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Simple Distillation and Gas Chromatography Chester Li (803-800-018), David Teng (003-922-448) Post-lab report Contributions

C.L. was responsible for power control and temperature reading. D.T. was responsible for collection of fractions. Jerry (TA) was responsible for injecting samples for GC data collection. Abstract (David)

Basic fractional distillation techniques and the concept of gas chromatography are explored in this experiment. Using Raoult’s Law as a theoretical basis, fractional distillation is performed with an mixture of unknown composition and fractions are obtained at 55-65°C, 65-75°C, 75-85°C, and 85-100°C temperature intervals if possible. The two fraction mixtures obtained from the 75-85°C and 85-100°C temperature ranges are run through a gas chromatography instrument and results are analyzed to determine the components of the unknown mixture based on retention time. The results show peaks at the retention times of 1.25+ minutes and 2.35+ minutes, which can be interpreted as two different species in the mixture. Based on these respective times, it is concluded that the identities of the unknown species in the composition mixture are ethyl acetate and n-propyl acetate. Experimental (Chester)

In this experiment, a 15mL liquid mixture of unknown composition or concentration of each constituent was taken. This liquid is placed inside a small round bottom flask and heated to distill it. Four 50-mL beakers are used for different temperature ranges to perform this fractional distillation. Fractions are collected at: 55-65°C, 65-75°C, 75-85°C, and 85-100°C. After the fractional distillation is complete, samples of each fraction are injected into a Vernier GC gas chromatograph and chromatograms are generated via computer link. Results (Chester)

Fractions #1 and #2 (at 55-65°C and 65-75°C respectively) did not collect any condensation and were not passed through the GC. The first drop of condensed vapour was collected at 75.0°C.

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Chromatograph of Fraction #3: 75-85°C

Figure 8.1: Chromatograph generated by a sample of Fraction 3. Qualitative analysis of the samples are conducted and calculated as follows: First, a mathematical approximation of the area under the curve is found by multiplying the width of the peak at half-height by its height.

The total area of all peaks is taken. Composition is done simply as percentage area.

Area = w1/2 ! h

%composition = 100% !AreaArea"

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tr (min) Peak (mV) w1/2 (cm) h (cm) Area (cm2) % Comp. Most Likely Species 1.25 950 0.175 4.990 0.873 70.00% ethyl acetate 2.35 275 0.250 1.500 0.375 30.00% n-propyl acetate

Ethyl Acetate

n-Propyl Acetate

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Chromatograph of Fraction #4: 85-100°C

Figure 8.2: Chromatograph generated by a sample of Fraction 4. The constituents table for Fraction 4 is:

Discussion (David)

For Fraction 3 (75-85°C), the percent composition at retention time=1.25 minutes is approximately 70% and the percent composition at retention time=2.35 minutes is approximately 30%. For Fraction 4 (85-100°C), the percent composition at retention time=1.40 minutes is approximately 23% and the percent composition at retention time=2.60 minutes is approximately 77%. Fractional distillation separates compound based on volatility of compound: the more volatile the compound, the more of it will equilibrate to gas phase at relatively lower temperatures. Condensing the gas mixture back into liquid prepares it for compositional analysis in a gas chromatography instrument. In context of the described concept and this experiment, it can be determined that the gas (retention time=1.25-1.40

tr (min) Peak (mV) w1/2 (cm) h (cm) Area (cm2) % Comp. Most Likely Species 1.40 450 0.150 2.350 0.353 23.12% ethyl acetate 2.60 600 0.375 3.125 1.172 76.88 n-propyl acetate

Ethyl Acetate n-Propyl Acetate

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minutes) with greater composition at the lower temperature range is more volatile than the gas (retention time=2.35-2.60 minutes) with a greater composition at the higher temperature range. Comparing the retention times to the standards provided, it can be quite conclusively determined that the identity of the chemical with the lower retention time is ethyl acetate while the chemical with the higher retention time is n-propyl acetate. The given standards for ethyl acetate and n-propyl acetate are retention times of 1.27 minutes and 2.43 minutes respectively, which fit in quite well with the data. The other two possible unknowns had retention times that lay outside of the retention time ranges obtained in this experiment, so they were discounted in this consideration. Conclusion (David)

Ethyl Acetate has a boiling point temperature of around 77°C and n-Propyl Acetate has a boiling point temperature of around 102°C. From observing the boiling points, it can be concluded that ethyl acetate is the more volatile compound and should therefore appear in higher percent composition at temperatures around 77°C, and n-propyl acetate should appear in higher percent composition at temperatures around 102°C. According to the percent composition data of the gas chromatograms, we can conclude that the experiment produced results that are expected according to the theory of simple distillation. One possible error was in very rapidly heating the mixture through the temperature ranges of the first two fractions up to around 70°C. Had there been a chemical with a boiling point lower than 70°C in the mixture, there would have been less-than optimal results in the fraction(s) obtained as the best separation is produced through slow heating of the mixture. However, this does not apply to our particular experiment as the mixture composition chemicals had boiling points that exceeded 70°C, after which the solution was heated much more slowly. Based on comparison of the boiling points and retention times, in can be extrapolated that the lower the boiling point of a volatile compound, the lower its retention time in a gas chromatography experiment. While the distillation provided very good results, still better results may have been produced. First is the heating of the solution, which although was slowed after exceeding 70°C, was still at a moderate pace. Had the solution been heated even more slowly, purer fractions may have been extracted. In addition, insulating the neck of the heated flask may be useful in preventing reflux and thus promote better results and separation. Increasing the fractional plates will increase the resolution of the distillation, and perhaps increasing the pressure (if possible) may allow better separation.