Experimental Procedure

Lab 704

Overview

A known mass of starting material is used to synthesize the potassium alum. The synthesis requires the careful transfer solutions and some evaporation and cooling techniques.

Major works Reaction of aluminum foil with 4 M KOH and Filtration Addition of 6 M H 2 SO 4 Heating for Dissolution of Al(OH) 3 Crystallization with stirring Suction Filtration and Drying Drying in a oven Melting point determination Estimated Time En route (min) 10 ~ 15 ~5 20~30 40 5~10 20 20~30

A. Potassium Alum Synthesis

1. Prepare the aluminum sample 1) Obtain a piece of aluminum foil and weigh out about ( ± 0.01 g) of aluminum pieces.

1.0 g 2) Tear the foil into small pieces and place the pieces in a 250 mL beaker.

2. Dissolve the aluminum pieces 1) Move the beaker to a fume hood. Carefully add

50 mL of 4 M KOH to the aluminum pieces

, and swirl the reaction mixture. Notice that the reaction is exothermic.

2) Allow the reaction to proceed until all of the foil is dissolved.

3. filter the reaction mixture 1) When no further reaction is evident, return the reaction mixture to the laboratory desk.

2) Carefully pour the reaction mixture through your Bűchner funnel and filter flask setup, and rinse the filter paper with a small amount of distilled water.

4. Allow the formation of aluminum hydroxide 1) Add a stir bar into the beaker.

stirrer/hot plate , slowly add With constant stirring on a

30 mL of 6 M H

2

SO

4 to the cool solution. Solid aluminum hydroxide may form.

5. Dissolve the aluminum hydroxide GENTLY heat on a hot plate for 20 ~ 30 min the reaction mixture with stirring until all of the aluminum hydroxide dissolves.

At the end of the heating period the volume of the solution should be about 60 ml.

1) If there are no solid impurities present, proceed to the next step. If solid impurities are present, pour the mixture through the the mixture.

clear Bűchner funnel and filter flask to separate undissolved solid from

6. Crystallize the alum 1) Remove the solution from the heat.

2) Cool the solution in an ice bath for 40 minutes. Alum crystals should form within 20 minutes . If crystals do not form, use a hot plate (Figure 15.2) to gently reduce the volume by one third to one-half (do not boil!) again and return to the ice bath.

7. Isolate and wash the alum crystals 1) Using Bűchner funnel, isolate the alum crystals form the solution.

2) Wash the crystals on the filter paper with temperature) 10-mL portions of a 50% (by volume) ethanol water solution.

two (cooled-to-ice 3) Maintain the vacuum suction for crystals appear dry.

at least 5 minutes until the And place a aluminum dish with the crystals into a pre-heated oven (~70 o C) for 20 minutes.

4) Weigh and record the mass (0.01g) of the crystals.

8. Percent yield Calculate the percent yield of your alum crystals.

CLEANUP:

Rinse all glassware twice with tap water and twice with distilled water. All rinses can be discarded in the waste container, followed by a generous amount of tap water.

Clay triangle DISPOSAL:

Discard the filtrate in the container labeled “Student Prep Alum”.

Bunsen burner

B. Melting Point of the Alum

1. Prepare the alum in the melting-point tube.

1) Place finely ground, dry alum to a depth of about 0.5 cm in the bottom of a melting point capillary, 2) To do this, place some alum on a piece of dry filter paper and “tap-tap” the open end of the capillary tube into the alum until the alum is at a depth of about 0.5 cm (Figure 15.3, page 196).

3) Invert the capillary tube and compact the alum at the bottom of the tube (see Figure 15.4).

2. Determine the melting point of the alum Use the apparatus in Figure 15.6.

1) Place the capillary tube containing the sample into the melting-point apparatus.

2) Mount the capillary tube containing the sample beside the digital temperature probe (Figure 15.6 insert) with a rubber band. Transfer the sample/thermometer into the water bath.

3) Slowly heat the sample at about 3 o C per minute while carefully watching the alum sample .

4) When the solid melts, note the temperature. Allow the sample to cool to just below this approximate melting point; at a 1 o C per minute heating rate, heat again until it melts.

5) Repeat the cooling/heating cycle until reproducibility is obtained-this is the melting point of your alum.

6) Record this on

the Report Sheet

.

DISPOSAL:

Dispose of the melting point tube in the Glass Only Waste box

.

Bunsen burner

The Next Step Other alums (Table 15.1) can be similarly synthesized.

(1) Design a procedure for synthesizing other alums .

(2) Research the role of alums in soil chemistry, in the dyeing industry, the leather industry, water purification, or food industry .

(3) “ Growing” alum crystals can be a very rewarding scientific accomplishment , especially the “big” crystals! How is it done?