Differential Scanning Calorimetry Lab Report

Abstract

Pure phenacetin and a mixture containing both phenacetin and benzamide were analyzed using Differential Scanning Calorimetry (DSC) to determine their enthalpy of fusion and melting points. A total of eight runs were conducted in aluminum pans sealed with lids, and the spectra were collected and observed. Runs one through three, involving pure phenacetin, obtained fusion enthalpies of 69.9 _kJ/mol, 57.1 _kJ/mol, and 75.3 _kJ/mol, respectively, with an average value of 67.4 _kJ/mol and a standard deviation of 9.35. Runs four through eight yielded fusion enthalpies of 21.

7 _kJ/mol, 21.6 _kJ/mol, 21.2 _kJ/mol, 21.0 _kJ/mol, and 20.3 _kJ/mol, respectively, with an average value of 21.2 _kJ/mol and a standard deviation of 0.56.

Introduction

Differential Scanning Calorimetry, also known as DSC, can determine the melting point and enthalpy of fusion of compounds. During the experiment, DSC graphs exhibit a sharp melting peak at a specific temperature, characterizing the material. Materials with minor impurities display a depressed and broadened melting point. Calorimetric purity is assessed through the measurement of melting point depression.

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DSC relies on the following equation and relationship:

_TS = _T0 - ^{X2 R T0°2}/_ΔHf x ^1/F

The linear equation generated from _TS versus ^1/F should yield a straight line. The slope should be equal to the melting point depression ^{X2 R T0°2}/_ΔHf, and the y-intercept should equal _T0°.

Experimental

The first three runs were conducted using pure phenacetin, while the last five runs utilized a provided mixture of phenacetin and benzamide. In the first three runs, pure phenacetin was weighed on an analytical balance, with a mass of 0.

005 g each. These three samples were placed in aluminum pans and sealed. The same procedure was followed for the last five runs with pure phenacetin. An empty aluminum pan was used as a reference. The required parameters were configured using the Thermal Advantage software for the TA DSC 2010 apparatus, and the experiments were executed. After each run, spectra were collected and subsequently analyzed.

Results

Discussion

The two methods used to determine the enthalpy of fusion were the direct fusion method and the freezing point depression method. During the experiment, both pure phenacetin and the mixture of phenacetin and benzamide were placed on an aluminum pan and analyzed using the DSC apparatus to determine phase transitions. Runs one through three were within the range of the literature melting point of 135°C, with an average value of 134.4°C.

The direct fusion method was the first approach, which allowed the use of equation 7 to calculate the fusion enthalpies found in Table 1. The enthalpies of fusion for pure phenacetin in runs one through three were 69.9 kJ/mol, 57.1 kJ/mol, and 75.3 kJ/mol, respectively. The average value of the three runs was 67.4 kJ/mol, with a standard deviation of 9.35, indicating that there was no significant deviation observed within the values.

The freezing point depression method was the second approach, which resulted in the following values for runs four to eight: 21.7 kJ/mol, 21.6 kJ/mol, 21.2 kJ/mol, 21.0 kJ/mol, and 20.3 kJ/mol, respectively. The five runs resulted in an average value of 21.2 kJ/mol, with a standard deviation of 0.56. The average value did not differ significantly from the literature value of 30 kJ/mol, and the small standard deviation indicates purity. Of the two standard deviations, the freezing point depression method yielded a very small value compared to the direct fusion method, signifying a larger error associated with the method rather than with the extrapolation from the slope. This could be due to the fact that the direct fusion method requires more variables and calculations, including both TB and ΔT, whereas the freezing point method requires only a TB value. The fewer variables needed to calculate can result in little to no calculation errors.

Differential scanning calorimetry (DSC) has both advantages and disadvantages. Some advantages include the use of small samples, the ability to operate at high temperatures, and instrument simplicity. Some disadvantages are relatively low accuracy and precision, inability to be used for overlapping reactions, and sensitivity.

In run five, slight impurities were observed due to the change in temperature being a negative value. This could be attributed to experimental errors such as improper weighing of the product on the analytical balance, failure to clean the analytical balance of any substances before use, not ensuring that the doors of the balance are properly closed while weighing, and the possibility of product escaping when sealing the aluminum pan. Therefore, correct techniques and the use of non-contaminated glassware are crucial for obtaining valuable data as well as accurate, pure, and precise results.

Conclusion

After analyzing the phase transitions of a mixture containing both benzamide and phenacetin reagents, as well as pure phenacetin, the following enthalpies were obtained from the direct fusion method: 69.9 kJ/mol, 57.1 kJ/mol, and 75.3 kJ/mol, with an average value of 67.4 kJ/mol and a standard deviation of 9.35. The enthalpies from the freezing point depression method (slope shown in Figure 1) were 21.7 kJ/mol, 21.6 kJ/mol, 21.2 kJ/mol, 21.0 kJ/mol, and 20.3 kJ/mol, with an average value of 21.2 kJ/mol and a standard deviation of 0.56. Further calculations of both standard deviations revealed that the direct fusion method obtained a higher value in deviation, indicating associated error in comparison to the low value of deviation obtained by the freezing point method. Despite the presence of deviation, the enthalpies were within the range of literature values. Runs one through three were within the range of the literature melting point of 135°C, with an average value of 134.4°C, and runs four through eight were within the range of the literature value of 30 kJ/mol. Moreover, both methods, direct fusion and freezing point depression, were evidently successful in determining the enthalpy of fusion for phenacetin.

References

1. Bradley, J.-C.; Lang, A.; Williams, A.. Jean-Claude Bradley Double Plus Good (highly Curated and Validated) Melting Point Dataset, 2014. DOI: https://doi.org/10.6084/m9.figshare.1031638.v1
2. Cammenga, H.; Eysel, W.; Gmelin, E.; Hemminger, W.; Höhne, G.; Sarge, S. The Temperature Calibration Of Scanning Calorimeters. Thermochimica Acta 1993, 219, 333-342.
3. Deangelis, N.; Papariello, G. Differential Scanning Calorimetry. Journal of Pharmaceutical Sciences 1968, 57 (11), 1868-1873.
4. Prenner, E.; Chiu, M. Differential Scanning Calorimetry: An Invaluable Tool For A Detailed Thermodynamic Characterization Of Macromolecules And Their Interactions. Journal of Pharmacy and Bioallied Sciences 2011, 3 (1), 39.
5. Saint-Guirons, H.; Xans, P. Differential Thermal Analysis And Differential Scanning Calorimetry Adapted Apparatus For Thermal Studies At High Pressure. Journal of Physics E: Scientific Instruments 1981, 14 (11), 1332-1336.
6. Wendlandt, W. Thermal Analysis Techniques: Part II. Differential Thermal Analysis And Differential Scanning Calorimetry (Continued). Journal of Chemical Education 1972, 49 (12), A671.

Appendix

Average in Table 1 for Temp at the bottom of the peak, ^Tw (°C):

^x' = ^{(136.59 + 137.53 + 136.63) / 3} = _136.92

Standard deviation in Table 1 for Temp at the bottom of the peak, ^Tw (°C):

^s = √^1/3 ∑_i=1³ (_xi - ^x')² = _0.532