Experiment #10: Analysis of Antacids

Experiment #10: Analysis of Antacids Purpose: In this experiment you will prepare one solution that is approximately 0.1 M NaOH. Then you will standardize this solution, which means that you will experimentally and accurately measure its concentration. To do this, you will conduct acid/base titrations with a solution of HCl that has a known concentration. The accuracy of your buret and pipet will make it possible to measure the concentration of the NaOH solution to three or four significant figures. Once you have standardized this solution, you will analyze the neutralizing power of two non-prescription antacids. Introduction: By the Arrhenius Theory (the first acid/base theory), an acid is a substance that produces H + in water and a base is a substance that releases or produces OH - in water. If you mix an acid and a hydroxide base together, they will react together to produce water and a salt. This is called a neutralization reaction, as the acid and base neutralize each other or consume each other to make products. However, the ph is not necessarily 7! This reaction is also an example of a double displacement reaction. HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) There is a more general type of acid/base rxn where an acid and a base react together to form an ionic salt and a weak electrolyte, or a nonelectrolyte, or even a gas. These are also double displacement rxns and they are also called neutralization reactions although water is not necessarily made. Note that the reacting base is usually written as an ionic salt. 2HCl(aq) + Na 2 CO 3 (aq) 2NaCl(aq) + H 2 CO 3 (aq) where H 2 CO 3 (aq) H 2 O(l) + CO 2 (g) In the lab, we do neutralization rxns all the time as titrations. In a titration, a known volume of an acid (or base) is added to a flask, and then a known concentration of a base (or acid) is added slowly from a buret (titrated). The titration is followed with a ph meter and/or an acid/base color indicator so that it is known when the neutralization is complete (UV-Vis or voltage may also be followed). The point at which the acid and base have neutralized each other stoichiometrically is called the equivalence point. If an acid/base color indicator is used, there is a color change (and ph change) when the endpoint is reached, which should be very close to the equivalence point (if you picked the right indicator). When the titration is complete, the volume of base (or acid) added via the buret is recorded. What is very important to remember for any acid/base titration is this: at the equivalence point, the acid/base reaction is complete, that is the acid and base reactants have been consumed. Titrations are useful for several purposes: To determine the K a or K b of an acid or base To determine the molecular weight of an acid or base To standardize, or determine accurately, the concentration of an acid or base. In this experiment, you will be conducting titrations in order to determine the concentrations of acids and bases. On day 1, you will be standardizing the NaOH solution. For these titrations, you will use the acid/base color indicator called thymol blue which is pink in highly acidic solutions, then yellow in weakly acidic solution, and is green to blue in basic solution. Thus, when the titration is complete, a yellow solution will turn just barely green or greenish-blue. Chem 400 Exp #10 11/4/15 Revision M Samples p. 1

On day 2, you will acidify (add excess acid) 2 different antacids solutions with HCl, and then titrate with NaOH. Antacids are simply bases, so when you take an antacid, it will undergo an acid/base neutralization reaction in your stomach. Different antacids contain different bases. For the antacid titrations, you will use the color indicator bromocresol green, which is also yellow under acidic conditions, and then turns green and then blue when the solution becomes more basic. The color change you will be watching for will be from yellow to green or greenish-blue. Based on your Day 2 data, you will be able to calculate how many ml of stomach acid each of the antacids is able to neutralize. Chemicals: 0.1000 M HCl 6 M NaOH DI water thymol blue aq sln Maalox or generic substitute Tums or Rolaids or generic substitute bromocresol green aq. sln (bromcresol green) Materials: 1 L plastic bottle 25 ml graduated cylinder Funnel 125 ml Erlenmeyer flask 50 ml buret 10, 20 ml volumetric pipet 150 ml beaker 100 ml beaker Stir rods Ring stand & buret clamp Mortar and pestle ph paper Plastic Box Waste beaker Safety/Disposal: 6 M NaOH should be handled with care. Wear goggles, gloves, and a lab coat at all times. If spilled, immediately inform the instructor. All titration wastes go into the liquid waste container. At the end of the two-day lab, excess NaOH may be poured into the liquid waste container; or it may be neutralized so the ph is between 4 and 10 before pouring down the drain with excess water. Procedures: General Titration Notes: 1) Always check the buret stopcock before using to make sure that it is screwed in securely (but not too tight or it is hard to open it), and it is in the off position. 2) Don t fill buret to 0.0 ml or titrate past 49.8 ml (on a 50 ml buret); estimate between the lines! 3) Place a piece of white paper under the titration flask so that small changes in color may be seen. 4) Gently swirl the flask while titrating. 5) Use a DI wash bottle to rinse the sides of the titration flask as necessary during the titration. 6) As the endpoint is neared, half-drops may be added. To get a halfdrop, slowly start to open the stopcock so that a drop begins to form. When about a half-drop has formed on the buret tip, close the stopcock. Gently flick the buret near the tip with your finger. The half-drop may need to be washed into the flask with the DI wash bottle. 7) Also record the buret volume before adding the next drop or half-drop. Thus, if the next drop is past the endpoint, the prior reading would be the endpoint. If the endpoint seems to be between 2 buret readings, you can average the two buret readings. Chem 400 Exp #10 11/4/15 Revision M Samples p. 2

Day 1 Procedure: Preparing NaOH Sln & Standardizing the NaOH Sln Note: Although you will be conducting all of the titrations individually, you will conduct Steps 1-3 with another student (your solution partner) and share the NaOH solution for all of the titrations. Obtain a piece of colored tape and mark it with your names and put on both bottles. 1. Obtain a 1 L prelabeled plastic bottle labeled "NaOH". Fill it half-full with DI water using a beaker to get roughly 500 ml. 2. Using a 25 ml cylinder, add 8-9 ml of 6 M NaOH to the bottle marked "NaOH". Cap the bottle and swirl or gently shake the solution for 5 minutes. This solution should have a concentration of roughly 0.1 M; but to determine a more accurate and precise concentration value, you will standardize it. For the rest of the lab, you will work individually, but you will continue to share this solution with your solution partner. 3. Put your bottle with the NaOH solution in a plastic box (plastic shoebox) to give it stability and to contain spills. 4. To standardize the NaOH solution, obtain 40-50 ml of a 0.1000M HCl solution in a clean and dry 100 ml beaker. Try not to get more than 50 ml! Label this beaker as 0.1000M HCl. 5. Obtain a 10 ml volumetric pipet and rinse it with DI water, and then the 0.1000M HCl solution. 6. Pipet 10.00 ml of this HCl solution into a clean 125 ml Erlenmeyer flask. Add 3 drops of the aqueous thymol blue indicator to the flask. The HCl solution should now be pink as this is a highly acidic solution. 7. Get a 50 ml buret, and rinse it with DI water. Then rinse the buret with a small amount of your NaOH sln, draining this rinsate into a waste beaker (or your 400 ml beaker). Fill the buret with the NaOH sln using a funnel and a labeled 150 ml beaker. Allow a small amount to drain from the tip into the waste beaker to clear any air bubbles. Record the initial volume of NaOH, making sure it is below the 0.0 ml line. 8. Make sure that you have a piece of white paper underneath the titration flask to more accurately see the color change during the titration. While swirling the HCl solution in the flask, add the NaOH sln from your buret to the contents of the flask until the pink solution mixture acquires a pale green or greenish-blue color that lasts for at least 30 seconds. Note: it will first go through peach, then yellow, before turning green or green-blue or blue. 9. Record the final volume of the NaOH sln in the buret. 10. Pour the contents of the flask into the labeled waste container. Rinse the 125 ml Erlenmeyer flask 3 times with tap water and 2 times with DI water. 11. Do Data Analysis Step 1. 12. Repeat the titration of 10.00 ml of the HCL solution with your NaOH solution two more times. Hopefully, you will obtain 2 concentration values that are within 0.0020 M. Calculate the average of those 2 concentration values or the closest concentration values. If all three titrations are close, average all three values. 13. Pour any remaining NaOH in the buret & the labeled NaOH beaker back into your 1 L NaOH bottle. Extra HCl may be given to another student or poured into the liquid waste container. Cap & store your NaOH sln, making sure it is labeled with colored tape with your name. Clean the buret with DI water & put it back in the container with the tip up. Rinse the 10 ml pipet with DI water and place it back in the container with the tip up. Day 2 Procedure: Preparing & Titrating the Antacid Slns 14. You will titrate one liquid antacid and one solid antacid. For the solid antacid, use the mortar and pestle to crush one antacid tablet, or use some that another student has already crushed. Tare a labeled 150 ml beaker labeled Solid Antacid and transfer 0.12-0.15 g of the crushed tablet into this beaker, recording the exact mass transferred. Make sure you record the name of Chem 400 Exp #10 11/4/15 Revision M Samples p. 3

the actual antacid tablet used. As one tablet will weighs over 1 g, you may share the crushed antacid powder with several other students. Make sure that the mortar and pestle get rinsed and dried before putting back. 15. Obtain 50-60 ml of a 0.1000M HCl solution in a labeled, clean and dry 100 ml beaker. 16. Obtain a 20.00 ml volumetric pipet and rinse it with DI water, and then the 0.1000M HCl solution. 17. Add 20.00 ml of the HCl sln using the 20.00 ml volumetric pipet. Stir with a stirring rod, and leave the rod in the beaker, then allow it to sit and digest for at least 45 minutes before titrating, stirring occasionally. 18. While the solid antacid is digesting (just like stomach acid digests food in your stomach), you will now prepare the liquid antacid solution. Then you will titrate both antacid solutions. 19. Tare a labeled 125 ml Erlenmeyer flask labeled Maalox or Liquid Antacid and transfer 0.15-0.20 g of the liquid Maalox into this flask, recording the exact mass transferred. To do this, keep your tared flask on the balance, then half-fill a plastic pipet with the Maalox. Then drip the contents of the pipet into the tared flask as you watch the mass. Stop adding when you have about the right amount. Then close the balance doors, and record the mass of the Maalox. Make sure you record what brand of liquid antacid you used. 20. Add 20.00 ml of your HCl sln using your prerinsed 20.00 ml volumetric pipet. Stir with a stirring rod, and leave the rod in the beaker, then allow it to sit and digest for at least 30 minutes before titrating, stirring occasionally. 21. While waiting, get your buret ready for the NaOH Solution. 22. Get a 50 ml buret, and rinse it with DI water. Then rinse the buret with a small amount of your NaOH sln, draining into a waste beaker (or your 400 ml beaker). Fill the buret with the NaOH sln using a funnel and a labeled 100 or 150 ml beaker. Allow a small amount to drain from the tip into the waste beaker to clear any air bubbles. Record the initial volume of NaOH, making sure it is below the 0.0 ml line and that you have at least 25 ml of NaOH in the buret. 23. After the antacid solutions have digested, you are ready to begin titrating with NaOH! 24. Now add 3 drops of the aqueous bromocresol green indicator to the 150 ml beaker containing the digested antacid solution. The solution should now turn yellow as it is weakly acidic. 25. While swirling the antacid mixture in the flask, add the NaOH sln from your buret to the contents of the flask as in Step 8, Day 1. The endpoint color is also a green to greenish-blue color that lasts 30 seconds. Record the final volume of NaOH in the buret. 26. Make sure that you have at least 35 ml of the NaOH solution in the buret, and then proceed with the liquid antacid titration as you did with solid antacid titration. Record the initial and final NaOH volume in the buret. 27. When you and your solution partner are done with all of the titrations, pour any unused solutions into the liquid waste container. 28. Clean the buret with DI water and put it back in the container with the tip up. Rinse the pipets with DI water and place them back in the container with the tip up. Thoroughly rinse the NaOH plastic bottle with water, then DI water, and put back in the storage cabinet, loosely capped. 29. Write down the active ingredients of the two antacids from the bottle labels. 30. Your instructor will give you the mass of an average teaspoon of Maalox and the average mass of the solid antacid. WRITE THESE DOWN! This is the normal dose. 31. Do Data Analysis Step 2-4. Chem 400 Exp #10 11/4/15 Revision M Samples p. 4

Data: For each day, record volume of HCl added to beaker or flask, the NaOH buret readings, observations on endpoint color, and the masses of antacids. Be sure to label each trial. Data Analysis: 1. For each day 1 trial, use the volume of the HCl solution and volume of NaOH solution to calculate the molar concentration of the NaOH solution as shown in the Appendix. Calculate the average of the good trials. 2. Calculate the volume of the HCl solution that was neutralized by the antacid. Then calculate the volume of stomach acid (0.14 M HCl) neutralized per gram of each of the above antacids. 3. Given the actual dose is one tablet or one teaspoon and that you wrote down the average mass of a dose in Procedure Step 30, convert your volume of stomach acid neutralized per gram antacid to volume of stomach acid neutralized per dose antacid. For example, if your antacid neutralizes 55 ml stomach acid per g antacid; and a tablet of this antacid weighs 1.2 g; how many ml of stomach acid would be neutralized by this tablet? This would be expressed as ml stomach acid per tablet or ml stomach acid per dose. Do for both antacids that you analyzed. 4. Write individual balanced equations for the acid/base neutralization reaction of each of the antacid active ingredients with HCl. So if the solid antacid has three active ingredients, write three balanced equations. If the liquid and solid antacids have the same active ingredient(s), do not repeat a balanced equation. Postlab Question: Maalox is a liquid antacid, while Tums and Rolaids are solids that dissolve slowly in water. After an antacid tablet is chewed or swallowed, it typically will remain in the stomach for about an hour. Based on this information, what advantage might Maalox have over the tablet antacids? Discussion/Errors: List the antacids in decreasing order of the amount of stomach acid neutralized per g of antacid. Then list the antacids in decreasing order of the amount of stomach acid neutralized per dose of antacid. Are these two rankings the same? Discuss any factors that could affect these rankings. Discuss any errors or possible errors that could affect your results. Conclusion: Restate the purpose of the experiment; briefly state the method of achieving the purpose. State the average concentrations of your NaOH solution. For the two antacids, state the ml of stomach acid neutralized per gram of antacid. Rank the 2 antacids, and restate any major errors or possible errors. Chem 400 Exp #10 11/4/15 Revision M Samples p. 5

Appendix: Equations to use for Data Analysis Data Analysis Step 1: Finding the Molarity of the NaOH Solution M NaOH = 10.00mL HCl sln 1000mL / L 0.1000mol HCl 1L HCl sln 1mol NaOH 1mol HCl 1 x ml NaOH 1000mL 1L where x ml NaOH is the volume of NaOH which were added from the buret during the titration. If the molarity of the HCl solution was not 0.1000M, put in the exact molarity. Data Analysis Step 2: Finding the Volume of HCl NOT Neutralized by the Antacid, then Finding the Volume of HCl Neutralized by the Antacid, and then Finding the ml of Stomach Acid Neutralized by an Antacid Part a) Find the ml of HCl which was not neutralized by the antacid. 1L y mol NaOH 1 mol HCl 1 L HCl e ml HCl not neutralized = ( x ml NaOH) 1000mL 1 L NaOH 1 mol NaOH 0.1000 mol HCl 1000mL 1L x is the ml of NaOH titrated; y is the average NaOH molarity from day 1. Part b) The above gives ml HCl not neutralized. You want the ml of HCl which WAS neutralized by the antacid. To find this, subtract the above answer (ml HCl which was NOT neutralized) from the ml of HCl which you added to the antacid tablet. (This was 20.00 ml.) c ml HCl neutralized = 20.00 ml HCl in flask - e ml HCl not neutralized where e is the answer to Part a above. Part c) Then use the ml of HCl neutralized as follows to find the ml of stomach acid neutralized per gram of the antacid: ml stomach acid neutralized = c ml HCl neutralized f g antacid 1L 1000mL where f is the mass of the antacid, and c is the answer to Part b above. 0.1000 mol HCl 1L HCl 1L stomach acid 0.14 mol HCl 1000mL 1L This gives ml of stomach acid neutralized per gram of antacid. Note: you have to do all of these calculations for each type of antacid analyzed. Chem 400 Exp #10 11/4/15 Revision M Samples p. 6