Phosphoric acid is deliberately added to soft drinks to give them a sharper flavor. It also slows the growth of molds and bacteria, which would otherwise multiply rapidly in the sugary solution.

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.