Analysis of an antacid tablet

Most people at one or another have suffered from indigestion or heartburn. In many people, these symptoms arise when the pH of the stomach falls below 3.0. The observation suggests that one way to avoid or overcome these distressing symptoms is to neutralize any excess acid with a non toxic chemical reagent that maintains the contents of the stomach at pH above 3.0.

A number of readily available commercial preparations have been designed to fill this need. One of the most familiar is milk of magnesia. Your objective of this part of the experiment is to determine the magnesium hydroxide content in these tablets. You, like the manufacturer, assume that stomach acid is approximately 0.10M HCl acid. In the stomach, some of the hydrogen ions combine with other species and keep the pH above 3.0. The procedure involves dissolving a weighed milk of magnesia tablet in an excess of simulated stomach acid whose volume has been carefully measured. The antacid tablet neutralizes a specific amount of the acid. You then determine the volume of unneutralised acid by titrating the resulting solution with standard NaOH solution.


Theory

For magnesium hydroxide

pKb1 = pKb2

When an excess standard solution of HCl is added, to a sample of milk of magnesia,

 

Procedure

Weigh the antacid tablet. Grind the tablet and put all the powdered material into a beaker. Dissolve this in approximately 200mL of 0.1M HCl acid. The solution may contain a small amount of undissolved, unreactive residue but this does not interfere with the analysis. Filter the solution in to a 250 mL volumetric flask and dilute up to the mark with 0.1M HCl. Pipette 25.00 mL into a titration flask, add few drops of a suitable indicator and titrate with 0.1M NaOH solution.

I. Calculate the percentage of magnesium hydroxide in the tablet
II. Why magnesium hydroxide cannot be titrated directly with HCl? (Hint: think of solubility, rate of the reaction and indicators)