrained fitting procedure described by Ambrose, Counsell, and Hicks@’ with different critical temperatures until the resultant values of T, and pc conformed fairly well with incremental schemes used for correlation of these properties. (‘-*’ In these schemes the increments for the nitro group depend solely on the data for nitromethane, and in this study the criticalpressure increment was adjusted so as to be in accord with the value of the critical pressure of nitromethane suggested’@ as being more consonant with the vapour pressures at lower temperatures than the one currently to be found in reference books. Equation (1) should not be applied outside the range 416 to 513 K but equations (2) and (3) are applicable from the triple point to the normal boiling temperature ; in the experimental range the values of p and dp/dT calculated from them differ negligibly, but below 416 K small differences will be found. Equation (3) may also be used at higher temperatures but it is doubtful whether the compound would have more than a transitory existence in these conditions. The critical values proposed allow calculation of a value for the acentric factor w = 0.422 and application of correlations based on the principle of corresponding states. According to this work the normal boiling temperature of p-nitrotoluene is (511.825+0.003) K, where the uncertainty given allows for the difference between
THE
VAPOUR
PRESSURE
OF p-NITROTOLUENE
561
equation (2) and equation (3), and dp/dT = 2.28 kPa K-’ at that temperature. Hence the enthalpy of vaporization AvapH = 46.6 kJ mol-’ and the entropy of vaporization A”,,H/T = 91.0 J K-’ mol- ‘. For calculation of AvapH, the second virial coefficient was estimated”’ as B = - 1890 cm3 mol- ’ and the volume of the liquid was taken as V. = 150 cm3 mol-’ (an uncertainty of 100cm3 mol-’ in the estimated value of (B- V,) gives rise to an error of about 0.1 kJ mol -I in A,,,H). One of the constraints in the procedure for fitting equation (3) is designed to improve its reliability when extrapolated downwards, and for such extrapolation the equation is insensitive to the exact position of the critical point. Similar equations obtained with the critical temperature assigned values of 730 K and 750 K showed negligible differences from equation (3) in the range from room temperature to 550 K (p = 200 kPa). Rounding the critical pressure, however, caused unnecessary differences to appear so the apparently over-precise 3207 kPa yielded by the fitting program was retained. Extrapolation of equation (3) to the triple point gives p = 67.27 Pa and dp/dT = 4.52 Pa K-r at 324.79 K. Combination of these values with the enthalpy of fusion allows calculation of an equation for the vapour pressure of the solid: ln(p/Pa)
= 32.2514-9108/(T/K).
(4)
This calculation has recently been discussed in detail”“’ with respect to the vapour pressure of solid acetic acid-here the vapour is assumed to be essentially monomeric and the effect of the estimated non-ideality of the vapour is negligible. From equation (4) we get p = 5.5 Pa at 298.15 K. Use of equation (2) for the extrapolation gives a value about 5 per cent higher, and the vapour pressure of the supercooled liquid obtained by direct extrapolation of equations (2) or (3) is 9 to 10 Pa. A recent paper from this laboratory (rl) shows that small quantities of p-nitrotoluene in a differential scanning calorimeter may exist in a metastable form, and account may need to be taken of this fact in the use of equation (4). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Andon, R. J. L.; Connett, J. E.: Martin, J. F. To be published. Ambrose, D. ; Ellender, J. H.; Sprake, C. H. S.; Townsend, R. J. Chem. Thermo&uzmics 1976,8. Ambrose, D. J. Phys. E. 1968, I, 41. Ambrose, D.; Counsell, J. F.: Davenport, A. J. J. Chem. Thermodynamics 1970, 2, 283. Wagner, W. Ctyogenics 1973, 8,470. Ambrose, D.; Counsell, J. F.; Hicks, C. P. J. Chem. Thermodynamics 1978, 10. 771. Ambrose, D. Correlation and estimation ~f’vapour-liquid critical properties. 1. Critical temperatures organic compounds, NPL Report Chem. 92, 1978. Ambrose, D. Correlation and estimation qf vapour-liquid critical properties. II. Critical pressure.v critical volumes qf organic compounds, NPL Report Chem. 98, 1979. Pitzer, K. S.: Curl, R. F. J. Am. Chem. Sot. 1957, 79, 2369. Ambrose, D. J. Chem. Thermodynamics 1979, 11, 183. Richardson, M. J. : Savill, N. G. Thermochim. Arta 1979, 30, 327.
165.
of and