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How can we determine the effectiveness of commercial antacids?

Introduction Stomach acid is hydrochloric acid, HCl. It is a strong acid and, in the stomach, its concentration ranges from approximately 0.10 M to 0.001 M. Although HCl is necessary for food digestion, excess HCl can cause indigestion and heart burn when the stomach pH remains below 3.0 ([H+] > 0.001 M) for an extended period of time. More than 600 tablets, liquids, powders, lozenges, gums and pills to "soothe indigestion" are available to the public. After evaluating the safety and effectiveness of antacid ingredients, the U.S. Food and Drug Administration has reached the conclusion that there is no ideal antacid on the market. Some brands of antacids will neutralize greater quantities of acid than others, and some act more quickly, but most produce undesirable side effects. Some antacids contain sodium bicarbonate as the active ingredient. Sodium bicarbonate is fast-acting, but it can affect the bladder and kidneys upon prolonged use. Also, because of their high sodium content, these antacids are not recommended for people with kidney disorders or for people on salt-restricted diets. Sodium bicarbonate neutralizes stomach acid in the following reaction:

NaHCO3(aq) + HCl(aq) H2O(l) + CO2(g) + NaCl(aq) Other antacids contain calcium carbonate as the active ingredient. Calcium carbonate is reasonably fast-acting, but it may stimulate the stomach to secrete more HCl (called acid rebound), and prolonged use tends to cause constipation and impaired kidney function. Calcium carbonate neutralizes stomach acid in the following reaction:

CaCO3(s) + 2HCl(aq) H2O(l) + CO2(g) + CaCl2(aq) Other antacids contain aluminum hydroxide and magnesium hydroxide as the active ingredients. Antacids containing aluminum hydroxide and magnesium hydroxide are relatively safe and have minimum side effects. Aluminum hydroxide is slow acting, but it provides long lasting antacid action. They neutralize stomach acid in the following reactions:

Al(OH)3(s) + 3HCl(aq) 3H2O(l) + AlCl3(aq)

Mg(OH)2(s) + 2HCl(aq) 2H2O(l) + MgCl2(aq) Some antacids contain aspirin, which can further cause stomach irritation in people suffering from ulcers. Other brands contain caffeine, which stimulates the stomach to secrete more stomach acid. The purpose of this experiment is to measure how much HCl is consumed by different types of antacids, and to compare their relative effectiveness at neutralizing HCl. Further, the relative economy of use and the active ingredients will be evaluated. With this data, you can decide which brand of antacid is most effective, economical and has the least side effects.

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The straight-forward method to neutralize the antacid would be by direct titration with 0.10 M HCl. Unfortunately, titration by this method will not work because most antacids contain buffers that make it difficult to determine when all the antacid has reacted. An approach called a "back titration" will be used in this experiment. It avoids the above mentioned buffer problem by adding a known volume of 1.00 M HCl that is in excess of that needed to neutralize the antacid and also overwhelms whatever buffers may be present. Therefore, only part of the HCl added actually neutralizes the antacid. All that is required is to then neutralize the unreacted HCl with NaOH. The difference between the moles of HCl added initially to the antacid and the moles of unreacted HCl determined by the titration with NaOH equals the moles of HCl that the antacid neutralized. Because a strong acid-strong base titration is being performed, the indicator phenolphthalein can be used to quantitatively determine the endpoint.

Using the titration results and the 1-to-1 stoichiometry of the HCl-NaOH reaction, the amount of HCl that is neutralized by the antacid is easily calculated by subtraction:

molesHCl(antacid) = molesHCl(initial) molesNaOH(used in backtitration)

In the above calculation, molesNaOH(used in backtitration) is equal to the moles of HCl which remained unneutralized by the antacid.

Using the molar mass of HCl, it is therefore easy to conform to the commercial description of antacid effectiveness, grams of acid neutralized. Procedure Today a technique called back titration is used to determine the effectiveness of an antacid sample in neutralizing excess stomach acid. Back titrations are often used when samples are buffered (more on that later!). Since many antacids are buffered, a precisely known, but excess amount of acid will be added to the antacid to overcome the buffering. The amount of acid added in excess will be accurately determined and the amount of excess acid consumed by the antacid sample calculated. Stepwise instructions are provided here.

1. Record the data clearly in your lab notebook in neat tables as you obtain it from the

experiment. Label everything! Include units! 2. Choose an antacid and record the brand of antacid to be tested. Crush one antacid

tablet using a mortar and pestle. Weigh the resulting powder on a weigh boat to the nearest 0.01 gram and record the weight of the antacid in your lab notebook as antacid #1. Transfer all of the antacid powder to a clean 250 mL Erlenmeyer flask.

3. Dissolve the powder in approximately 25 mL H2O. Use some of the water to rinse the

weigh boat if necessary. Observe and record the color of the solution. Note that some antacids are not too soluble and will not completely dissolve.

4. Add 8 drops of phenolphthalein indicator solution and observe the color of the solution.

Was your prediction (in 3.) that the solution is acidic, basic or neutral confirmed, or would you like to change your description of the solution? Briefly, explain your reasoning.

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5. Using a volumetric pipet, add 20.00 mL of ~1.0 M HCl to the flask and mix well. Record

the exact concentration of HCl as shown on the stock bottle. DO NOT PUT PIPETS IN THE STOCK BOTTLE. Pour ~25 mL into a beaker or flask, and take that to your station before filling the pipet. Record any color changes observed in the solution. If the solution is colorless, proceed to step 6. If the solution is not colorless, use a Mohr Pipet to add 1 or 2 mL aliquots of ~1.0 M HCl until the solution stays colorless. At this point, the solution is acidic and we have added an excess of 1.00 M HCl for our back titration. Record the total volume of HCl added in the table.

6. Fill a 50 mL buret with ~1.0 M NaOH. Record the exact concentration of the NaOH as

shown on the stock bottle. Run some solution thru the buret tip to remove any air bubbles and record the initial reading of the volume of solution in the buret (Vi).

7. Titrate the solution in the flask with the 1.0 M NaOH solution until the endpoint is

observed. The faintest color should persist for at least 30 seconds. Record the final reading of the volume of solution in the buret (Vf). At this point the unreacted HCl added in step 1 has been exactly neutralized by the back titration.

8. Determine the volume, Vf Vi, of 1.00 M NaOH used to neutralize the unreacted HCl. Record this volume of 1.00 M NaOH. Post lab report should include followings in addition to summary of data

1. Determine the number of moles of HCl added to the antacid in step 5. 2. Determine the number of moles of NaOH added to the acidified antacid in step 8. 3. Write an equation to determine the number of moles of HCl neutralized by the antacid

solution you prepared.

4. Determine the mass of HCl neutralized by the antacid solution you prepared. 5. Calculate the mass of HCl needed to neutralize 1.00 g of the antacid sample. This

calculation provides a way to compare the effectiveness of different antacids. Record this as mass of HCl/gram antacid, in units of g HCl per 1.0g antacid.

6. Record the cost per tablet in cents per tablet as shown on the container from which you took your sample.

7. Calculate the cost per gram of the antacid tablet and record it in cents per 1.0 g of tablet. 8. Calculate the economy of use: Divide the mass HCl for 1g antacid (question 5) by the

cost per gram of antacid. This will give us the mass of HCl consumed by the antacid per penny. Record your answer.

9. Which is the most effective antacid tablet? 10. Which is the most effective antacid per gram antacid? 11. Which is the most economical antacid?

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Pre Lab Report

1. Given the following data, answer the questions.

Brand of antacid CHEM1212K Weight of antacid tablet 2g Major base in the CHEM1212K sodium bicarbonate

(sodium hydrogen carbonate)

Volume of 1M HCl added into the antacid 20 mL Initial volume reading of 1M NaOH in buret 3 mL Final volume reading of 1M NaOH in buret 15 mL

a. How many mL of 1M NaOH used in back titration?

b. How many moles of 1M NaOH used in back titration?

c. How many moles of 1M HCl was added into antacid?

d. How many moles of 1M HCl was neutralized by 1M NaOH?

e. How many moles of 1M HCl was neutralized by antacid?

f. How many moles of sodium bicarbonate in the 2g of antacid?

g. How many grams of sodium bicarbonate in the 2g of antacid?

h. What is the % mass of sodium bicarbonate in the antacid?