Write experiment on sublimation process with the diagram
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Answer:
Under Atmospheric Pressure
The sublimation pictured in this section shows purification of 0.29g of ferrocene, which grew in long needles on the bottom and top of the petri dishes ( 90% recovery).
Figure 6.30: a) Ferrocene before sublimation, b) Sample with top petri dish in place, and on a wire mesh on a hotplate, c) Petri dishes with cold beaker.
Spread the crude, dry solid to be sublimed in a thin layer on a "bottom" petri dish (Figure 6.30a). If chunky, first crush with a mortar and pestle. Determine the empty mass of the top petri dish.
It is important that the solid to be purified is dry: if the sample is wet with solvent, condensation may form on the top petri dish during the sublimation. In the beginning stages of the sublimation, small amounts of condensation can be wiped off the top petri dish with a paper towel. However, too much condensation may wash crystals off from the top dish.
Cover the bottom petri dish with the top dish and set atop a wire mesh on a hotplate in the fume hood (Figure 6.30b) set to the appropriate temperature (depending on the sublimation point of the compound of interest, perhaps medium low). The wire mesh helps dissipate the heat evenly to the dish and minimizes charring.
Place a large 600mL beaker filled with ice water atop the petri dish (Figure 6.30c).
Figure 6.31: Time-lapse sublimation of ferrocene from the bottom petri dish onto the top petri dish.
Over time the sample will sublime and collect on the upper petri dish (Figure 6.31). Monitor the sublimation as compounds may char during the process (if it starts to blacken, turn down the heat). Continue the sublimation until it appears as if little (or no) solid remains on the bottom petri dish. It is very common for crystals to also grow along the sides of the bottom dish.
Delicately remove the petri dishes from the hotplate using cotton gloves (Figure 6.32a) or a silicone hot hand protector. Jostling the dishes will cause sublimated crystals to fall from the top petri dish.
Safety note: Allow the two dishes to cool intact on a ceramic tile in the fume hood (Figure 6.32b). Do not remove the top petri dish right away or noxious fumes may escape.
The crystals on the top petri dish are purified and should be retained (and their mass determined). Sometimes material on the bottom dish may also be saved if it appears crystalline (signifying it underwent a sublimation process) and doesn't appear contaminated with char (Figure 6.32c).
Figure 6.32: a) Removal of the sublimation apparatus from the hotplate, b) Cooling on a ceramic tile, c) Separated petri dishes: bottom dish (left), top dish (right).
Under Reduced Pressure (Vacuum Sublimation)
The sublimation in this section shows purification of camphor on two scales 2.28g (large scale, 77% recovery), and roughly 0.2g (small scale).
Figure 6.33: a) Camphor ground in a mortar, b) Adding camphor to a small scale sublimator, c) Large scale sublimator with camphor in the bottom, d) Greasing the ground glass joint.
If the solid to be sublimated is chunky, first crush with a mortar and pestle (Figure 6.33a). Then place the crude, dry solid in the bottom of the sublimation apparatus (Figure 6.33b).
It is important that the solid is dry: if the sample is wet with solvent, condensation may form on the cold finger during the sublimation. Too much condensation may wash crystals off the cold finger.
Secure the apparatus to a ring stand or latticework (Figure 6.33c+d). For small scales, support the apparatus with a platform (Figure 6.33d). A large scale sublimator is shown in Figure 6.33c.
Lightly grease the joint that connects the two pieces of sublimation glassware. Grease can be easily applied with a syringe full of grease. If using ground glass, lightly grease the joint near the end that will not be in contact with the sample (Figure 6.33d).
Figure 6.34: a) Small scale sublimation apparatus greased appropriately, b) Large scale sublimation apparatus with the cold finger filled with ice water, c) Heating the apparatus with a heat gun, d) Solid deposits on the outside of the glassware.i