|MadSci Network: Chemistry|
Why does Coca-Cola make a penny bright and shiny? Briefly, the phosphoric acid in the Coca-Cola removes the oxide coat and leaves the metal fresh and shiny. When a penny is new it is bright and shiny. But like most metals, after exposure to oxygen, and especially in the presence of moisture and salts, it is soon covered by a dull oxide coat. With some metals, like Copper and Aluminum, the oxide layer tends to "seal" the metal. With other metals, like iron, the oxide layer flakes away and offers no protection. Thus iron rusts away. Only the least active metals like gold and platinum avoid surface corrosion and stay nice and shiny. What can you do to get rid of the oxide layer? You can polish it away or put the metal in an acid. With a penny, for instance, the copper oxide will react with phosphoric acid to produce copper phosphate. Although copper phosphate is not soluble, it does not adhere to the penny so the penny is left bright and shiny. I found a site on the web that said that Coke is 0.055% phosphoric acid. That makes it 0.0055 molar phosphoric acid. This would give Coke a pH of about 2.3. I just checked some cola with my pH meter and got 2.6. Not exact, but it is in the correct range. By comparison, lemon juice has a pH of 2.2 and orange juice has a pH of 4.2. Phosphoric acid is added to Coke to add to its a zingy taste. Making it acidic also keeps the CO2 ready to pop out, adding to the fizziness. Since this increased acidity reduces ones sensitivity sweetness, they need to add more sugar to Coke to keep up the sweet taste. So remember to brush your teeth! The phosphoric acid will also dissolve away the calcium in your teeth - so Coke provides a double whammy against teeth. A tooth left in a glass of Coca-Cola for a few days is supposed to become soft and mushy. I haven't tried that (yet) but it makes sense to me. Remember to brush your teeth after drinking Coke! Girls and women should also take note that the high phosphate levels in colas and diet colas (and the EDTA in other soft drinks) will grab calcium from their bodies. So if they drink colas, they need to get their calcium to prevent bone loss and osteoporosis. Males tend to have a higher calcium balance, so they don't need to worry about bone loss as much. But hey, I drink a Coke now and then. I prefer it to coffee any day. But I think that we all should think more about what we eat and drink. Cheers! Mike Conrad
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