Almost all of the acidity of soda pop comes from the phosphoric acid and not from the carbonic acid from the dissolved CO2. You can verify this by measuring the pH of fresh and flat soda pop; there's very little difference.
The phosphoric acid is corrosive, but actually the acid concentration in soda pop is lower than that in orange juice or lemonade. Try submerging identical strips of magnesium (or iron staples) in each of these beverages overnight. Which beverage dissolves more metal? Which dissolves the metal fastest?
Fruit juices and drinks are also tart, but they don't use phosphoric acid as a flavor additive. Phosphoric acid would cause many ions present in fruit juices to settle out as insoluble phosphates. These beverages get their tang from citric acid, a substance found in oranges, limes, lemons and grapefruits. Malic acid, found in apples and cherries, is added to many fruit juices. Fumaric acid is used in noncarbonated soft drinks, and tartaric acid gives grape-flavored candies a subtle sour flavor. All of these substances impart only tartness, without overpowering other flavors present.
"... tartaric acid gives a very true flavor, but Mother Nature does not intend for tartrates to stay in solution long. When KH-tartrate precipitates out of a juice, looking very much like glass or metal shavings, and the consumer passes their bottle of juice to the FDA, one really does not care about "true" flavor. We in the juice industry usually use malic or a malic citric blend."
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/print-why-phosphoric-acid-in-soda-pop.shtml