Determination of the magnesium and calcium content in effervescent tablets
Are you a teacher looking for experiment ideas? The Chemistry Olympiad is at your service! Our volunteers have broken down an experiment from the final round into three bits suitable for a lesson which will be published here. This is the third and last part.
In the laboratory at the Chemistry Olympiad final. (Source: Luca Ferrari, ETH Zurich)
Duration: ca. 2 h Difficulty: High
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 or Plastic pipettes
Hot plate and stir bar
Mortar and pestle
pH paper
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
Effervescent tablets containing magnesium and calcium
Murexide
Procedure
Part I: Dissolving the Tablet
Dissolve the 1st 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 HCl until the tablet is completely dissolved.
Fill the volumetric flask up to the mark with deionized water.
Part II: Determination of the total divalent cation content
The following operations are carried out using the solution obtained in Part I.
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.
Let the solution cool to room temperature. Neutralize the solution with 0.1 M aqueous sodium hydroxide until the pH is 7.
You may also dilute the solution a little more using deionized water for better visual monitoring later.
Add 1 ml buffer solution and a very small amount of Eriochrome black T.
Fill a burette with 0.1 M EDTA solution.
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.
Repeat steps 2-7 at least two more times. You should end up with at least three titration results for a tablet.
Part III: Specific determination of individual ion contents
Continue using the tablet solution you prepared in Part I.
Transfer 10 ml of said solution into a 200 ml Erlenmeyer flask. Again, add 1 ml ofM HCl and heat the solution to 90°C for 10 min while stirring.
Let the solution cool to room temperature. Neutralize the solution with 0.1 Maqueous sodium hydroxide until the pH is 7.
Fill a burette with 0.1 M EDTA solution.
Work quickly now. Carefully add more 0.1 M aqueous sodium hydroxide until youreach a pH of ≥ 12. Monitor with pH paper.
Add a very small amount of murexide with the tip of a spatula.
Titrate the solution with 0.1 M EDTA solution while stirring. A color change from redto violet should occur. The equivalence point is reached when the last hue of reddisappears.
Repeat steps 2-7 at least two more times.
Theoretical questions
Question 1
Why do we add 2 M HCl and heat the mixture in Parts II and II?
Question 2
Write down the reaction equation for the titration in Part II.
Question 3
Which divalent ion content is determined in Part III and why only this one in Part III?
Question 4
Calculate the individual contents of calcium and magnesium in the tablet.
Question 5
Calculate the individual contents of calcium and magnesium in the tablet.
Would you like to discover all three parts of the experiment - and more? You will find what you are looking for in the Chemistry Olympiad teaching material archive.