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0.4 Antacids

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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 CaCO 3 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 ) size 12{ ital "BiO" \( ital "HOC" rSub { size 8{6} } H rSub { size 8{4} } ital "COO" \) } {}
Milk of Magnesia Mg ( OH ) 2 size 12{ ital "Mg" \( ital "OH" \) rSub { size 8{2} } } {}
Rolaids NaAl ( OH ) 2 CO 3 size 12{ ital "NaAl" \( ital "OH" \) rSub { size 8{2} } ital "CO" rSub { size 8{3} } } {} (newer tablets: CaCO 3 size 12{ ital "CaCO" rSub { size 8{3} } } {} )
Tums CaCO 3 size 12{ ital "CaCO" rSub { size 8{3} } } {}
Alka-Seltzer II NaHCO 3 size 12{ ital "NaHCO" rSub { size 8{3} } } {} and KHCO 3 size 12{ ital "KHCO" rSub { size 8{3} } } {}
Maalox Mg ( OH ) 2 size 12{ ital "Mg" \( ital "OH" \) rSub { size 8{2} } } {} and Al ( OH ) 3 size 12{ ital "Al" \( ital "OH" \) rSub { size 8{3} } } {}
Gaviscon 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 ) 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" \( s \) } {}
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 \( l \) } {}
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 \( l \) } {}
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 \( l \) } {}
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{ - {}} } \( ital "aq" \) } {}
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 \( l \) } {}

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

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Source:  OpenStax, Honors chemistry spring. OpenStax CNX. Mar 10, 2008 Download for free at http://cnx.org/content/col10512/1.9
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