edlted by
J. DUDLEY HERRON Purdue University
West LafayeIte, Indiana 47907
The Effects of HCI and Aspirin on the Stomach: An Equilibrium Review Ronald DeLorenzo Middle Georgia College Cochran, Georgia 31014 The relative effects of hvdrochloric acid, a strong acid, and aspirin, a weak acid, on rhk stomach, provide the background for a nand review of chemical equilibrium. This author spends a f d e c t u r e period analyzing this comparison with emphasis on reviewing pH, percent ionization, equilibrium calculations, common ion effects, and other basic concepts of solution chemistry. The discussion beeins ., bv.describine the series of events that occurred on June 6, 1822, when Alexis St. Martin accidently shot himself in the stomach while cleanine his s h o m n in Fort Mackinac, Michigan. Dr. William ~ e a u m o n was t the attending physician who stitched the wound. The edges of the wound eventually healed but did not knit. A hole remained in St. Martin's stomach which was covered hv a f l a of ~ skin that could he easily lifted for a first hand vidw of what was haooenina inside a human stomach. Dr. Beaumont attached a &e ofmeal to the end of a string and lowered it through the hole in St. Martin's stomach and was able to observe the churning, kneading, and oozing of juices which followed. The juices were removed by suction through a rubber tube and found to be, among other things, 0.1 M HC1. (Class question: What is the p H of this solution?) Hvdrochloric acid is a strone acid and is c a ~ a b l eof dissolvfng a wide variety of metals-including zinc and iron, and vet it doesn't attack the stomach cells. Hvdrochloric acid helos higest food proteins, but not the proteindin the stomach's o& cells. Dr. Horace W. Davenport of the University of Michigan Medical School has demonstrated that the stomach is protected from hydrochloric acid by thin outer memhrane bells containine fat (lioidi). Hvdrorhloric acid of a known concentration was &ced inside a dog's stomach and the p H of the solution, after standing, was measured. The pH reading agreed with the calculated value based on the concentration of the known acid.'rhen the dog's stomach was washed wirh detergent (detergents dissolve fats and grease) and the experiment was repeated. The acid attacked the unprotected stomach and a change in the p H was observed. (Class question: Which p H meter reading would be higher?) Aspirin is a weak acid with an equilibrium constant of approximately When most people take two aspirin, they lose a milliliter or two of blood from the stomach wall.Some people lose a half a quart or more of blood and become anemic. (Class auestion: How would vou know if vou lost a sienificant amouni of hlood after taking aspirin? inswer: o n & stools would turn black.) A brief review is given at this point concerning the generalization that hke dissolves like. It is exolained that nsoirin can exist in both an essentially nonpolar form or in an ionized form. The lipid barrier is nonpolar. The nonpolar lipid layer
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306 1 Journal of Chemical Education
will not dissolve ionic substances such as hydrogen ions from HCI. When aspirin is taken, the high concentration of hydrogen s of the asoirin in the molecular ion in the stomach k e e ~ most form. (Class question: why? Calculate ;he percent ionization of a 1 M solution of a weak acid. K = 10-4. in water and in 1.0 M HCI.) In the unionized form. a s ~ i r i ncan dissolve in the lipid laver and pass through it. onhe
