Determination of the magnesium content in magnesium Tablets

Duration: ca. 1 h
Difficulty: Medium

A PDF download of this task sheet as well as the answers and instructions for preparing the experiment can be found here.

Materials

• 1 x 100 ml volumetric flask
• 1 x 50 ml Burette
• 3 x 200 ml Erlenmeyer flask
• Glass funnel
• Glass pipettes
• Hot plate and stir bar
• Mortar and pestle
• pH paper
• Plastic pipettes
• Spatula

Chemicals

• 0.1 M aqueous sodium hydroxide
• 0.1 M EDTA solution
• 2 M aqueous hydrochloric acid
• Buffer solution pH 10 (NH4Cl/NH3)
• Deionized water
• Eriochrome Black T, 1% in NaCl
• Magnesium effervescent tablets

Procedure

Part I: Dissolving the Tablet

1. Dissolve the effervescent tablet in deionized water in a 100 ml volumetric flask. You may use a mortar and pestle to crush the tablets. Don't fill the volumetric flask up right away but try to add a smaller amount of deionized water initially and swirl to dissolve the tablet. You may add 2 M hydrochloric acid until the tablet is completely dissolved.
2. Fill the volumetric flask up to the mark with deionized water.

Part II: Determination of the Magnesium content

1. You should continue using the pre-dissolved effervescent tablet from Part I in the 100 ml volumetric flask. 
2. Transfer 10 ml of the solution from the volumetric flask into a 200 ml Erlenmeyer flask. Add 1 ml 2 M HCl and heat the solution to 90°C for 10 min while stirring.
3. Let the solution cool to room temperature. Neutralize the solution with 0.1 M aqueous sodium hydroxide until the pH is 7.
4. You may also dilute the solution a little more using deionized water for better visual monitoring later.
5. Add 1 ml buffer solution and a very small amount of Eriochrome black T.
6. Fill a burette with 0.1 M EDTA solution.
7. Titrate the solution while stirring constantly. A color change from red to blue should occur. The equivalence point is reached when the last hue of red disappears.
8. Repeat steps 2-7 at least two more times. You should end up with at least three titration results for a tablet.

Theoretical questions

Question 1

Why do we add 2 M HCl and heat the mixture in Part II.2?

Question 2

Write down the reaction equation for the titration in Part II.7.

Question 3

Calculate the whole magnesium content in your tablet. Show your work.

Would you like to discover all three parts of the experiment now - and more? You will find what you are looking for in the Chemistry Olympiad teaching material archive.