MadSci Network: Chemistry

Re: What is in Coca Cola that cleans pennies so quick and well???

Date: Mon Mar 20 22:23:56 2000
Posted By: Mike Conrad, Post-doc/Fellow, Microbiology, UNC
Area of science: Chemistry
ID: 952903020.Ch

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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