Wednesday, January 16, 2013

How Safe Are Post 1982 Pennies To Eat?

I remember being very young and having my mother yell at me when I got near change. Why?  My sister swallowed a penny when she was a toddler.  Of course my contacted the doctor who said keep a watchful eye in the toilet for the next 24 hrs.  As it so happens, my mother was able to retrieve the coin without too much fanfare but did not want to go through the experience with me.  This was in the late 1960's - early 1970's and stomach acid does not react with copper.

Prior to 1982 pennies were made of pure copper.  However, the price of copper rose to such price that the copper was worth more than the penny.  To avoid massive amounts of people getting rich by turning in their copper Lincoln's for cash at the scrapyard, the U.S. Mint made pennies out of a cheaper metal, zinc, and just plated them with copper.

Why the nostalgia, my students recently did a fun activity where they calculate the percent composition of a post 1982 penny (97.5% Zn, 2.5% Cu) by putting four small niches on the sides of a penny and soak it in 6.0 molar hydrochloric acid (HCl) for 24 hrs.  The next day what is left is a shell of a penny as HCl dissolves zinc (Zn) but does not touch copper (Cu).  Stomach acid is also made of HCl (pH around 2), thus if you swallowed a new penny with a notch in it, the stomach acid would dissolve the zinc and release hydrogen gas in the process.  Avoid sparks when you burp!

Although stomach acid with a pH of 2 corresponds to a 0.01 molar solution of HCl and we used 6.0 molar HCl, it is still strong enough to dissolve some of the zinc.  You probably don't have to worry about flaming burps but it could cause ulcers (source: Chicago Tribune).

Zn + 2HCl  -->  ZnCl2 + H2

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