CHEM 43A UC San Diego Distillation and Gas Chromatography theory

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Apply your understanding of the theory behind the techniques to identify the new key steps in the experiment, analyze, and briefly explain these key steps worked to help achieve the objective.

Note: Try to explain why a technique was used and how it worked rather than listing the procedures/steps that you followed. The procedures "add up" to a particular technique with a specific goal - what is that goal and why are the steps necessary?

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19W CHEM 43A Klosterman Experiment 5 Distillation and Gas Chromatography Tasks 1. Separate a mixture of two hydrocarbons by simple and fractional distillation 2. Evaluate the separation using Gas Chromatography Experiment 5 Learning Objectives 1. Explain the observed boiling point of a mixture in terms of partial pressure and mole fraction using a phase diagram. 2. Explain how distillation works to separate liquids 3. Explain how gas chromatography works, its advantages and limitations Reading Assignment - Distillation and Gas Chromatography Techniques in Organic Chemistry 4th edition: pages 173-192; 291-3081 Organic Chem Lab Survival Manual 10th ed. pages 140-149; 150-156; 164-168;227-235 Video Assignment - Distillation MIT Digital Lab Techniques Manual Weizman CHEM143A "Distillation" Videos Introduction Part A Distillation Distillation is a separation and purification technique that relies on differences in boiling points (and vapor pressure) to separate components of a liquid mixture. The mixture of miscible liquids is heated and begins to boil and the liquids transition to the gas phase. The vapors contain a greater percentage of the more volatile component (i.e. lower boiling point or higher vapor pressure). The vapor is then cooled and condensed into a separate flask. The resultant liquid now contains an increased percentage of the more volatile compound and a decreased percentage of the less volatile compound. This is a simple distillation (Fig 1.). To complete separate and purify (highly enrich) liquids via simple distillation, the distillation must be repeated numerous times, each time increasing the purity of the collected liquid. Eventually one can even separate (purify) a mixture of two or more liquids with quite close boiling points. However, this is very tedious and requires a large volume of the initial liquid mixture. To save time, you will analyze the results of a simple distillation carried out by a member of the lab staff. Figure 1 Simple distillation apparatus Fractional distillation is a technique for accomplishing a large number of simple distillations in a single continuous operation. A fractionating column (fractional distillation column) has an extensive surface area that induces the vapors to condense before the collection flask (Fig 2). The cooled condensate falls down the fractionating column where it encounters hot vapors rising up. The hot vapors transfer heat to the condensate causing the liquid components to return to the gas phase according to their boiling points and the vapor becomes enriched in the more volatile component. As the vapors heat the condensate, the vapors cool and condense according to their boiling point such that the percentage of the less volatile component in the condensate increases. Eventually, the vapor that reaches the top of the column is highly enriched in the more volatile component. Whereas the condensate that has returned to the distillation flask contains primarily less volatile component. 12nd edition: pages 127-137; 142-145; 190-205; 3rd edition: pages 141-161; 256-273 ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 2 In this experiment, you will work with a partner to conduct a fractional distillation of a mixture of two hydrocarbons, hexane and heptane. You will compare your separation results with the results from a simple distillation of the same mixture that was carried out by a member of the lab staff. Setting up the Distillation Apparatus The distillation setup is fairly simple. Set up the apparatus in the hood. First, set the round bottom flask containing the liquid/s to be distilled in a cool disconnected heating mantle. Clamp the flask gently but securely. Insert the distillation head into the flask for a simple distillation. For the fractional distillation you will first insert a fractionating column containing a copper mesh. Attach the condenser to the distillation head and hold it securely in position with a plastic keck clip. Place a bent adapter on the end of the condenser using a plastic keck clip to hold it in position. You may need to clamp the distillation head or condenser for greater stability. It is quite common to chill the condenser by circulating cold water to aid condensation and recovery. In this experiment, you will use a water-filled condenser that does not circulate. Figure 2 Fractional distillation apparatus Carefully position an alcohol thermometer in the thermometer adapter and attach the adapter to the top of the distillation head. Make sure that the rubber thermometer adapter fits tightly over the lip of the glass thermometer adapter. Position the thermometer so that the thermometer tip (bulb) is positioned just below the side arm of the distillation head. This will provide an accurate temperature of vapors just before they enter the chilled condenser and begin cooling. Use a clamped graduated cylinder as a receiving flask. Finally, insulate the fractionating column with cotton wool. This will ensure a steady distillation. Use aluminum foil to help keep the wool positioned about the fractionating column. Occasionally it is useful to wrap the top of the round bottom flask as well to control heat transfer. Running the Distillation Increase the temperature of the heating mantle (~40%) to bring the solution to a boil. Once the solution is boiling, decrease the heat (~25%) so that the solution maintains a gentle boil. You will continue to adjust heat to keep a constant rate of distillation; approximately one drop per second from the tip of the bent adapter into the receiving flask. As the distillation progresses you will need to slowly raise the temperature as the liquid in the round bottom contains more of the less volatile component. The thermometer will help you monitor the progress of the distillation; The temperature of the more volatile vapors will be lower than the less volatile component. If the distillate does not appear (condense) within 15-20 minutes, your fractionating column is probably too cool. Check the insulation and move away from any air flow. You may also have a leak at a joint in the apparatus. You will analyze your results using gas chromatography (GC). Part B Gas Chromatography Gas chromatography (GC) is an analytical technique that uses a gas as the mobile phase in a chromatographic separation. As the separation occurs in the gas phase, the compounds must first be vaporized. Typically, a solution containing the compounds to be separated and analyzed is injected into the GC instrument where the sample is rapidly heated under reduced pressure to vaporize everything. Next an inert gas, (typically helium or nitrogen) carries the vaporized compounds through the stationary phase. A detector at the exit reports the time it takes for each component to pass through the column (Fig. 3). The retention time of a compound is characteristic for a compound (under specific conditions), similar to the Rf values in TLC. GC analysis involves the comparison of known and unknown retention times. Unlike TLC, GC analysis is quantitative. Thus, you can determine the relative amounts of each component injected by calculating the relative area under each ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. Figure 3 Representative GC trace of a 1:1 mixture of a and b (V:V). W19 CHEM 43A Klosterman 3 "peak" in the GC trace. In part C, you will use GC to determine the contents of each fraction. Setting up the GC There are many parameters that need to be tuned for a GC analysis. In this experiment, the GC settings are pre-set. You will want to record the settings used as the retention times are only reproducible under the exact same settings. • Column packing material • Injector temperature • Column length • Column temperature • Inert gas (mobile phase) • Detector temperature • Inert gas flow rate • Detector current Make sure you use the same GC instrument for all data collection. Retention times will be different. Injecting the Sample into the GC The syringes used for injecting the sample to the GC are very delicate and must be handled with care. First rinse the syringe with several portions of clean solvent and the sample to be injected. Hold the syringe barrel with one hand and direct the needle with the other hand through the rubber septum and into the injector port of the GC. With one smooth motion, depress the plunger and withdraw the needle from the instrument. Immediately start the chart recorder that will also begin the timer (indicating retention times). Immediately clean the syringe after use by rinsing several times with clean solvent (acetone). Be gentle; Never force the syringe plunger or you will bend it. If it offers resistance or seems to be stuck, it will need to be cleaned. If a peak appears with a flat line at the top, it means that you have injected too much sample and the detector is saturated. If this happens, inject a less concentrated sample. You need to see the entire peak on the chromatogram, including its sharp apex, for quantitative analysis. Analyzing the Chromatogram First, the retention times can be compared against a standard to identify each component (similar to TLC). Second, we can use the peak area to quantify the relative amount of each component. The area beneath each peak is proportional to the absolute amount of compound that was injected. This area is calculated by an “integrator” and is represented in arbitrary units (because only the ratios are meaningful). But we must first determine relative response factors. Different compounds respond differently to the GC detector; For example, the compounds in Fig 3 are at equal concentrations but peaks are visibly different sizes and integration of the peak area gives slightly different values. Detector Response Factor Rather than finding absolute response factors, we commonly use relative response factors, where one of the components is set to have a value of one (1) and the other is relative to that number. The first step is to inject a mixture of standards where the ratio between the components is known to be 1:1 (by volume). In Figure 3, a 1:1 mixture (by volume) of compound a and b was injected. As you can see, the ratio between the peaks is not 1:1 even though a and b are present in a 1:1 ratio. This is due to the different response of the GC detector to various compounds. In other words, the detector is more sensitive to one compound. We need to compensate for this difference by applying a correction factor known as “relative response factor.” The first step is to choose the reference peak – typically we choose the smaller value and normalize the area to 1. This ensures all correction factors are equal to or greater than 1. Using Figure 3, we select peak b as the reference. Next, we divide the area of each peak by the reference area to get the response factor. peak area (a.u) Response factor 2500 2500 a = 1.075 2325 2325 2325 b = 1.000 2325 The relative response factor for a is 1.075 because we set the relative response factor of b to be 1.000. This means the peak area for a appears larger than it should be (7.5% larger) and we will use the correction factor to reduce the observed peak area for a. Component Percentage Suppose a GC run of 1.0 μl injection of a fraction containing a and b gave the following results: peak a: 3300 (a.u.) peak b: 4700 (a.u.) ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 4 First, we use the correction factor to adjust the areas. component a: 3300 ÷ 1.075 = 3070 component b: 4700 ÷ 1.000 = 4700 The percentage of each component in the mixture can be calculated by )*+) ) /010 % compound a = )*+) )-)*+) . = /010-2100 = 0.395 or 39.5% % compound b = )*+) . )*+) )-)*+) . = 2100 /010-2100 = 0.605 or 60.5% Remember that the relative response factor was determined based on a 1:1 mixture by volume. So the component percentage is also by volume. Thus the 10 mL fraction (from which we injected 1.0 μl) is a 60:40 mixture, by volume. More precisely: volume component a: 10mL × 0.395 = 3.95 mL volume component b: 10mL × 0.605 = 6.05 mL We can convert volume into moles; using density (d) and molecular weight (MW): moles component a/b = 3.95 mL × d (g/mL) ÷ MW (g/mol) For each of your fractional distillation samples, you will convert the integrated peak areas to a mole fraction. You will then plot the mole fraction of each component in each fraction as a function of the fraction temperature midpoint. These graphs enable you to assessment of the efficiency of the fractional distillation in the terms of the theory and underlying principles for distillation. Lab Notebook Prelab write-up Write n-hexane and n-heptane into the reagent table. Include the chemical structures, molecular weights, boiling points and densities. Use the SDS to find and enter the missing information into the chemical hazard sheet and staple onto the cover of your notebook. Write out the steps you will perform under the procedure section. Observations Write down what you did and what you observed directly into the notebook. No data that was not written into the notebook can be used for the lab report. Calculations All calculations must be written directly into the lab notebook. Submit scanned copies of all gas chromatograms with your report. Procedure Safety • Perform up all distillations in a chemical (fume) hood to minimize exposure to any vapors. • Never to heat a closed vessel. Always make sure that your distillation apparatus always has an opening to the atmosphere. (the collection port in this experiment) • Never heat a flask to dryness. Stop the distillation before the round-bottom flask goes dry. • Never leave a distillation unattended (see above). • Always use a lab jack underneath a distillation or refluxing reaction. In an emergency, the lab jack can be used to quickly remove the heat source by lowering the heating mantle (or hot plate) from a hot flask. Never grab a hot flask. Waste • Used aluminum foil, cotton, and any contaminated paper and gloves goes in the solid waste trash can by hood #6. • All solvent waste goes into liquid organic waste container in hood #6. • Dispose of collected fractions in liquid organic waste. ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 5 Teamwork For this experiment, you will work in teams of two and perform a simple distillation on day 1 and a fractional distillation on day 2 using a 50:50 (V:V) mixture of hexane and heptane. The setups are slightly different, but the procedures are the same. You will run 11 GC’s: one of the original mixture (for calculating relative response factors) ten from the simple and fractional distillation (five fractions each) to determine the mole fraction composition of each sample. Part A - Distillation Your TA will demonstrate a simple distillation apparatus on day 1 and a fractional distillation on day 2. You will be given a solution containing hexane and heptane. Set aside about 10 drops of the mixed solution (1:1 by volume) for GC analysis. Obtain a set of four vials with caps. Number them. You will transfer each fraction into these vials for analysis. Add ~50 mL of the mixture solution and one or two boiling chips to a 100 mL round bottom flask. Position the flask in a cool disconnected heating mantle and clamp the flask securely. Raise a lab jack under the heating mantel to provide additional support. The jack should be raised far enough such that upon lowering it, the mantel will be lowered completely away from the distillation flask. Assemble the remaining components of the simple or fractional distillation apparatus. Use a 10 ml graduated cylinder as your collection flask. Do not start the distillation until the TA has approved your setup! Begin the distillation and record the temperature at the distillation head vs. the volume of distillate collected every milliliter. When 10 mL of distillate has been collected, switch graduated cylinders and transfer the fraction to the appropriate vial. Cap the first vial tightly; record the vial number and the sample temperature range in your notebook. Collect the next 10 mL fraction while monitoring the temperature at the distillation head. Cap the vials tightly. After you have collected a total of 4x10 mL = 40 mL of distillate, remove the heat source and stop the distillation. Turn off and unplug the heating mantle. Allow the equipment to cool before disassembling. PART B Gas Chromatography Table 1. E5 Hydrocarbons Compound MW (g/mol) Boiling Point* n-Hexane 86.2 69˚C n-Heptane 100.2 98˚C *Sigma Aldrich Catalog: Your TA will demonstrate how to use the GC syringe and inject a sample Use the same instrument for ALL of your GC analyses. Rinse the syringe a couple of times with the sample to be analyzed and inject ~ 1 μL (one microliter not mL) of the sample into the injection port of the instrument. Press the start button of the recorder immediately after you remove the syringe. Make sure you recap all samples immediately. First analyze the original 50:50 mixture and identify the two components. Compare of the observed retention times with the labelled GC trace above each instrument. Record the peak areas and use them to calculate the relative response factors. Next, analyze the four distillation fractions. Inject ~ 1 μL (one microliter not mL) of each sample for each run. You will have 4 chromatograms, one for each fraction. Each team member should run at least two GC’s. Share the data with your lab partner. Submit scanned copies of all gas chromatograms with your report. Each student must do the calculations and data analysis for all the data alone. No sharing of calculations. ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 6 Experiment 5 Chemical Hazards - Fill in the missing information CAS Number IUPAC Name Chemical Structure mp / bp (°C) n-Hexane n-Heptane Water Acetone Hazard Statements - list and define any new hazard codes - ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. GHS Hazard Codes GHS Pictogram ...
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