Back titration of antacid tablets calculations

Antacid Analysis and Titration Hands-On Labs, Inc. 42-0139-00-02

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

Experiment Summary:

You will use a back-titration technique to determine the amount of acid that a commercial antacid is capable of neutralizing. You will be introduced to experimental controls, and use a control to validate the antacid neutralization analysis.

EXPERIMENT

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Learning Objectives Upon completion of this laboratory, you will be able to:

● Identify and explore the causes of acid reflux disease.

● Investigate the relationship between antacid and gastric acid and define how antacids neutralize gastric acid.

● Define titration, equivalence point, and pH indicator.

● Compare and contrast titrations and back titrations.

● Review back titration calculations and explain how control experiments are used to support experimental results.

● Perform a titration, back titration, and control experiment.

● Determine how much acid an antacid is able to neutralize.

Time Allocation: 2.5 hours

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Experiment Antacid Analysis and Titration

Materials Student Supplied Materials

Quantity Item Description 1 Bottle of distilled water 1 Dish soap 1 Metal spoon 1 Pair of scissors 1 Roll of paper towels 2 Sheets of white paper 1 Source of tap water

2-6 Thick textbooks

HOL Supplied Materials

Quantity Item Description 1 Digital scale 1 Glass Beaker, 100 mL 1 Graduated cylinder, 10 mL 1 Pair of gloves 1 Pair of safety goggles 1 Short stem pipet 1 Syringe, 10 mL 1 Stopcock 1 Test tube cleaning brush 1 Test tube clamp 1 Experiment Bag: Antacid Analysis and Titration

2- HCl, 1 M, 30 mL in dropper bottle 1- Phenolphthalein solution, 1% - 0.5 mL in pipet 2- Sodium hydroxide, 1 M - 30 mL in dropper bottle 2- Antacid tablets

Note: To fully and accurately complete all lab exercises, you will need access to:

1. A computer to upload digital camera images.

2. Basic photo editing software, such as Microsoft Word® or PowerPoint®, to add labels, leader lines, or text to digital photos.

3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.

Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit.

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Experiment Antacid Analysis and Titration

Background Acid Reflux Disease

If you have ever wondered what antacids are, or more specifically the chemistry behind how an antacid works, you are not alone. Antacids are used to neutralize gastric acid, a substance secreted in the stomach to promote the digestion of food. Gastric acid is produced and secreted by specialized glands in the stomach, where it functions to break down the food we consume into smaller nutrient particles so they can be absorbed by the small intestine. Gastric acid is composed primarily of hydrochloric acid (HCl), glycoproteins, and enzymes, and has a pH close to 2.0. The stomach is lined with mucus, a natural secretion that withstands and protects the stomach from direct contact with the otherwise corrosive HCl. In a healthy digestive system, the gastric acid remains in the stomach. See Figure 1.

Figure 1. Digestive System. © Leonello Calvetti

The lower end of the esophagus, the muscular tube that transports food from the mouth to the stomach, is surrounded by a ring of muscles known as the lower esophageal sphincter. The lower esophageal sphincter acts to prevent the stomach contents from moving upward into the esophagus. When this sphincter malfunctions or is otherwise compromised, gastric acid refluxes (moves back) into the esophagus resulting in acid reflux disease. Acid reflux disease causes inflammation and irritation of the esophageal lining. See Figure 2.

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Experiment Antacid Analysis and Titration

Figure 2. Gastric acid refluxes past the lower esophageal sphincter (as noted by the two arrows in the top image) and enters the esophagus, causing what is commonly known as acid reflux

disease. © Alila Sao Mai

Antacid Neutralization and Titration

While there are many ways to treat acid reflux disease, including prescription drugs, surgery, and diet modifications, the initial treatment for controlling the symptoms of acid reflux disease is through the use of over-the-counter medications, including antacids. Antacids are basic substances that neutralize, or raise the pH, of gastric acid (primarily HCl). In a neutralization reaction, the acid and the base first dissociate in solution, producing hydrogen (H+) and hydroxide (OH-) ions respectively, which then react to produce a salt and water. Commercial antacids contain a wide variety of basic substances as their active ingredient, including aluminum hydroxide (Al(OH)3), magnesium hydroxide (Mg(OH)2), sodium bicarbonate (NaHCO3), and calcium carbonate (CaCO3). See Figure 3.

Figure 3. Antacid neutralization reactions. (Top Reaction) Antacid neutralization reaction with aluminum hydroxide (Al(OH)3) as the active ingredient. (Bottom Reaction) Antacid neutralization reaction with calcium carbonate (CaCO3) as the active ingredient. Note that both reactions react

with the HCl to form a salt and water, and in the case of calcium carbonate, a gas (CO2).

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Experiment Antacid Analysis and Titration

An antacid’s effectiveness is based both qualitatively, by the physical relief it provides, and quantitatively, by calculating the amount gastric acid neutralized by the antacid. The technique used to calculate the amount of gastric acid neutralized by an antacid is titration, or more specifically, back titration. Titration is a direct, quantitative, volumetric technique, where a solution of a known concentration (titrant) is added to a solution of an unknown concentration (analyte) until the equivalence point is reached. The equivalence point of a titration, also known as a stoichiometric point, is the moment in a titration where exactly enough titrant has been added to completely react with the analyte.

A back titration is an indirect, quantitative volumetric technique where a known quantity of reagent is added to a known volume and concentration of analyte, and allowed to react. It is expected that the reaction is not complete and some analyte remains in the solution. The amount of analyte remaining is determined in a second step, by a titration reaction. A solution of known concentration (titrant) is added to the solution until the equivalence point is reached, which is indicated by a change in color. Consider, for example, a back titration to determine the amount of gastric acid neutralized by an antacid with the active ingredient aluminum hydroxide (Al(OH)3). A known mass of the Al(OH)3 containing antacid is mixed with an excess known volume and concentration of HCl and allowed to react. The remaining HCl, which was not neutralized by the antacid, is then titrated with a known concentration of NaOH until the equivalence point is reached. In this example, the antacid is the substance, the HCl is the analyte, and the NaOH is the titrant. The most effective antacid will leave the fewest HCl molecules after the reaction, and require less of the NaOH in the back titration. This back titration method can easily be used to compare the effectiveness of different antacids. The most effective antacid will leave the fewest HCl molecules after the reaction and require less of the NaOH solution in the back titration step.

In either a direct titration or indirect titration, the equivalence point can be identified through use of a pH indicator. A pH indicator is a substance that changes color when the pH of a solution changes, allowing scientists to qualitatively measure the moment when the analyte has completely reacted with the titrant. A common indicator for a titration between a weak acid and a strong base is phenolphthalein. Phenolphthalein is a pH indicator, which turns bright-pink in solutions with a pH of 8.2 or higher. Thus, equivalence points in titrations are marked by the analyte changing color from colorless to bright pink. See Figure 4 for a schematic representation of the back titration process.

There are many commercial advertisements for both

prescription and over-the-counter drugs to help with symptoms of

gastroesophageal reflux disease (GERD). GERD is simply acid reflux disease that occurs chronically, resulting in similar

symptoms and treatment as acid reflux disease.

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Experiment Antacid Analysis and Titration

Figure 4. Schematic representation of the back titration process.

Calculating Results

The quantity of HCl neutralized by the antacid is calculated indirectly by 1) calculating the amount of HCl present in the initial sample and 2) calculating the amount of HCl neutralized by the NaOH in the back-titration step. The difference between these two is the amount of HCl neutralized by the antacid.

For example, calculate a back titration with 0.5 g of antacid (Al(OH)3), 20 mL of 1.5M HCl as the analyte, and NaOH with a concentration of 1.0M as the titrant. To reach the equivalence point, 27 mL of NaOH was required.

Step 1) Calculate the initial amount of HCl available for neutralization by the antacid.

Note: The molecular weight of HCl is calculated by adding the molecular weights of the two elements in the compound: H + Cl (1.008g + 35.45g), thus 1 mole of HCl is equal to 36.46g.

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Experiment Antacid Analysis and Titration

When an HCl solution is neutralized to the stoichiometric point with NaOH, the number of moles of NaOH required to reach the stoichiometric point is equal to the amount of moles of HCl present.

Step 2) Calculate the number of moles of NaOH required to reach the stoichiometric point (neutralize the excess HCl) after initial neutralization with the 0.5 g of Al(OH)3.

Step 3) Calculate the amount of HCl neutralized by the 0.027 moles of NaOH.

Step 4) Calculate the amount of HCl neutralized by the antacid.

Step 5) Calculate the gram per gram neutralization of HCl by the antacid.

3 3

0.11g HCl = 0.22 g HCl neutralized / 1g Al(OH) 0.5g Al(OH)

To double check the results of the calculations and to confirm the results of the back titration, a control experiment may be run. In a control experiment the variable tested (the antacid) is removed from the experiment as a tool to quantitatively confirm that results of the experimental design were set up to evaluate a single variable. A control experiment for the back titration of an antacid is to perform a titration between 20 mL of 1.5M HCl and 1.0 M NaOH, removing the antacid from the experiment. A positive confirmation in the control experiment would be for the moles of NaOH required to reach stoichiometric quantities to be equal to the initial number of moles of HCl present in the experiment. This result in the control titration would confirm that the antacid was indeed neutralizing some of the HCl in the test, since without the antacid, more NaOH is needed for the neutralization.

For example, assume that in a control experiment, 30 mL of 1.0M NaOH was required to reach stoichiometric quantities when titrated into 20 mL of 1.5M HCl.

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Experiment Antacid Analysis and Titration

As shown in the above equations, in this example control experiment, the moles of NaOH required to reach stoichiometric quantities and neutralize the HCl is equal to the number of moles of HCl present in 20 mL of 1.5M HCl. Additionally, the number of grams of HCl neutralized by 30 mL of 1.0M NaOH is equal to the total number of grams of HCl initially present in the back titration experiment. Furthermore, as the back titration required 27 mL of 1.0M NaOH to neutralize the excess HCl, following the initial neutralization with the antacid, the difference in NaOH volume (30 mL – 27 mL) between the back titration and control experiment should equal a neutralization of 0.11 g HCl.

As the calculations show, the control experiment verifies that the back titration was successful in quantitatively determining the amount of HCl neutralized by 0.5 g of antacid (Al(OH)3).

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Experiment Antacid Analysis and Titration

Exercise 1: Back Titration of Antacid Neutralization In this exercise, you will perform a back titration to determine the amount of acid that a commercial antacid is able to neutralize.

Procedure

Note: Please read all steps and safety information before starting the procedure.

1. Gather the test tube holder, small stopcock, 10-mL syringe (titrator), and either 2 thick textbooks and the lab kit box or 5-6 thick textbooks. See Figure 5.

Figure 5. Titrator and small stopcock.

2. Remove the plunger from the titrator and place it back in your kit.

3. Attach the stopcock to the tip of the titrator by placing the larger, clear, plastic end of the stopcock into the tip of the titrator and then twist the stopcock into place. The stopcock should fit tightly into the titrator so that the liquid will not leak. See Figure 6.

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Experiment Antacid Analysis and Titration

Figure 6. Fitting the stopcock into the titrator.

4. Stack the 5 textbooks or stack 2 textbooks on top of the lab kit box.

5. Clamp the test tube holder around the middle of the titrator and slide the long end under the top 2 books in the stack. Place a sheet of white paper next to the bottom of the stack and set the 100-mL beaker on the sheet of white paper. The end of the stopcock should be located near the top of the beaker, approximately 1 cm above to 1 cm below the top of the beaker. See Figure 7.

Figure 7. Titration setup. Note the location of the end of the stopcock. It is important that the placement of the titrator allows for the white knob to be easily adjusted. If this is not the case, then either adjust the location of the books in the stack or slightly adjust where in the test tube

clamp the titrator is located.

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Experiment Antacid Analysis and Titration

6. Use the pipet to fill the titrator with 7 - 9 mL of distilled water.

Note: You must use distilled water for this step and not tap water.

7. Using both hands, one on the titrator and one on the stopcock, practice releasing water from the titrator into the beaker. The goal is to be comfortable releasing only one drop at a time from the titrator. See Figure 8.

Figure 8. Proper hand positioning for titration. When the open circle is facing you, the titrator is closed, when the open circle is directly under the titrator spout, the titrator is open and liquid

will flow.

8. When you are comfortable using the titrator, pour the water in the beaker down the drain, remove the titrator from the test tube clamp, and remove the stopcock from the titrator. Thoroughly dry each of these 3 items with paper towels.

9. When all items are completely dry, reassemble the titration setup, as shown in Figure 7.

10. Put on your safety gloves and goggles.

11. With the stopcock in the closed position, fill the titrator with 9 - 10 mL of the 1.0M NaOH.

12. Move the beaker away from the titrator and place a crumpled paper towel directly below the titrator.

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Experiment Antacid Analysis and Titration

13. Using the stopcock, allow a few drops of the NaOH to flow through the titrator into the paper towel. This will fill the tip of the titrator with NaOH solution and remove any air bubbles from the titrator.

14. Place the paper towel with the NaOH drops into the trash.

15. On a sheet of clean, white paper, use a clean spoon to crush an antacid tablet into a fine powder. See Figure 9.

Figure 9. Crushing and antacid tablet. A. Use the back of a spoon to carefully crush the tablet. B. Tablet should be crushed into a fine powder, as shown.

16. Place the clean, dry beaker on the scale and tare the scale so it reads 0.0 g. See Figure 10.

Figure 10. Tared scale with 100-mL glass beaker.

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Experiment Antacid Analysis and Titration

17. Use the clean spoon to carefully weigh 0.5 g of the crushed antacid tablet into the glass beaker on the tared scale. Record this mass of the crushed antacid in Data Table 1 in your Lab Report Assistant.

Note: Save the remaining crushed antacid powder and second tablet for the second trial.

Note: The glass beaker is acting as the weigh boat in this step.

18. Put on both the safety gloves and goggles.

19. Use the graduated cylinder to measure exactly 5 mL of 1M HCl.

20. Pour the 5 mL of HCl into the 100-mL beaker containing the 0.5 g of crushed antacid.

21. When the HCl is added to the antacid a fizzing reaction (formation of gas) will occur. While carefully holding the beaker, swirl the beaker to thoroughly mix the antacid into the HCl until the fizzing subsides.

Note: There may be a few granules of antacid that do not incorporate into the HCl, this is normal.

22. Cut off the tip of the phenolphthalein pipet with scissors and add 1 drop of phenolphthalein to the antacid/HCl mixture in the beaker.

23. Carefully swirl the mixture in the beaker to ensure that the indicator is incorporated into the antacid/HCl mixture; the solution will be colorless and clear.

24. Place the beaker containing the antacid/HCl/indicator solution back in the titration setup, under the titrator.

25. Read the volume of NaOH in the titrator and record in Data Table 1 next to “Initial NaOH Volume (mL)” under Trial 1.

26. Open the stopcock and add 1 drop of NaOH to the colorless and clear antacid/HCl/indicator sample in the beaker. After the drop is added, gently swirl the beaker and observe the color for 5 seconds.

Note: It is important to add the NaOH 1 drop at a time to accurately identify the equivalence point and avoid overshooting the titration.

27. Continue adding NaOH to the beaker, 1 drop at a time, swirling and observing after each drop until the color changes to a pale-pink color for at least 5 seconds. See Figure 11.

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Experiment Antacid Analysis and Titration

Figure 11. Endpoint of titration. The titration is complete when the color changes to a bright pink for at least 5 seconds.

28. Read the volume of the NaOH solution remaining in the titrator and record this volume in Data Table 1 next to “Final NaOH Volume (mL)” under Trial 1.

29. Calculate the total volume of NaOH used by subtracting the final NaOH volume from the initial NaOH volume and record the total volume in Data Table 1.

30. Leave the titrator assembly intact. You will need it for future titrations in this experiment.

31. Pour the contents of the beaker down the drain and flush the drain with water. Thoroughly wash the beaker with soap and water to remove all of the antacid/HCl/NaOH/indicator solution from the beaker. When the beaker is clean, rinse the beaker with distilled water and then thoroughly dry.

32. If necessary, add more NaOH to the titrator.

Note: It is only necessary to add more NaOH to the titrator if there is less than 1 mL more than the total volume of NaOH used in the previous trial. For example, if the total volume of NaOH used in Trial 1 was 2.1 mL, then needs to be at least 3.1 mL of NaOH in the titrator.

33. Repeat steps 16 through 31 one additional time (Trial 2).

34. Average the results from the 2 trials (mass of antacid and total volume of NaOH used) and record the two averages in Data Table 1.

Note: Leave the titration setup intact (including the remaining NaOH in the titrator) for use in Exercise 2.

Using the averages from Data Table 1 and the sample calculations provided in the Introduction section:

35. Calculate the initial amount (grams) of HCl available for neutralization by the antacid. Record in Data Table 2 in your Lab Report Assistant.

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Experiment Antacid Analysis and Titration

36. Calculate the number of moles of NaOH required to reach the stoichiometric point (neutralize the excess HCl), after initial neutralization with the antacid. Record in Data Table 2.

37. Calculate the amount (grams) of HCl neutralized by the antacid. Record in Data Table 2.

38. Calculate the gram per gram neutralization of HCl by the antacid. Record in Data Table 2.

39. Leave all materials set up for Exercise 2.