The Analysis of Alum, Chemistry tutorial


After a compound has been synthesized, tests must be taken out to verify that the compound formed is certainly the compound desired. There are a number of tests which can be performed to confirm that the compound is the one desired. Previously we prepared alum crystals, KAl(SO4)2•12H2O. Now, we will conduct a series of tests to find out if our crystals are really alum. The first test is to determine the melting temperature of the compound and compare this value by the accepted (published) value for alum (92.5°C). The second test finds out the water of hydration present in the alum crystals. The third test is a chemical test to find out the percent sulphate in your sample of alum.


Part I: Melting Temperature test materials

Melting point apparatus:

  • Capillary tubes   
  • Alum crystals, KAl(SO4)2•12H2O  
  • Hot plate  
  • Bunsen burner 
  • Mortar and pestle

Part II: Water of Hydration test materials

  • Crucible with cover: Alum crystals, KAl(SO4)2•12H2O
  • Balance:                   Ring stand, ring and clay triangle
  • Lab burner:              Tongs or forceps


Part I: Find out the Melting Temperature of Alum

1) Obtain and put on goggles.

2) Make use of a mortar and pestle to pulverize about 0.5 g of dry alum and put it in a small pile in the mortar. Push the open end of the capillary tube to the pile of the alum powder. Pack alum to the capillary tube to a depth of around 1 cm via tapping the tube lightly on the table top.

3) Insert the capillary tube to the aluminum sample block of the melting point equipment. Adjust the angle of viewing for the utmost comfort.

4) To start the melt cycle presses 'Start' key. The pump will stop and the unit will heat at maximum rate to the pre-programmed plateau temperature. The screen will exhibit 'Heating' all along by the current temperature of the block. 

5) Whenever melt has occurred press 'Stop'. The display will change to 'Cooling to Ambient' and the cooling pump can be listened.

6) Conduct a second test by a new sample of alum in a latest capillary tube.

Part II: Find out the Water of Hydration of Alum

7) Heat a crucible by cover over a burner flame till it is red hot. Let the crucible to cool, and then measure the net mass of the crucible and cover. Handle the crucible by tongs or forceps to ignore getting fingerprints on it.

8) Place around 2 g of your alum crystals in the crucible, and then compute the mass of the crucible, cover and alum. Record this dimension in the data table.

9) Set up the ring, ring stand and triangle over a lab burner. Make use of tongs or forceps to set the crucible at an angle on the triangle and put the cover loosely on the crucible. Make use of a lab burner to very gently heat the crucible of alum till you can see no vapor escaping from the crucible. It is significant that the vapor doesn't carry any alum with it. After the vapor is gone, heat the crucible more strongly for 5 minutes, and then cool the crucible.

10) Evaluate and record the mass of the crucible, cover and alum after drying. 

11) Reheat the crucible and alum sample for 5 extra minutes. Cool and calculate the mass of the crucible again. If the two masses are similar (or very nearly so), the test is done. If not, repeat the heating and weighing until a constant mass is obtained.

Part III: Find out the Percent sulphate of Alum 

12) Get a clean, dry Gooch crucible and evaluate its mass. If you are not employing a Gooch crucible, then utilize the finest-grain filter paper available, like Whatman No. 42, as the very tiny particles of barium sulfate precipitate will pass via most paper filters. Evaluate and record the mass of the filter paper. 

13) To clean a Gooch crucible, assemble the crucible, Walters adapter and filter flask. Make use of suction to draw distilled water via the filter pad. Put the crucible in a beaker and dry it in an oven. 

14) Measure the mass of around one gram of your alum sample to a 250 mL beaker. Add around 50 mL of distilled water to the beaker of alum and stir the mixture to dissolve the sample. 

15) Compute the volume of 0.20 M Ba(NO3)2 solution which is required to fully precipitate the sulphate ions in the beaker of alum solution. Evaluate twice the volume which you have computed, and slowly add it to the beaker of alum solution. Stir the mixture to make sure complete mixing of the reagents.

CAUTION: Handle the barium nitrate solution with care. This solution is toxic. 

16) Choose one of the two processes below for preparing the precipitate: 

  • Set up a ring stand, ring and wire gauze for heating over the lab burner. Put a watch glass over the beaker and heat the beaker of your reaction mixture over the lab burner. Heat the mixture to near boiling for around 15 minutes. This step helps in collecting the particles of precipitate to a bigger size and eases the filtering method.
  • Make use of a watch glass to cover the beaker of the reaction mixture. Store the beaker in a safe place during the night.

17) Let the mixture to cool. Filter the beaker of precipitate via the Gooch crucible with suction (or make use of a filter paper and funnel). Add liquid to the crucible very slowly.

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