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Why do citric acids clean pennies best


Q. My ten year old daughter and I read some of your letters about "What cleans pennies the best". She has 5 different juices: lemon, lime, pineapple, orange and grapefruit, she has actually found that lime has cleaned the best. However, with all of your letters, there is still not a research site or references given, so that she can back up her findings. She knows that the citric acid is reacting with the copper, but why or how is this reacting in terms that she can understand and then write her report on. Thank you for any help.

Paticia H deleted
- Cleveland Heights, Ohio


A. We applaud her for actually doing the experiment instead of just asking for the answer, Patricia! It's also good that she is looking for high quality references, rather than the tossed salad of information, misinformation, and disinformation that is the internet. However, I do not personally agree with the structure & nature that you and perhaps the teacher seem to be trying to give to this investigation.

What is important for a ten year old student to get from a project of this nature (in my opinion) is the concept of "the scientific method" and the mechanics of a science project. She should be trying to learn the vocabulary: hypothesis, independent variable, dependent variable, control, lab book, observation, and conclusion. She should be learning that her efforts do not become a scientific experiment unless she follows a scientific protocol. She should be learning what "junk science" is, why it is so poisonous, and how to avoid it. Please start with our FAQ on Cleaning Pennies as I think it will help.

The discoloration on old pennies is a copper oxide tarnish on them. Your daughter could also have "cleaned" the penny by applying Sandpaper [linked by editor to product info at Rockler] to it, scraping away that copper oxide tarnish and exposing the underlying copper metal. Citric acid is an acid, and acids have the power to "clean" pennies, by dissolving this copper oxide tarnish. The tarnish dissolves into the acid and is washed away, exposing the "clean" non oxidized metal lying under it. Personally, I think that is enough depth for a ten year old. But the reaction is roughly:

Acid + copper oxide => copper salt + water

balancing the equation before proceeding:

2 acid + 1 copper oxide => 1 copper salt + 1 water

2 H+ [citrate anion]-- + Cu++ O-- --> Cu++[citrate anion]-- + H20

Acid has a 'sour' taste. So whichever fruit tastes the most sour may do the best job of cleaning. I agree with you that lime juice is quite good at removing tarnish from a penny. I'm not sure that it's better than lemon juice, though. It may depend on season, ripeness of individual piece of fruit, country where it was grown, etc. There is enough random variation in technique, tarnish depth, dirt on the pennies, etc., that the experiment may not have enough precision to resolve whether limes are better than lemons. But if repeated experiments demonstrate it, then we can perhaps assert it as true.

Citric acid is not just an acid, but also a chelating agent. Even after college chemistry I don't fully understand chelation, and I doubt that you or the teacher do, and I don't believe that a 10 year old should be expected to. But it does aid in dissolving of the metal. There ought to be leads to references in the teacher's guide; or you might write to the publisher's website, since they should have the answer to the question they wrote.

Shake a few granules of salt into the lemon or lime juice and it will turn the penny salmon pink in a heartbeat. Why? Because the chloride from the salt is a very powerful copper corroder; but it is difficult to understand why this is so, too -- let alone explain it to a 10-year old.

I hope this reply may get you to slightly rethink what this experiment is supposed to be teaching your daughter. Not that I claim any special insight -- just that I have faced the question a thousand times now and I think I'm starting to understand the issues :-)

Good luck!

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 23, 2008

Q. Anion, Cation, Salt, Confusion...

I'm doing my 7th grade science project on what is the best way to clean a penny. For my research report to accompany it, I need to write about ions and salt and how it reacts with the copper/copper oxide. Please don't simplify... I may not be able to totally comprehend it but I guess I could ask my mom for help... Please respond ASAP! My teacher keeps telling me to explain more and more. I'm on my fourth draft. Please help me!

Sofia Fdeleted
student - Berkeley, California

April 24, 2008

A. Hi, Sofia. As an aside, think about this: there are 3000-page books in the library that talk about ions; so how will you decide which pages of such books to leave out from your report? Easy: your research should be written in terms that explain or comment upon your own results. But you haven't told us your results! This may be the source of the confusion.

Try this:

"My hypothesis was . . . In order to try to prove that hypothesis, I conducted the following experiment . . . The results of my experiment tended to confirm my hypothesis but . . . I believe that the reason that this happened is because . . . This reasoning is supported by these excerpts from . . . which explain that . . ."

Ions are charged particles that are dissolved in a water-based solution. Positively charged ions are called cations, negatively charged ions are called anions.

Let's look at vinegar for an example of cations and anions. Vinegar is dilute acetic acid, and the formula for acetic acid can be written as HC2H3O2. Vinegar will ionize as H+ + C2H3O2- , and the H+ are cations and the C2H3O2- are anions. As another example, citric acid from citrus fruits can be written as HCOOH, which will ionize as H+ + COOH- , where the H+ are cations and the COOH- are anions.

In either case the cations can react with copper to form copper salts as explained in the formulas in my previous response to Patricia. Good luck.


pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

November 13, 2008

Q. Hi my name is Monica and I need help. I'm doing the same project and I was wondering does lemonade or Dr. Pepper clean the penny best? And why? Please answer ASAP.

Monica Tdeleted
student - Duluth, Georgia


Award Winning Science Fair Projects

November , 2008

A. Hi, Monica. It is your job to do the experiment. After you tell us which worked better, then we'll try to help you with why. Nobody can explain why you got the result you got until you tell us what result you got. And no one should try because that is called "junk science": deciding what the answer should be before doing the experiment, then talking yourself into discounting any contrary observations and giving far too much weight to observations which support the answer you want to get.

Good luck.


pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

January 20, 2010

Q. Hi my daughter is doing the cleaning the penny project. The 6 juices that she used was Apple, grape, grapefruit, lemon, cranberry, and orange. After reading on line the majority of the juices that cleaned the best was lemon. But, ours turned out to be that the cranberry juice in fact cleaned the best. Could you help us as to why ours is different?

Jodi Harris
student - millbrook, Alabama

January 20, 2010

A. Hi, Jodi. Maybe your cranberry juice was not pure juice, but a cranberry cocktail that had the tiniest smidgen of salt in it? Even "100% juice" cranberry juice isn't cranberry juice but "juice from apple, cranberry, pear and grape . . . citric acid . . . grape skins, pomegranate . . . ascorbic acid . . ."

It may be hopeless to try to explain why one juice manufactured to a secret formula cleans better than another juice manufactured to another secret formula, but you could try pH paper and see if it demonstrates anything :-)

Please see our FAQ on Cleaning Pennies and try to have your daughter learn how to conduct experiments and learn from them . . . and to not worry too much about the results. Good luck!


pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 5, 2010

Q. I'm a freshman and doing an project about which solution can "clean" a penny the best, and why. My results were that vinegar, salt and vinegar, ketchup, and lemon juice worked the best (over approx. a 10 hour time span). As far as I can tell, these all work in about the same way. (all acids. I've been researching why this is, but I've mostly been able to find only that acids dissolve the copper oxide. The only more "in depth" explanations I've found for both the role salt plays in this, and "how" the acids dissolve the copper oxide are TOO in depth, and written w/ the symbols that are past my understanding. Could you either give me, or direct me to somewhere I can get a "medium" explanation? I just need a simple "the copper bonds w/ the salt" or the acid breaks down into whatever and this can dissolve the copper oxide. I'm not even sure that you (or anyone) can give me this type of an explanation, but I'm at least trying! Thanks, you're site has already been really helpful!

Christian Gdeleted
- College Park, Maryland

April 6, 2010

A. Hi, Christian. Two common acids are
HCl, which is muriatic (hydrochloric) acid, used for dissolving mortar & grout, and pickling metal, and
H2SO4, which is sulfuric acid, used in car batteries and a host of other applications.

If you note what they have in common, they start with H for hydrogen. In water, HCl will dissolve or ionize into the charged ions H+ and Cl-; and sulfuric acid will ionize into H+, H+, and SO4-2

Most other acids are written in rather similar form:
H3PO4, phosphoric acid
HNO3, nitric acid
HC2H3O2, acetic acid or vinegar
H3C6H5O7, citric acid

We measure acidity on the pH scale, which tells us how much H+ has ionized into the solution. Basically, the lower the pH, the more acidic the solution is, and the more capable of attacking metals, rusts, and oxide tarnishes.

A penny, whether it is an old solid copper penny or a newer zinc-core penny, has a skin of copper. The copper metal (Cu) has reacted with oxygen in the air (O) to form copper oxide tarnishes like Cu2O. When you remove the tarnish from a penny with an acid, the chemical reaction is something like:

2HCl + Cu2O => H2O + 2CuCl, with the copper oxide now dissolved and the "clean" copper metal exposed again.

Basically, the stronger the acid, the faster and better it will work.

Salt is sodium chloride, NaCl. It is difficult to explain why salt helps as much as it does, except to say that chloride salts like CuCl are very soluble in solution, so the reaction tends to easily proceed, without much impediment or backsliding (chemical reactions don't actually proceed entirely in one direction: there tends to be an equilibrium reaction, with the reaction trying to proceed in the "other" direction to some extent). Perhaps if you quickly get the generated copper "out of play" by tying it up as highly soluble CuCl, the reaction proceeds faster?

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

April 6, 2010

thumbsup2Thank you so much for your help! Hope I didn't "bombard" you with the same question again, it was just one I still had left over after reading the FAQ. Thanks a bunch!

Christian Gdeleted
- College Park, Maryland

December 5, 2014

Q. I need some dash notes on which juices clean pennies better.

Kerry Lynn C
- Cowpens, South Carolina, USA

December 2014

Hi Kerry. Please see the previously mentioned "FAQ on Cleaning Pennies". I'm happy to try to help people over the hurdles, but not to actually lay out the dash notes for them.
Best of luck.


pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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