Experiment Number 6

 

Analysis of Alum or Potassium Aluminum Sulfate

 

 

 

 

 

INTRODUCTION

 

When a compound has been synthesized it is important to verify that the product is actually the intended compound. In this laboratory exercise we will analyze the alum made in the previous experiment using a variety of tests, including melting point, water of hydration and the percent aluminum and the percent sulfate.

 

The easiest test for purity is melting point.  In this test, pure alum and the alum to be test are heated in a capillary tube adjacent to a thermometer.  The temperature at which each undergo the phase change from solid to liquid and the "sharpness" of the transition.

 

In the water of hydration test, alum is tested for the water of hydration in its crystal structure.  Chemical structures often contain a discrete number of water molecules as a part of their crystalline structure and these compounds are called hydrates.  Hydrates occur commonly among ionic compounds and the water in the molecule is present in a direct proportion to the ionic salt in the compound.  For example, a common hydrate is copper (II) sulfate pentahydrate.  This compound has as a part of its crystal structure five water molecules per molecule of copper (II) sulfate. If a hydrate is heated strongly, the water can be removed and the compound becomes anhydrous or without water. 

 

In the last test we will perform a gravimetric analysis of the sulfate present in the alum.  When two soluble salts are mixed and an insoluble product is produced it is called a precipitation reaction.  Depending on the nature of the salts employed this type of reaction can be used to either identify components in a reaction and/or determine the amount of a component in a reaction.  The later can be accomplished because the precipitate can easily be separated from the other reactants either by filtration or by centrifugation; thus allowing its mass to be determined independent of the other reactants.  In this experiment we will react excess BaCl2 with an unknown sulfate containing compound that varies in its sulfate ion concentration.  The expected reaction will be:

 

                        (NH4)2SO4 (aq)   +   BaCl2 (aq)   -à    BaSO4 (s)  +   2 (NH4)Cl  (aq)

 

The mass of the precipitated barium sulfate will then be determined and from this you should be able to ascertain the concentration of the unknown.

 

 

 

 

 

 

MATERIALS – Melting Point

 

Capillary tubes

Rubber band

Thermometer

Hot plate/stirrer

Beaker, 250 mL

Magnetic stirrer

Ring stand

Alum standard

Glass mortar & pestal

 

MATERIALS – Water of Hydration

 

Ring stand

Bunsen burner

Crucible

Balance

Crucible tongs

Crucible triangle

Spatula

 

 

 

MATERIALS – Spectrophotometric Aluminum Analysis

 

Alum standard

Test tubes

Test tube rack

Spectrophotometer

Al Test Reagent

5 ml Pipet

Pipet pump

 

 

 

MATERIALS – Gravimetric Sulfate Analysis

 

0.2 M Barium chloride

Balance

Funnel

Ethanol/Water (50/50)

Beaker, 250 mL

25 mL Grad. Cylinder

Hot plate/stirrer

250 mL Flask

Filter Paper, fluted

 

Stirring rod

 

 

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SAFETY

 

Goggles must be worn at all times during this experiment. Should anything get on your hands or clothing wash off immediately.

 

 

 

 

PROCEDURE – Melting Point

 

1.         Prepare a powder of your alum crystals using the glass mortar and pestle. 

2.         Pack two glass capillaries with alum, one from the pure alum and one of your sample. Depth of powder should be approximately 2-3 mm.  Tap the tubes on the table to pack the powder.  Be sure to mark tubes so that they can be differentiated from one another.

3.         Rubber band the tube to the bottom of the thermometer.

4.         Heat the tubes and the thermometer slowly in the 250 mL beaker while stirring and record the temperature of the phase transition.

 

 

 

 

PROCEDURE – Water of Hydration

 

1.         Obtain crucible and weigh to nearest hundredth of a gram. 

2.         Add a spatula of potassium aluminum sulfate and reweigh the crucible to the nearest hundredth of a gram.

 

Alternative #1

3.         Assemble a ring stand with the crucible triangle mounted on the ring of the ring stand

4.         Place the crucible at an angle in the crucible triangle and place the crucible lid on the crucible so that .the lid is not completely on the crucible. Ad just the ring so that Bunsen burner is about 3 cm above the top of the burner.  Have the instructor check your set up before proceeding to the next step.

5.         Heat the crucible with the Bunsen burner from five to ten minutes (Should be about a minute or two after escaping steam is no longer evident).

6.         Use the crucible tongs to carefully  remove the crucible lid.  Use the tongs to then place the crucible on the ring stand base (NOT THE COUNTER TOP !).  Allow the crucible to cool for 5-10 min.

 

Alternative #2

3.         Place crucible on hot plate.

4.         Heat a setting #10 until no water vapor rises from the crucible.

5.         Use crucible tongs to move crucible to the metal plate of a ring stand.

6.         Allow crucible to cool.

 

7.         Reweigh the cooled crucible.

8.         Using a pipet add several drops of water to the now anhydrous alum and describe in your notebook what happens.

9.         Repeat this procedure using your synthesized sample of alum. (Your instructor may provide a table of data with this step already completed)

 

 

 

PROCEDURE – Spectrophotometric Aluminum Analysis

 

1.         Prepare a standard curve and samples as directed by you instructor. 

2.         Add the aluminum test reagent and let sit for 5 minutes.

3.         Read tubes in the spectrophotometer and record the absorbance.

4.         Prepare a standard curve and evaluate the amount of aluminum in the sample.

 

Your instructor will provide more details for this procedure.

 

 

 

 

PROCEDURE – Gravimetric Sulfate Analysis

 

1.         Weigh an alum sample on weighing paper.  Use about 2 g and weigh to the nearest mg

2.         Dissolve sample in approximately 25 mL of distilled water.  Heat sample to 90°C and stir.

3.         Add 40 mL of 0.25 barium chloride or barium nitrate.  Add drop by drop with stirring over a ten minute period while continuing to heat sample.

4.         Continue to heat tube for about 30 min so that large crystals will form.  Add water as necessary to maintain volume at about 50-60 mL

5.         Pre-weigh and flute filter paper.

6.         Place in funnel and place funnel in 500 mL flask.  Transfer heated solution to filter paper and allow to filter overnight. 

7.         Dry filter at 60° overnight and weigh.

8.         Calculate actual and expected sulfate values.

 

Your instructor may have an alternative for this procedure.

 

 

 

 

QUESTIONS

 

1.         What is the purpose of heating the sample while precipitating the barium sulfate and afterwards (You may have to read about gravimetric analysis of sulfate in various Internet articles)?

2.         What mass of precipitate of barium sulfate would be produced from 0.85 g of potassium sulfate?

3.         Gravimetric analysis could also be done using sodium sulfide to precipitate the aluminum ion as aluminum sulfide.  What advantages or disadvantages do you think might be incurred using this method instead?

4.         Another type of analysis that could be performed is called atomic absorption spectrophotometry.  Give a description of this teachnique.

5.         Suppose lead(II) nitrate were substituted for barium.  How much lead (II) sulfate would be made from 2.00 g of potassium aluminum duodecyl hydrate.?