Step 3: Crystallize and recover the alum |
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In a medium-sized beaker, make a 50/50 mixture of ice and water. Ice will be provided by your TA. Place your beaker into the ice water bath. Cool the solution until the temperature of the liquid in your beaker is below 10°C. Measure the temperature of the solution with the thermometer provided in your lab drawer. |
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| When the solution has been cooled to 10° C, add 3 mL of ethanol to your solution using a clean graduated cylinder and cool for 5 more minutes. During this time, you should see white crystals of alum begin forming in your beaker. |  |
| Set up a Buchner funnel as shown here. The black ring underneath the Buchner funnel is called a Filter-Vac. It allows a seal to be made between the funnel and the filter flask, and is a necessary part of this set-up. Don't forget to place it beneath your funnel. Secure both filter flasks with iron rings and ring stands, as well. |  |
| After you have completely set up your apparatus, place a piece of filter paper inside of the Buchner funnel. Use your deionized water bottle to wet the paper. |  |
| Use your deionized water bottle to wet the paper. |  |
| Turn on the vacuum. Begin pouring the contents of your beaker through the funnel. Swirl the liquid in the beaker gently to suspend the alum crystals in solution. Continue swirling the beaker's contents and pouring into the funnel until you have filtered all of the solution through the Buchner funnel. |  |
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Make a 10 mL solution containing a 50/50 mixture of ethanol and
distilled water in a 50 mL beaker. Again, use a clean graduated
cylinder to measure both liquids. Cool this mixture in your ice
bath. After it has cooled for a few minutes, use small portions (5 mL)
of this ethanol/water mixture to rinse the inside of the beaker.
Some crystals are probably stuck to the sides and bottom of the
beaker. Rinsing the beaker will help you to remove them. Simply
add a little ethanol/water rinse solution to the beaker, swirl,
and pour this solution into the funnel. The ethanol/water mixture
also serves to wash the isolated crystals as they dry. After you are done rinsing, keep the crystals in the Buchner funnel with the vacuum attached and on to allow air to flow through the filter for 10 minutes. This will help to dry your crystals and remove excess water. |
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Weigh a clean, dry watch glass on the pan balance. Record this
value to the nearest hundredth of a gram in your laboratory notebook. Turn off the vacuum. Using a clean, dry spatula, remove the alum crystals from the
Buchner funnel and onto the watch glass described above. Weigh the
crystals and watch glass. Record this value to the nearest hundredth
of a gram in your laboratory notebook. |
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| Record the mass of the crystals in your laboratory notebook. |
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