Last Updated February 25 th, 2005/MM
Click here to go to the Power Point
Click here to go to the Data Sheet
Objective
The goal of this experiment is:
You will be assessed on:
The Report Form is to be filled out and turned in your next lab session.
Link to Data Sheet to be posted
Introduction
The parietal cells in the stomach secrete hydrochloric acid at a concentration of about 0.155 M HCl, quite concentrated! The flow of HCl increases when food enters the stomach. When you eat or drink too much, your digestive system may generate too much acid. You may develop a condition called "heartburn" or indigestion. Antacids are swallowed to neutralize this excess acid and "relieve" but not eliminate the condition. The reaction that takes place is an acid/base reaction. A little bit of NaOH might be equally effective, but it's quite rough on the rest of the digestive system, so antacids need to be formulated to reduce acidity while avoiding physiological side-effects. Many antacids use CaCO3 for this purpose.
In addition to the active ingredient (base), tablets may also contain flavors, sweeteners, binders, fillers, antifoam agents, pain relievers (aspirin), etc. In this experiment, the tablets will be analyzed only for their ability to neutralize acids. The base in antacids varies with the brand. The tablet lists the active agents in several brands.
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Acids are neutralized by these bases as illustrated below.
BiO(HOC6H4COO) + 3 H+(aq) -> Bi3+(aq) + H2O(l) + HOC6H4COOH(s)
Mg(OH)2(s) + 2 H+(aq) -> Mg2+ (aq) + 2 H2O(l)
Al(OH)3(s) + H+(aq) -> Al(OH)2+ (aq) + H2O(l)
Al(OH)2+ (aq) + H+(aq) -> Al(OH)2+ (aq) + H2O(l)
CaCO3(s) + H+(aq) -> Ca2+(aq) + HCO3- (aq)
HCO3- (aq) + H+(aq) -> CO2(g) + H2O(l)
In this simple experiment you will find the neutralizing capacity of various commercial antacid tablets. To test their capacity to neutralize acid, we will first dissolve an appropriate and measured amount of the sample in a simulated stomach environment. This is a solution containing a known quantity of HCl that will react with all of the antacid and still leave some extra HCl. Then we will determine how much of the original HCl remains by titrating it to neutrality with a standardized solution of NaOH. Simple subtraction will reveal how much of the acid was neutralized by the antacid tablet. This general method of analysis is called back-titration.
If NA is the number of moles of HCl that we use for dissolving the antacid sample, and NB is the number of moles of NaOH needed to back-titrate the excess HCl, then Nsample, the number of moles neutralized by the sample, is given by:
Remember that the number of moles in a volume of solution equals the concentration multiplied by the volume.
Analytically, it is important to remove carbon dioxide.
Dissolved carbon dioxide is converted into the weak acid, carbonic acid (H2CO3) which reacts with NaOH. Although the deionized water is free of most impurities, it does contain dissolved carbon dioxide that must be removed by boiling to give accurate titration results. All of the antacids that you titrate contain carbonates. When you acidify the antacid sample with standard HCl, the carbonates are converted into carbonic acid that must be boiled off in the form of CO2.
In this experiment you will titrate one commercial antacid twice. Each lab section will compile their results and decide which antacid is the "best buy" in terms of neutralizing ability per dollar. The general approach to this quantitative determination is volumetric.
EXPERIMENTAL
Part 1:Titration of Antacids
1. Your TA will randomly assign you on of the commercial brands of antacid tablets; which you will analyze it in duplicate.
2. Weigh one whole tablet using the analytical balance. Record the mass of the intact tablet. Then break or cut the tablet to obtain pieces roughly the size indicated in the table below, and weigh two of these pieces separately on sheets of weighing paper. (Don't forget to subtract the weight of the paper.) After weighing, fold over the paper and gently crush the sample fragments using the round portion of your padlock.
3. Use a transfer pipet to dispense 25.00 mL of standardized HCl into a clean 125 mL Erlenmeyer flask and add about 20 mL of deionized water. This solution will be used to dissolve the antacid tablet sample.
4. Transfer one of your duplicate crushed samples (without any spilling) into a flask containing the HCl solution. Label each flask to keep track of which sample and which portion is being titrated.
5. Warm gently to dissolve the sample and then boil for one or two minutes. Some components of the samples may remain undissolved, but these will not cause problems.
6. Cool the outside of the flask with tap water and add a few drops of methyl purple indicator solution. The flask should now be purple in color. (If it is green instead of purple, you have used too large a fragment and neutralized all of the HCl.)
7. Titrate with your standardized NaOH solution until you reach the endpoint, a change in color from purple to green (or from purple to clear). Some samples may not give color changes as sharp as for the HCl standardizations; for these use your best judgment to estimate the endpoint. Endpoints will generally be sharper for quick titrations than for slow ones.
8. Repeat the above procedure until you have obtained duplicate determinations. Enter your data in an excel spreadsheet in the workrooms, check the web page on Friday and make a final comment on the overall cost to neutralize one mole of HCl for various brands.
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