Practical Chemistry

Experiment 2: Analysis of an indigestion medicine


To analyse the magnesium hydroxide content of a medicine by a back titration method


Back titration allows analysis to be performed in situations where direct titration is not possible. Magnesium hydroxide is only slightly soluble in water, which makes direct titration impossible. In back titration the magnesium hydroxide sample is neutralised by a known quantity of a strong acid. The excess acid is subsequently titrated against standardised sodium hydroxide solution and this excess acid calculated from the results. From a knowledge of the original quantity of acid and the excess, the acid used in neutralising the magnesium hydroxide may easily be calculated, and hence the quantity of magnesium hydroxide in the sample.


Prepare a standard sodium hydroxide solution and use this to find the strength of the bench sulphuric acid by titration (the bench acid must first be diluted to around 0,1M) using phenolphthalein indicator.

Assuming that the quantity of magnesium hydroxide in a 5ml sample contains about 0,05 moles of Mg(OH)2 calculate the volume of sulphuric acid needed that will allow the excess to be made up to a 250ml soution of about 0,1M acid. (if you cannot do this here is the help sheet)

Add your sample of medicine to the measured quantity of acid and stir well until clear (gently warming may be necessary)

Make up the resultant solution to 250ml using a volumetric flask and titrate against the standard sodium hydroxide solution using phenolphthalein indicator.


Experimental report

The following factors must be considered:

  • Accurate recording of raw data
  • Presentation of raw data
  • Data analysis and presentation
  • Evaluation of errors both qualitative and numerical


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