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Antacids

Module by: Mary McHale

Analysis of Commercial Antacids

Objectives

  • Measure the quantity of commercial antacid required to neutralize a simulated stomach acid (0.15 M hydrochloric acid) and compare the effectiveness of several brands of antacids in neutralizing acids.
  • Learn and practice the back-titration method.

Grading

You grade will be determined according to the following:

  • Pre-lab (10%)
  • Lab Report Form (80%)
  • TA points (10%)

Introduction

The parietal cells in the stomach secrete hydrochloric acid at a quite high concentration of about 0.155. 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 CaCO3CaCO3 size 12{ ital "CaCO" rSub { size 8{3} } } {} 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. Below is an example of active agents in several brands.

 Brand Active Agent Base
Pepto-Bismol BiO ( HOC 6 H 4 COO ) BiO ( HOC 6 H 4 COO ) size 12{ ital "BiO" \( ital "HOC" rSub { size 8{6} } H rSub { size 8{4} } ital "COO" \) } {}
Milk of Magnesia Mg ( OH ) 2 Mg ( OH ) 2 size 12{ ital "Mg" \( ital "OH" \) rSub { size 8{2} } } {}
Rolaids NaAl(OH)2CO3NaAl(OH)2CO3 size 12{ ital "NaAl" \( ital "OH" \) rSub { size 8{2} } ital "CO" rSub { size 8{3} } } {} (newer tablets: CaCO3CaCO3 size 12{ ital "CaCO" rSub { size 8{3} } } {})
Tums CaCO 3 CaCO 3 size 12{ ital "CaCO" rSub { size 8{3} } } {}
Alka-Seltzer II NaHCO3NaHCO3 size 12{ ital "NaHCO" rSub { size 8{3} } } {} and KHCO3KHCO3 size 12{ ital "KHCO" rSub { size 8{3} } } {}
Maalox Mg(OH)2Mg(OH)2 size 12{ ital "Mg" \( ital "OH" \) rSub { size 8{2} } } {} and Al(OH)3Al(OH)3 size 12{ ital "Al" \( ital "OH" \) rSub { size 8{3} } } {}
Gaviscon Al ( OH ) 3 Al ( OH ) 3 size 12{ ital "Al" \( ital "OH" \) rSub { size 8{3} } } {}

 Acids are neutralized by these bases as illustrated below.

BiO ( HOC 6 H 4 COO ) + 3H + ( aq ) → Bi 3 + ( aq ) + H 2 O ( l ) + HOC 6 H 4 COOH ( s ) BiO ( HOC 6 H 4 COO ) + 3H + ( aq ) → Bi 3 + ( aq ) + H 2 O ( l ) + HOC 6 H 4 COOH ( s ) size 12{ ital "BiO" \( ital "HOC" rSub { size 8{6} } H rSub { size 8{4} } ital "COO" \) +3H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "Bi" rSup { size 8{3+{}} } \( ital "aq" \) +H rSub { size 8{2} } O \( l \) + ital "HOC" rSub { size 8{6} } H rSub { size 8{4} } ital "COOH" \( s \) } {} (1)
Mg ( OH ) 2 ( s ) + 2H + ( aq ) → Mg 2 + ( aq ) + 2H 2 O ( l ) Mg ( OH ) 2 ( s ) + 2H + ( aq ) → Mg 2 + ( aq ) + 2H 2 O ( l ) size 12{ ital "Mg" \( ital "OH" \) rSub { size 8{2} } \( s \) +2H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "Mg" rSup { size 8{2+{}} } \( ital "aq" \) +2H rSub { size 8{2} } O \( l \) } {} (2)
Al ( OH ) 3 ( s ) + H + ( aq ) → Al ( OH ) 2 + ( aq ) + H 2 O ( l ) Al ( OH ) 3 ( s ) + H + ( aq ) → Al ( OH ) 2 + ( aq ) + H 2 O ( l ) size 12{ ital "Al" \( ital "OH" \) rSub { size 8{3} } \( s \) +H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "Al" \( ital "OH" \) rSub { size 8{2} } rSup { size 8{+{}} } \( ital "aq" \) +H rSub { size 8{2} } O \( l \) } {} (3)
Al ( OH ) 2 + ( aq ) + H + ( aq ) → Al ( OH ) 2 + ( aq ) + H 2 O ( l ) Al ( OH ) 2 + ( aq ) + H + ( aq ) → Al ( OH ) 2 + ( aq ) + H 2 O ( l ) size 12{ ital "Al" \( ital "OH" \) rSub { size 8{2} } rSup { size 8{+{}} } \( ital "aq" \) +H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "Al" \( ital "OH" \) rSub { size 8{2} } rSup { size 8{+{}} } \( ital "aq" \) +H rSub { size 8{2} } O \( l \) } {} (4)
CaCO 3 ( s ) + H + ( aq ) → Ca 2 + ( aq ) + HCO 3 − ( aq ) CaCO 3 ( s ) + H + ( aq ) → Ca 2 + ( aq ) + HCO 3 − ( aq ) size 12{ ital "CaCO" rSub { size 8{3} } \( s \) +H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "Ca" rSup { size 8{2+{}} } \( ital "aq" \) + ital "HCO" rSub { size 8{3} } rSup { size 8{ - {}} } \( ital "aq" \) } {} (5)
HCO 3 − ( aq ) + H + ( aq ) → CO 2 ( g ) + H 2 O ( l ) HCO 3 − ( aq ) + H + ( aq ) → CO 2 ( g ) + H 2 O ( l ) size 12{ ital "HCO" rSub { size 8{3} } rSup { size 8{ - {}} } \( ital "aq" \) +H rSup { size 8{+{}} } \( ital "aq" \) rightarrow ital "CO" rSub { size 8{2} } \( g \) +H rSub { size 8{2} } O \( l \) } {} (6)

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.

Note: The standardized solutions of NaOH and HCl have been prepared for you. However you need to understand how and why it is done this way. (See Supporting Information on this) Also see notes on Titration Tips.

If NANA size 12{N rSub { size 8{A} } } {} is the number of moles of HCl that we use for dissolving the antacid sample, and NBNB size 12{N rSub { size 8{B} } } {} is the number of moles of NaOH needed to back-titrate the excess HCl, then NsampleNsample size 12{N rSub { size 8{ ital "sample"} } } {}, the number of moles neutralized by the sample, is given by:

N sample = N A − N B N sample = N A − N B size 12{N rSub { size 8{ ital "sample"} } =N rSub { size 8{A} } - N rSub { size 8{B} } } {} (7)

Remember that the number of moles in a volume of solution equals the concentration multiplied by the volume. (n=M×V)(n=M×V) size 12{ \( n=M times V \) } {}

Dissolved carbon dioxide is converted into the weak acid, carbonic acid (H2CO3)(H2CO3) size 12{ \( H rSub { size 8{2} } ital "CO" rSub { size 8{3} } \) } {} 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 CO2CO2 size 12{ ital "CO" rSub { size 8{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

Part 1: Titration of Antacids

Your TA will randomly assign you one of the commercial brands of antacid tablets and you will analyze two samples of it. Before starting, calculate the acid/base ratio of the tablet that you have been assigned that you will neutralize. Start thinking how this may be an error in your final calculation.

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 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 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 flaks 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. Put a clean magnetic stir bar inside flask and stir solution on stir plate. (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 (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.

10. Repeat the above procedure with a new sample of the same antacid. Enter your data in an excel spreadsheet in the workrooms, check the web page later on the week and make a final comment on the overall cost to neutralize one mole of HCl for various brands. THIS IS PART OF YOUR REPORT AND IF DATA IS NOT FOUND ON THE WEB YOU SHOULD CONTACT YOUR TA. (No excuses)

Retail Cost Maximum

 Label Antacid Brand Price (cents per tablet) Fraction
B Walgreens Antacid Tablets 2.66 1/6
C Titralac Plus 5.99 1/4
D Titralac 5.89 1/4
J Tums Regular 2.86 1/6
K Tums Plus 5.32 1/6
M Eckerd Antacid 2.19 1/6

 *Active ingredient in all of these antacids is CaCO3CaCO3 size 12{ ital "CaCO" rSub { size 8{3} } } {}.

Supporting Information on Standardization:

Molarity is the most commonly used concentration term when one is interested in the amount of materials involved in a chemical reaction in solution. Molarity (M) is defined as the number of moles of solute per liter of solution.

M = moles(solute)Liters(solution)moles(solute)Liters(solution) size 12{ { {# ital "moles" \( ital "solute" \) } over { ital "Liters" \( ital "solution" \) } } } {} (1)

The number of moles is calculated by dividing the mass of the sample in grams by the gram formula weight (GFW or molar mass). One GFW is the same as one mole.

Number moles = grams(solute)GFWgrams(solute)GFW size 12{ { { ital "grams" \( ital "solute" \) } over { ital "GFW"} } } {} (2)

For example in a 0.150 M HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} solution, there are 0.150 moles of HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {} in one liter of this solution. The following factors may then be used in chemical calculations: 

0.150molHNO31L(solution)0.150molHNO31L(solution) size 12{ { {0 "." "150" ital "molHNO" rSub { size 8{3} } } over {1L \( ital "solution" \) } } } {} or 1L(solution)0.150molHNO31L(solution)0.150molHNO3 size 12{ { {1L \( ital "solution" \) } over {0 "." "150" ital "molHNO" rSub { size 8{3} } } } } {}

In a chemical reaction that takes place in solution, the volume and the molarity of one reactant and the molarity of the second reactant can be used together with the stoichiometry of the equation to find the volume of the second reactant needed to react completely with the first reactant.

Titration is a process in which a solution of one reagent, usually the base, is added to an accurately measured volume of another solution, usually the acid, until the reaction is complete. The concentration of one of the reagents is known. From the known concentration and the measured volumes, the concentration of the second solution can be calculated.

In acid-base reactions the end of the reaction or equivalence point is detected by adding a compound that undergoes a color change as it changes from its acid form to its basic form. This compound is called an indicator. An indicator is an organic dye that changes color at a characteristic H+H+ size 12{H rSup { size 8{+{}} } } {} ion concentration. A dye can be an indicator if it has an intense color that changes when it gains or loses H+H+ size 12{H rSup { size 8{+{}} } } {} ions.

HIn+OH−→In−+H2OHIn+OH−→In−+H2O size 12{ ital "HIn"+ ital "OH" rSup { size 8{ - {}} } rightarrow ital "In" rSup { size 8{ - {}} } +H rSub { size 8{2} } O} {} (3)

The