Lab Report 2 - Grade: 12/15 PDF

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Separation of Liquids by Simple Distillation; Analysis by Gas Chromatography....

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Waad Osman UIN: 676880108 Partner: Shahin Avokh

Separation of Liquids by Simple and Fractional Distillation; Analysis by Gas Chromatography

Methods and Background: The main objective of this lab is to be able to set up simple and fractional apparatuses to perform a distillation of a 1:1 mixture of acetone and 1-propanol using both simple and fractional distillation. The objective of this lab is to set up both simple and fractional distillation apparatuses to separate a 1:1 mixture of acetone and 1-propanol. The goal was to collect 3 fractions from each distillation and create a graph of volume vs. temperature during both distillations. These fractions are then analyzed through gas chromatography to determine the % composition of acetone and 1propanol of each fraction in both distillations. By definition, distillation is the process of heating a liquid mixture to form vapor and then cooling that vapor to get a liquid. Simple distillation allows separation of distillates from less-volatile substances with acceptable purity if the difference between the boiling points of each pure substance is greater than 40–50 °C (Gilbert 131). Organic chemists frequently use this technique to separate the desired reaction product from the solvents used for the reaction or its work-up. The solvents are usually more volatile than the product and are readily removed from it by simple distillation. As the liquid being distilled is heated, the vapors that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapors, within a relatively small temperature range (by 2 or 3°C); by carefully watching the temperature in the distillation flask, it is possible to get a reasonably good separation. As distillation progresses, the concentration of the lowest boiling component will steadily decrease. This is because the vapor has begun condensing in the condenser, forming into the liquid referred to as distillate. By Raoult’s law, which describes the quantitative relationship between vapor pressure and composition of homogeneous liquid mixtures, the decrease of the concentration of the lowest boiling component will cause the temperature within the apparatus to begin to change; as a pure compound is no longer being distilled. The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate will be primarily the compound that boils at the second-lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated. Fractional distillation follows the same theory as simple distillation, the main difference being that fractional distillation contains small rings in the Hempel column called Raschig rings. These rings give a greater surface area in the column for distillation. The greater surface allows for

multiple cycles of vaporization of the liquid mixture and condensation of the said mixture on the rings. The multiple vaporization/condensation cycles allow for a better distillation of the mixture, allowing the result from the final condensation to have a higher purity rate than that of the simple distillation. After collecting fractions from both distillations, the fractions will be taken to be analyzed for their composition using the gas-liquid chromatography (GLC). Also called gas chromatography (GC), it is a technique that can be used to separate mixtures of volatile compounds with a boiling points difference of as 0.5 °C. It can also be applied as an analytical tool to identify the components of a mixture or in preparative applications when quantities of the pure components are desired (Gilbert 196). Gas-liquid chromatography work on the idea of separating the components of a mixture between a mobile gaseous phase and a stationary liquid phase. In practice, a sample is injected into a heated chamber where it is immediately vaporized and carried through a column by a flowing inert gas such as helium or nitrogen, which is called the carrier gas. This gaseous mixture is the mobile phase. The column inside is packed with a finely divided solid support that has been coated with a viscous, high-boiling liquid, which serves as the stationary phase. In this lab, the liquid, stationary phase is carbowax. As the mobile phase moves through the column, its components are continuously separated between the two phases. Those components that show a higher affinity for the mobile phase move through the column more quickly, whereas those with a stronger attraction to the stationary phase migrate more slowly, and separation occurs. The retention time of a component is the time required for the compound to pass from the point of injection to the detector, and it may be used for purposes of identification. Four experimental factors influence retention time of a compound: (1) the nature of the stationary phase, (2) the length of the column, (3) the temperature of the column, and (4) the flowrate of the inert carrier gas. (Gilbert 197.) The first condition, the nature of the stationary phase, has to do a lot with the nature of the mixture that is going to be separated in this lab. The mixture that is going to be separated is a 1:1 mixture of acetone and 1-propanol. While both compounds have similar chemical structure (C3H6O for acetone and C3H8O for 1-propanol) as seen in Figure 0.1, the key difference between both compounds is the hydroxyl group in 1-propanol. Due to the hydrogen bonding of the hydroxyl group which is stronger than any covalent bond that exists in acetone, 1-propanol is more polar than acetone has a higher boiling than acetone (1-propanol has a boiling point of 97°C, while acetone has a boiling point of 56°C).

Figure 0.1: Chemical structures of 1-propanol (left) and acetone (right).

As stated before, the first condition of retention time is related to the stationary phase in the GLC. The GLC in this experiment will use carbowax as the stationary phase; given that carbowax is a polar compound it is most likely that 1-propanol will have a longer retention time than acetone. That is because 1-propanol is polar and with the rule “like dissolves like”, 1propanol is most likely to stay in the stationary phase with the carbowax. That is how they will be identified on the GLC graph, if there two peaks are visible, most likely in all 2 fractions of simple distillation and fraction 2 of fractional distillation, the peak with the longer retention time would be the 1-propanol peak and the peak with the shorter retention time would be the acetone peak. The peaks that are generated on the GLC graph are not generated by the GLC instrument, they are generated by the Thermal Conductivity Detectors (TCDs) attached to it. Seen in Figure 0.2, the detector works as a sample of solute passes through the cell with the mobile phase (carrier gas). Due to the solute’s conductivity, the ability of the substance to conduct heat, the temperature of the filament decreases. This decrease in temperature causes a change in the resistance of the filament and said change can be measured electrically by a computer. That is what we see as the peaks in the GLC graphs. The rule of thumb goes that the more concentrated the solute is, the greater the change in resistance would be and the computer would generate a larger area of the peak. The area of the peak can be measured using the computer and it is what is going to be used to calculate the purity of the fractions from the distillations to see which distillation was more effective at separating the mixture (Landrie 192).

Figure 0.2: Diagram of a Thermal Conductivity Detector. The equation to calculate the purity or composition of the compound based off the area of the peak in the GLC graph, also known as the Ideal Mol % is by using the equation below: Ideal Mol % (of compound A) = Area of compound A x 100 [(Area of compound A) + (Area of compound B)] However, the thermal conductivity of each substance is slightly different from one another, so a mole correction factor (Mf) is applied when using the TCDs to get more accurate results (Landrie 192). The equation below takes the mole correction factor into account in order to calculate a better composition of the compounds: Correction Mol % Composition (of compound A) = Area of compound A x Mf (A) [ [(Area of compound A x Mf) + (Area of compound B x Mf)] The mole correction factor for 1-propanol is 1.20, and for acetone, it is 1.17.

x 100

Given that both simple and fractional distillations take a long time to set up and distill the mixture, each lab table was divided into two pairs, with one pair working on simple distillation and the other pair working on fractional distillation. Each pair was responsible for assembling their distillation apparatuses, performing the distillation, collecting the GLC graphs per fraction and sharing their data with the other pair. However, the time was cut short by the fire drill that occurred in the middle of the lab, that caused students to restart their distillation. That is why for the fractional distillation, the temperature vs volume graph and the GLC graph on this lab report belong to another pair of students who were able to complete the entire lab on time. As for the simple distillation, the temperature vs volume graph belongs to the pair that experimented, but the graph belongs to another pair of students that also performed simple distillation due to experiencing the same problems mentioned previously. Procedure: In my lab table, one pair worked on Simple Distillation and the other pair worked on Fractional Distillation, with me and my lab partner being the latter pair. Simple Distillation The simple distillation, the apparatus was set up according to Figure 1.1. About 30 ml of the acetone and 1—propanol mixture is poured into a still-pot (round bottom flask) and a boiling chip is added into the still-pot to prevent flash boiling. The pot is then held up by an iron clamp on a clamp stand. The flask is then put into the heating mantle that is still off. Then connect a still-head to the flask, followed by securing a thermometer into the still-head using a thermometer adapter. The thermometer bulb must be slightly below the side opening of the stillhead. A west condenser is then connected, tilting slightly downwards to the side opening of the still-head, held together by a Keck clip. The west condenser is also held up by another iron clamp on a clamp stand. Two rubber tubes are then connected into the side openings of the west condenser, with the opening farthest from being connected to a rubber tube that connects to the faucet to let the water in the condenser and the other opening being connected to the tube that opens up to the other side at the sink to drain the water out of the condenser. A graduated cylinder was placed by the open end of the west condenser to collect three fractions. After everything was set up, the electric flask heater was turned on. On a separate table, the temperature shown on the thermometer was recorded for every 1 ml of liquid distilled into the graduated cylinder. Three fractions of distillate are to be collected from this distillation. Fraction one is collected when the still-head temperature remains close to the boiling point of acetone. Fraction two is collected when the still-head temperature difference per milliliter begins to increase, whether gradually or rapidly. Fraction three is collected when the still-head temperature stabilizes or at near the boiling point of 1-propanol. Fractional Distillation: The setup of fractional distillation is very similar to the setup of simple distillation with the addition of the Hempel column in between the still-head and the still-pot. To prepare the Hempel column for this lab, pipette bulbs are used to cover the side openings of the column. A small, coiled copper wire is dropped into the column, followed by Raschig rings to fill the small inner column of the Hempel column. After attaching the Hempel column to the still-pot which, and

then attaching the still-head to the Hempel column, the rest of the setup follows that of simple distillation. An example of the setup can be seen in Figure 1.1 below.

Figure 1.1: Simple Distillation (left) and Fractional Distillation (right) setups. Gas-Liquid Chromatography (GLC): All of the fractions were collected, they are taken to the GLC room to run and analyzed by the gas chromatography instrument. A needle is used to take a small sample (2-3 microliter) of the fraction to rinse out the needle with said fraction. Then another sample of the fraction is taken by the needle, to be injected into the GLC instrument after another person clicks record on the computer screen. The computer screen then shows a peak or two, and using the software, the area of the peak(s) is collected and the data is printed onto sheets of paper. The same procedure is the repeated for the other two fractions until there are 3 sheets that contain peaks and their corresponding area calculated by the software. The data from the peaks are then used for further calculations on the % composition of the distillation fractions. Data and Calculations: Equations:  Mol % (of compound A) = Area of compound A x 100 [(Area of compound A) + (Area of compound B)]  Correction % Composition Mol % (of compound A) = Area of compound A x Mf (A) x 100 [ [(Area of compound A x Mf) + (Area of compound B x Mf)]

Simple Distillation: Volume (ml) 0 1 2

Temperature (°C) 57 58 58

Fraction # Fraction 1

3 59 4 59 5 59 6 60 7 60 8 60 9 60 10 60 11 60 12 65 Fraction 2 13 69 14 74 15 75 16 75 17 89 18 90 19 90 20 90 21 90 22 90 23 90 Fraction 3 24 90 25 90 26 90 Table 2.1: The volume and temperature of the fractions during distillation.

Fractions Fraction 1 Fraction 2 Fraction 3 Peaks Peak 1 Peak 2 Peak 1 Peak 2 Peak 1 Peak 2 2050 7701 3331 5081 7371 16.69 Area (s*mV) Identity 1-propanol Acetone 1-propanol Acetone 1-propanol Acetone 21.0 79.0 39.6 60.4 99.8 0.2 Ideal % Compositio n (%) 78.5 40.2 59.8 99.8 0.2 Corrected % 21.4 Compositio n (%) Table 2.2: Analysis of Fractions from Simple Distillation and Gas Chromatography  Calculation for Ideal % Composition:  Fraction one: Peak one: Mol % = {(2050 s*mV)/ [(2050 s*mV) + (7701 s*mV)]} x 100 = 21.0 % Peak two: Mol % = {(7701 s*mV)/ [(2050 s*mV) + (7701 s*mV)]} x 100 = 79.0%  Fraction two:

Peak one: Mol % = {(3331 s*mV)/ [(3331 s*mV) + (5018 s*mV)]} x 100 = 39.6 % Peak two: Mol % = {(5081 s*mV)/ [(3331 s*mV) + (5018 s*mV)]} x 100= 60.4%  Fraction three: Peak one: Mol % = {(7371 s*mV)/ [(7371 s*mV) + (16.69 s*mV)]} x 100 = 99.8% Peak two: Mol % = {(16.69 s*mV)/ [(7371 s*mV) + (16.69 s*mV)]} x 100 = 0.2%  Calculation for Corrected % Composition:  Fraction one: Peak one Mol % = {(2050 s*mV x 1.20)/ [(2050 s*mV x 1.20) + (7701 s*mV x 1.17)]} x 100 = 21.4 % Peak two Mol % = {(7701 s*mV x 1.17)/ [(2050 s*mV x 1.20) + (7701 s*mV x 1.17)]} x 100 = 78.5 %  Fraction two: get the temperatures so that you can figure out which is which Peak one Mol % = {(3331 s*mV x 1.20)/ [(3331 s*mV x 1.20) + (5081 s*mV x 1.17)]} x 100 = 40.2 % Peak two Mol % = {(5081 s*mV x 1.17)/ [(3331 s*mV x 1.20) + (5081 s*mV x 1.17)]} x 100 = 59.8 %  Fraction three: Peak one Mol % = {(7371 s*mV x 1.20)/ [(7371 s*mV x 1.20) + (16.69 s*mV x 1.17)]} x 100 = 99.8 % Peak two Mol % = {(16.69 s*mV x 1.17)/ [(7371 s*mV x 1.20) + (16.69 s*mV x 1.17)]} x 100 = 0.2 % Fractional Distillation: Volume (ml) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Temperature (°C) 52 52 52 52 52 52 52 53 53 53 53 53 54 57 78 89 90 90 90

Fraction #

Fraction 1

Fraction 2 Fraction 3

19 90 20 90 Table 2.3: The volume and temperature of the fractions during distillation.

Fractions Fraction 1 Fraction 2 Fraction 3 Peaks Peak 1 Peak 2 Peak 1 Peak 2 Peak 1 Peak 2 Area 0 606.3 150.4 46.3 198.1 0 (s*mV) Identity 1-propanol Acetone 1-propanol Acetone 1-propanol Acetone 0 100 76.5 23.5 100 0 Ideal % Compositio n (%) 100 76.9 23.1 100 0 Corrected % 0 Compositio n (%) Table 2.4: Analysis of Fractions from Fractional Distillation and Gas Chromatography  Calculation for Ideal % Composition:  Fraction one: Peak one: Mol % = {(0 s*mV)/ [(0 s*mV) + (606.3 s*mV)]} x 100 = 0 % Peak two: Mol % = {(606.3 s*mV)/ [(0 s*mV) + (606.3 s*mV)]} x 100 = 100%  Fraction two: Peak one: Mol % = {(150.4 s*mV)/ [(46.30 s*mV) + (150.4 s*mV)]} x 100 = 76.5 % Peak two: Mol % = {(46.30 s*mV)/ [(46.30 s*mV) + (150.4 s*mV)]} x 100= 23.5%  Fraction three: Peak one: Mol % = {(198.1 s*mV)/ [(198.1 s*mV) + (0 s*mV)]} x 100 = 100% Peak two: Mol % = {(0 s*mV)/ [(240.7 s*mV) + (0 s*mV)]} x 100 = 0%  Calculation for Corrected % Composition:  Fraction one: Peak one Mol % = {(0 s*mV x 1.20)/ [(0 s*mV x 1.20) + (606.3 s*mV x 1.17)]} x 100 = 0% Peak two Mol % = {(606.3 s*mV x 1.17)/ [(0 s*mV x 1.20) + (606.3 s*mV x 1.17)]} x 100 = 100 %  Fraction two: get the temperatures so that you can figure out which is which Peak one Mol % = {(150.4 s*mV x 1.20)/ [(150.4 s*mV x 1.20) + (46.30 s*mV x 1.17)]} x 100 = 76.9% Peak two Mol % = {(46.30 s*mV x 1.17)/ [(150.4 s*mV x 1.20) + (46.30 s*mV x 1.17)]} x 100 = 23.1% Fraction three: Peak one Mol % = {(198.1 s*mV x 1.20)/ [(198.1 s*mV x 1.20) + (0 s*mV x 1.17)]} x 100 = 100% Peak two Mol % = {(0 s*mV x 1.17)/ [(198.1 s*mV x 1.20) + (0 s*mV x 1.17)]} x 100 = 0%

Fractional Distillation 11 4 5 100

21.4

0

59.8

23.1

40.2

76.9

0.2

0

99.8

100 100

Temperature (°C)

100

Temperature (°C)

Volume (ml) of Fraction 1 Volume (ml) of Fraction 2 Volume (ml) of Fraction 3 Mol % Acetone in Fraction 1 (%) Mol % 1-propanol in Fraction 1 (%) Mol % Acetone in Fraction 2 (%) Mol % 1-propanol in Fraction 2 (%) Mol % Acetone in Fraction 3 (%) Mol % 1-propanol in Fraction 3 (%) Temperature vs. Volume Graph

Simple Distillation 11 11 4 78.5

80 60 40 20 0

80 60 40 20 0

0

5

10

15

20

Volume (ml)

25

30

0

5

10

15

20

25

Volume (ml)

Table 2.5: A comparison of data between Simple and Fractional Distillation. Conclusion: The goal of this lab was to be able to separate a 1:1 mixture of acetone and 1-propanol and then analyze the collected distillates using gas chromatography and thermal conductivity detectors to analyze the purity of the distillates by identify the peaks on the GLC graph and calculating the mol % composition of the distillates. Table 2.5 sums up the differences between the two methods of distillations and the main data that was collected from both distillations. As hypothesized, fractional distillation was able to perform a better separation of the mixture, as the mol % of acetone was 100% in Fraction 1 and 1propanol was 100% in Fraction 3, in comparison to simple distillation where the mol % of acetone in Fraction 1 was 78.5 and 1-propanol had a mol % of 99.8 in Fraction 3. Tables 2.4 and 2.2 illustrate some of the data collected from the GLC graphs. As 1-propanol was polar, it had a longer retention time then acetone due to 1-propanol binding to the polar stationary phase. The reason behind the closer relative retention times between the simple and fractional distillations is because, in simple distillation, the compounds do not separate as

quickly as in fractional distillation, as simple distillation cannot perform as many vaporization/condensation cycles as fractional distillation can. In the temperature vs volume graph for simple distillation, the early leveling off around 75°C is due to the fire drill in which the flask heater was turned off to safely evacuate the lab. By the time the still-pot was reheating, it was starting to level off at that temperature. A simila...