MadSci Network: Chemistry

Re: Why is Coca-Cola corrosive?

Date: Fri Oct 23 12:27:34 1998
Posted By: Bob Peeples, PE, Chemical Engineer, Environmental Program Management, U S Postal Service
Area of science: Chemistry
ID: 909002523.Ch

Coca-Cola®, like any other soft drink, is acidic. This is because a small amount of the carbon dioxide dissolved in soft drinks (to make them effervescent) will react in solution to form carbonic acid. Carbonic acid may clean the rust from a nail, but it would not dissolve a nail in 24 hours. Try it—a can of Coke® and a nail costs less than a dollar. Many common foods are acidic. That does not mean that they are dangerous. Specialized cells in the human stomach wall secrete hydrochloric acid. Hydrochloric acid is much stronger than carbonic acid, and will dissolve an iron nail easily. The contents of your stomach are usually at a pH of 2. Please don’t try the nail in the stomach contents experiment. There is no safe way to get a nail in your stomach, and there is no safe (or polite) way to remove the contents of your stomach to apply it a nail. You will just have to trust me on that one.

Coca-Cola® products range in pH from 2.5 - 4.2. Coca-Cola Classic® is the most acidic at 2.5. Other food liquids you might use in your experiment include lemon juice, vinegar, salad dressings, and pickle solutions. It should be a fun project for you. When you are finished, I am sure that you will be glad that you do not have an iron stomach.

In your research, you will also discover that the human stomach is pretty capable of regulating its own pH. If things do get out of control, it is not usually the hydrochloric acid itself that is dangerous. Digestive enzymes work best at their optimum pH. One enzyme in particular, pepsin, works a little too well if the stomach is allowed to approach pepsin's optimum pH of 1.5 - 1.6. Excessive enzyme activity (in effect, digesting the wrong meat) is the real reason that excess stomach acid leads to gastric ulcers—not stomach acid itself.

Note: Another fun experiment is to drop an unopened can of Diet Coke® and an unopened can of Coca-Cola Classic® in a tank, bowl or bucket of water. Although both cans contain the same amount of liquid, one floats and the other sinks because of the difference in density between sugar and aspartame, the sweetener used in Diet Coke®.

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