15B Analysis of Alum

15B Analysis of Alum Purpose Confirm the identity of a sample of alum using melting point, water of hydration, and a test for sulfate. Background Once a synthesis has been completed, the identity of the compound produced should be confirmed. A number of tests can be used to confirm the identity of potassium

alum, KAl(SO4)2•12H2O.

Melting point, water of hydration, and presence of sulfate in the compound will be used in this lab to verify that the substance you have is actually potassium alum. Materials

Equipment • Xplorer GLX • Glass stirring rod • Stainless steel temperature probe • Hot plate or combination hotplate/ stirrer • Ring stand with ring • Watch glass or mortar and pestle • Buret clamp • Bunsen burner • Crucible • Porcelain triangle • Crucible tongs • Beaker, 250 mL

Consumables

• Potassium alum (KAl(SO4)212H2O) • Distilled water • 0.2 M Barium chloride solution • Capillary tubes

(BaCl2)

Safety Precautions • Wear safety glasses and follow all standard laboratory safety procedures Procedure Melting point determination 1) Setup the ring stand, and hot plate as shown in the illustration. Fill a 250 mL beaker three quarters full with water to use as a water bath. Place the beaker on the hot plate or a combination hot plate/stirrer. Do not turn on the hot plate yet.

2) Connect the temperature probe to one of the temperature ports () on the left side of the Xplorer.

3) Crush ~0.5 g of alum crystals to a fine powder in a watch glass using a glass stirring rod. Scrape the powdered alum into a pile in the middle of the watch glass.

4) Push the open end of a capillary into the alum, then invert the capillary tube and tap the closed end on the bench top to pack the alum into the end. Repeat this procedure until the capillary tube contains about 1 cm of firmly packed alum.

5) Attach the capillary tube to the end of the temperature probe with a small rubber band so that the alum is even with the end of the temperature probe. Be sure that the closed end of the capillary tube is down.

6) Clamp the temperature probe to the support stand using a utility clamp so that the tip of the temperature probe and the portion of the capillary tube containing the alum is immersed in the water bath. Be sure that the open end of the capillary tube is above the water level and no water is able to get into the capillary.

7) Turn on the hot plate to begin heating the water bath at a gradual rate. Stir the water continuously while the water heats.

8) Turn on the Xplorer and press  to open a digits display.

9) Observe the alum in the capillary tube until it has completely melted. You may need to peridoicaly tap the temperature probe gently to loosen bubbles that attach and may interfere with the view of the alum crystals. Record the temperature in your lab notebook. Water of hydration determination. 1) Setup the ring stand, ring, porcelain triangle, crucible with lid and bunsen burner as shown in the illustration.

2) Heat the crucible with the bunsen burner until it is red hot. Allow the crucible to cool.

3) Weigh the crucible and lid and record the mass in your lab notebook. Use crucible tongs and do not touch the crucible or lid with your fingers.

4) Put about 2 g of alum into the crucible and weigh the crucible, alum and lid. Record the weight in your lab notebook.

5) Place the crucible on the ring stand using the tongs and place the lid slightly ajar so that water vapor can escape.

6) Begin heating the crucible slowly with the bunsen burner.

7) When vapor can no longer be seen escaping from the crucible, heat the crucible strongly for 5 minutes. Allow the crucible to cool.

8) Weigh and record the mass of the crucible, alum and lid.

9) Reheat the crucible with alum for 5 more minutes then allow it to cool. Weigh the crucible, alum and lid and record the mass as before.

10) If the two masses are very nearly the same, the procedure is complete, otherwise repeat step 9 until a constant mass is obtained.

11) Calculate the change in mass due to drying of the alum.

Sulfate test 1) Place a few crystals of alum into a test tube. Add distilled water dropwise with stirring until the alum is dissolved.

2) Add one drop of 0.2 M barium chloride solution. Record your results in your notebook. Analyze Record calculations in your lab notebook as you complete your analysis. Data Table

Run 1 Run 2 Melting point (°C) Mass of crucible and cover (g) Mass of crucible, cover and alum before heating (g) Mass of crucible, cover and alum after 1st heating (g) Mass of crucible, cover and alum after final heating (g) Mass of anhydrous alum Water of hydration

Precipitate with BaCl2 ( yes/no)

Analysis Questions 1) Did your tests confirm the presence of alum? Explain using your results to support your answer.

2) What other tests could be used to confirm the identity of your sample?

3) If the melting point was the only data you had to confirm the identity of your sample, how confident would you be of your identification? Explain.