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Figure 1. This is the distillation column. I'm sure no one in this group would start up the column with that closed can of flammable 1-propanol sitting right next to the heating mantle for the pot. Start-Up Procedure for the Column 1. Pour the 1-propanol-water mixture from the three bottles (labeled 1, 2, 3) into the white bucket and mix them up with the aluminum stirring rod. Withdraw a few c.c. of the liquid with a dropper and analyze the concentration by the refractometer (read the procedure for using the refractometer).
Figure 2. This is the refractometer that is used to measure composition. 2. The mole fraction of the 1-propanol should be above 80% (refer to the refractive index calibration curve posted). If mole fraction is too high (e.g. close to 100%) try to dilute it with distilled water and analyze it with the refractometer. Use a small quantity only. This is to avoid over dilution of the 1-propanol. Repeat the procedure until you reach a refractive index reading of about 80%. 3. After the desired concentration of 1-propanol is reached, measure 5 to 6 liters of the solution and pour it into the still pot with a funnel through the port where thermocouple no. 1 is placed. Use the peristaltic pump provided to pump the liquid. Use great care when you remove the thermocouple from the pot and avoid bending it or it might break. Don't switch on the distillation column before filling it with the liquid. 4. The thermocouples is plugged into the computer for data acquisition. Each thermocouple is hooked up along the distillation column. No. 1 is for the still pot, 2 for the 1st tray, 3 for the 2nd tray and so on. A total of 7 probes are used and they are all labelled. Try to trace them on the distillation column. The aquisition program is based on Labview, similar to last semesters Cooling Tower experiment.
Figure 3. This is one of the thermocouple wells in the side of the column. I hope that isn't someone's beverage cup on the counter in the background! Eating in a chemical lab is a definite safety violation. 5. Turn on the flow of water in the condenser. Plug in the power to the lower heating unit at the pot. You do not require the second heating unit, midway to the top of the column.With these settings you should be able to establish a temperature gradient in the column. Use total reflux conditions. Steady state is achieved when the temperature readings do not change with time. When this happens, flush the outlet sample ports of the distillation column as you were instructed to earlier. Allow a few more minutes for the column to reach steady-state again. Record all temperatures. 6. Once steady state and temperature gradient are established at total reflux, take a sample (couple of c.c.) from each tray with a syringe through the port where the septum is located. Pour the sample into the plastic storage vials and allow it to cool to room temperature prior to reading it in the refractometer. Analyze the collected sample with the refractometer. Take several readings (at least three) of the same sample to ensure correct readings. Dry the syringe with air after each sampling to avoid contamination of the sample. Be careful while making these measurements and ensure that the values you obtain are "logically" consistent. Also withdraw a few c.c. from the pot and analyze its composition. Record the total quantity of liquid used. Pour the waste liquid back into the containers (1, 2, or 3).
Figure 4. These are the ports on the side of the column through which samples are extracted with a syringe. Shut-Down Procedure for the Column 1. Turn off the heating coils for the column. 2. Wait 30 minutes (or as long as it takes) for the column to cool and drain into the stillpot. 3. Using the peristaltic pump to drain off the liquid from the bottom, into the containers labeled 1,2,3. 4. Drain what ever liquid remains at bottom with syringe set-up. 5. Remove the contents of the vapor ports into a beaker, and then add all the liquid removed into one closable vessel/bottles. 6. Turn off the column condenser. N O T E: At all times, avoid leaving alcohol solutions exposed to air. Cover the container. Procedure for Using the Refractometer 1. Lift and pivot the top cover of the prism so that you can see the rectangular prism surface. Clean the surface with tissue paper if it is dirty.
Figure 5. Here's the prism of the instrument, this time open and ready to have a few drops of liquid placed on its surface. 2. Place a couple of drops of 1-propanol-water solution on the surface of the prism. 3. Cover the prism surface with the top cover gently. As you push the top cover down on the prism you will hear a "click" sound.
Figure 6. This is a close-up of the prism (closed) upon which the sample is placed. 4. Turn on the toggle switch located on the left side of the refractometer and adjust the light source by moving it up towards the prism. 5. When you look into the eye piece, you will see an orange rainbow pattern on the upper half of the circular field. 6. Focus the lower edge of the orange pattern with the dial that is located above the prism on the body of the refractometer. For 1-propanol-water mixture, the proper range should lie between 18 and 25. Adjust the lower edge of the orange pattern so that it passes through the cross by turning the metal knob (hand wheel) that is located on the right side of the refractometer. By pushing the toggle switch down the orange pattern will disappear and a scale will appear to show the refractive index. Remarks If you have trouble with focusing the orange rainbow pattern, it could be due to one of the following reasons: 1. Part of the liquid is evaporated and the liquid film is unevenly spread on prism surface. You should clean the prism and reapply the liquid. 2. The refractive index adjustment knob is either over or under adjusted. If you know the refractive index is in the neighborhood of 1.380, adjust the scale reading to that reading by pushing the toggle switch down to review the scale and turning the refractive index adjustment knob until it reads 1.380. Now go back to the beginning and repeat the procedure above. |
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