Chem 197 - Spring 1998
Antacid Analysis
I. Introduction
All antacids serve to relieve acid indigestion with varied degree of effectiveness, by neutralizing the excess hydrogen ions in the stomach. To decrease the possibility of the stomach becoming too basic from the antacid, buffers are added as part of the formulation of some antacids. In this experiment, the neutralizing powers of several antacids are determined using a strong acid-strong base titration. To obtain the quantitative data for the analysis, which requires a well-defined end-point in the titration, the buffer action must be destroyed. The buffer action is destroyed by adding an excess of HCl to the antacid solution. The antacid solution is then heated to remove carbon dioxide and the excess HCl is titrated with a standardized NaOH solution. This analytical technique is referred to as a back titration.
II. Procedure
1. Measure roughly 0.2 g (? 0.001 g) of the pulverized commercial antacid tablet. Transfer the sample to a small Erlenmeyer flask.
2. Pipet 25.0 mL of standardized 0.1 M HCl into the flask and swirl to dissolve. Heat the solution to a gentle boil for 1 minute to remove dissolved carbon dioxide. Add 4-8 drops of bromophenol indicator . If the solution is blue, pipet an additional 10.0 mL of 0.1 M HCl into the solution and boil again. Repeat as often as necessary. Record the total volume of HCl added.
3. Obtain about 75 ml of standardized 0.1 M (note: this is approximate) NaOH solution. It is important that you record the precise concentrations of the NaOH solution you are using. Prepare the buret for titration and fill it with the NaOH solution.
4. Once the antacid solution has been cooled, carefully titrate the sample with the NaOH solution to a blue endpoint. When a single drop of NaOH solution changes the sample from yellow to blue, stop. Wait for 30 seconds and then read the final volume of NaOH solution in the buret (? 0.02 mL).
5. Repeat the titration of the same sample.
III. Calculations
1. Calculate (mol base in antacid/ mass of antacid) (mol/g)
2. Calculate (average mol base in antacid/ mass of antacid) (mol/g) HINT The number of moles of base in the antacid of the commercial sample plus the number of moles of NaOH used in the titration equals the number of moles of HCl added to the original antacid sample: moles(antacid) + moles (NaOH) = moles (HCl) A rearrangement of the above equation provides the number of moles of the base in the antacid in the sample. The moles of base in the antacid per gram of antacid provides the data required for a comparison of the antacid effectiveness of commercial antacids.