Cleaning Pennies

A. 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.

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.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2001

A. 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.

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

A. 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 sulfides 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 that you understand it we hope the teacher grading your answer does too :-)

Note to students with science projects: Please see our FAQ on Cleaning Pennies; it will tell you everything you need to know!

A. Ashley:

If you are collecting coins, you should not use any chemicals at all - they do a good job, but an experienced collector can recognize an unnatural finish - the coins will be worth more if you leave them alone.

If you want them clean for some other reason, any copper cleaner should do the job - found in hardware stores, big box stores, etc. (wear rubber gloves ⇦ this on eBay or Amazon [affil links] , use in ventilated area).

Hi, Teryne. Vinegar plus salt will work instantly. So will lemon juice & salt, ketchup, taco sauce, etc.

But the purpose of your science project is not to learn how to clean pennies, it is to learn how to conduct an experiment properly. Our FAQ on Cleaning Pennies should be exactly what you need.

It is boring because it has no relevance to your perceived needs and interests. See if you can talk the teacher into you conducting a somewhat similar yet different experiment involving something that interests you. Good luck.

Regards,

A. Hi, Jake. You imply that it took many days for taco sauce to get the tarnish off of pennies. Just how much time did it take?

I don't foresee your head exploding if you brush your teeth after having tacos for dinner. But I guess you should get your little brother to try it before you brush your own. Assuming he doesn't explode or pass out, you can safely mix them.

But remember that the purpose of this experiment is not to learn how to clean pennies ... it's how to conduct science experiments. So think about what conclusion you will have if the mix works better than the individual ingredients, and what you will have learned that is of any actual value having to do with science rather than penny cleaning :-)

Regards,

A. Hi, Jessica.

I think your husband has a great idea, but one which must be verified first. I think you should put nail polish on one-half of one face of a half-dozen pennies of varying brightness, leave it for a measured amount of time, then remove it with nail polish remover and assure yourselves that nail polish and nail polish remover have no effect on the tarnish. We call a coating like that a "maskant"; it keeps the tarnish removal chemicals away.

I think immersing the pennies is better than rubbing them because it doesn't introduce extraneous variables like how hard you rub, or for how long, or how clean the cloth was, etc. Use plastic or glass bowls rather than metal bowls. The procedure probably doesn't matter, only the consistency of application -- like assuring that the pennies are in each solution for the same amount of time, that each of the solutions is at room temperature, etc.

Regards,

A. Hi Sherry. In what way would it be helpful ... what is your situation? See, the thing is, every explanation of chemistry is just an abstraction, a simplification appropriate for a grade level or a situation. As a quick example, citric acid is not just an acid (a liberator of hydrogen atoms), it is also a chelating agent. When you ask for the chemical reaction, do you mean let's pretend it's just a simple inorganic acid rather than a chelator and write a formula as if it was? The values won't work out right, but you can can Wiki "sodium citrate" for a general feel for how citric acid gives up its hydrogen atoms in three stages. Or do you mean let's really get into studying the chelation of copper by citric acid ?

I believe that ascorbic acid (essentially Vitamin C) is also a chelating agent, but I'm not readily seeing much info about it in relation to metals, except notes that it is effective as a metal stain remover because it's a "reductone". I'm not so sure that you will find "the actual chemical equation for the reaction between Copper Oxide and Ascorbic Acid" anywhere for free. There is a pay article entitled "From ascorbic acid to oxalate: Hydrothermal synthesis of copper oxalate microspheres and conversion to oxide" at
www.sciencedirect.com/science/article/pii/S0167577X1301687X
One of your two questions might be the basis for a good advanced project. Please get back to us with your situation and maybe someone can help. Good luck.

Regards,

Sherry,
The primary reaction one generally sees between a metal and an acid is a redox reaction as the metal is converted to ions and dissolved into the solution.

Ignoring the conjugate base of the acid, you're looking at something along the lines of

M + 2 H- ==> M++ + H2

That varies a bit depending on what oxidation state the metal in question tends to prefer, in this case it's a metal that likes the 2+ or (II) state. For others you just increase or decrease the number of participating hydrogen ions as needed.

Further reactions may or may not occur depending on if the conjugate base of the acid in question will want to react any with the metal ions. Chelating is less of a chemical reaction and more of a "capture", though, since no structures are changed or electrons exchanged.

Thanks for the clarification/correction on the chemical "reaction" of chelating, Ray.

Regards,