Practical investigations
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I have been asked to determine the % purity by mass of magnesium in a corroded sample of magnesium ribbon using any acid. The corrosion layer is mainly a mixture of magnesium oxide, magnesium hydroxide and magnesium carbonate. I am unsure as to how I am going to calculate this as both hydrogen and carbon dioxide are evolved. The task also does not mention alkalis so I am not sure if a titration is expected. Which method is best - help would be much appreciated!
Ulex replies
I can’t see that titration would be of much help here. The acid would react with magnesium and its oxide, carbonate and hydroxide. I would begin with some qualitative experiments designed to find out whether the carbonate is present in significant amounts – treat with acid, collect the gas evolved and test for carbon dioxide.
Reacting a weighed sample of the ribbon with excess acid produces hydrogen and carbon dioxide together with, inevitably, some water vapour. Passing the gas over calcium oxide removes both the water vapour and the carbon dioxide, leaving only the hydrogen which can be collected in a gas syringe and its volume measured. I’ll leave you to design the apparatus! The calcium oxide needs to be fresh – old stock will have absorbed water and carbon dioxide from the air.
I also have a suspicion that magnesium nitride might be present, Mg3N2. If it is, warming with sodium hydroxide solution produces ammonia. This is not important to your problem but it would be quite interesting to know!
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updated: 13 January 2007
