0.4 Indigestion? which is the best commercial antacid?  (Page 2/4)

Remember that the number of moles in a volume of solution equals the concentration multiplied by the volume. $(n=M\times V)$

Dissolved carbon dioxide is converted into the weak acid, carbonic acid $(H_2{\text{CO}}_3)$ 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 ${\text{CO}}_2$ .

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 Procedure

Titration of antacids

Your TA will randomly assign you one of the commercial brands of antacid tablets which you will analyze twice.

1. Weigh one whole tablet using the analytical balance. Record the mass of the intact tablet.

2. Break or cut the tablet to obtain pieces roughly the size indicated in the table below (e.g.. ½, ⅓, etc.) and weigh two of these pieces separately on sheets of weighing paper. (Don't forget to subtract or tare the weight of the paper.) After weighing, fold over the paper and gently crush the sample fragments using the round portion of your padlock, so it dissolves faster.

3. Use the burettes around the periphery of the lab 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 fraction you weighed of the antacid tablet.

4. Transfer one of the weighted crushed samples (without any spilling) into the flask containing the HCl solution. Repeat for the other weighed fraction in another flask and label each flask (1 and 2) to keep track of which sample and which portion is being titrated.

5. Warm gently to dissolve the sample and then boil solution for about a minute. Some components of the samples may remain undissolved, but these will not cause problems.

6. Let the flask cool down sitting on the bench for couple minutes, and then cool the outside of the flask with tap water.

7. When room temperature is reached 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 have neutralized all of the HCl. (In this case add 5mL of HCl and observe the color change. If needed add measured amounts of HCl little by little until solution is purple. Remember to record the amount of HCl added and add to the first 25mL of HCl.)

8. It is important that you read Titration Tips in Supporting Information below before you titrate, or you may have to repeat titration several times.)

9. Titrate with your standardized NaOH solution until you reach the endpoint, a change in color from purple to green. 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.