Letter 6998

Cleaning Pennies 

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Hello. My 7th grade son is doing some research and experiments on cleaning pennies with different fruit juices. He is finding several that work well, Pineapple and Lemon in particular. They seem to work too well and seem to be removing some plating from the penny. He soaks them for around 12 hours. Can you help explain this?

Thanks Much,

An interested Mom

Susan Luken
- Fairfield,OH. USA

Susan,

The pennies are not plated. They are, I believe, made from a copper alloy (copper mixed with some other metal). The pennies tarnish/rust (reaction to oxygen) which produces a film of copper oxide. The acid in the fruit juices dissolves this and what you're seeing is a rust colored liquid.

Hope this helps.

Peter Weber
- Creil, France

Maybe in France they aren't, Mr. Weber...

Here in the USA pennies before 1982 were solid copper and after 1982 they are a zinc core with copper plating.

Ted Mooney, P.E.  finishing.com Brick, NJ

Cleaning a penny is primarily using an acid to remove the oxide layer from the surface of the coin along with a tiny amount of other dirt or soils. The acid also attacks the copper plate on a modern penny. Rate of chemical attack is normally a function of

1. concentration of the chemical,

2. temperature of the chemical (crude rule is raise the temperature 20F and the reaction proceeds twice as fast)

3. Time

4. ratio of the amount of available acid to the amount of reaction products.

You are using organic acids which react a slight bit different than a mineral acid in that they tend to form complexes with the copper, but it is close enough for government work.

You have all kinds of additional possibilities. Shorter time, overnight in the fridge, overnight in the freezer, lots of different dilutions of the acid and a constant time.

Lemon juice is almost completely citric acid. I have no idea about pineapple. You might also try some vitamin C solution. It is ascorbic acid.

James Watts
- FL

i have done a experiment on titration

it is about citric acid in apple juice, and how the acid concentration gets affected by temperature

for example, as temperature increases, the acid concentration goes down....

and i need some back up information to prove my point.

thanks

judy

ps. please reply as soon as possible

Judy F
student - Napier, NZ, New Zealand

I don't think your hypothesis is correct, Judy. But give us the readings and your experimental propcedure and we can comment.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

Oxidation and oxidation removal of zinc (pennies)

Introduction: Hey guys. My name is Hanif. I'm from Turkey, Asia and a freshman in high school.

Interest: I have a science fair and this project seemed very interesting because i have a lot of pennies ( about $10.00 worth) that are dirty. This is why, after reading about the experiment i wanted to find out why the pennies are cleaned.

Experiment: put dirty pennies in acids and watch them become clean
I have tried the experiment and it works.

Question(s): 1. Why do dirty pennies become clean after being soaked in acids for a while?
2. Does this have anything to do with oxidation?
3. Whether it is or is not oxidation, why do the acids clean pennies?

!!!THANKS!!!

Hanif S
student - Baton Rouge, LA USA

Well, yes, Hanif: it has "something" to do with oxidation (but maybe not much) -- but it depends on what metal the coins are made of and what acids you are using.

When a coin is new it is shiny because it is metal inside, outside, and through and through, and metals are generally shiny. But most metals (except gold and precious metals) react with the air and the environment over time and form a thin layer of tarnish at the surface. This is an oxide of the metal (maybe also sulphides and carbonates, but let's keep it simple). These oxides are generally a bit "dirty" looking. The tarnishing is an oxidation process. For an example --

2Cu⁰ + O₂ --> 2Cu⁺⁺O^{- -}

That is, metallic copper (oxidation state 0) reacts with oxygen to form copper oxide, with the copper going to oxidation state +2. Thus an oxidation process.

When this copper oxide is treated with acid, it is not necessarily an oxidation process (depending on the acid used) because most acids can't dissolve copper or react with it, they only react with copper oxide. For an example,

Cu⁺⁺O^{- -} + 2HCl --> Cu⁺⁺Cl^-₂ + H₂O

Because the resulting copper chloride is far more soluble in water/acid than copper oxide is, it dissolves into the water/acid and you once again see the clean surface of pure copper metal. But in this acid treatment process the copper started in oxidation state +2 and ended in oxidation state +2 so there was no oxidation.

Now you understand it and we hope the teacher grading your answer does too :-)

Ted Mooney, P.E.  finishing.com Brick, NJ

; it will tell you everyhing you need to know!