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How Does Soy Sauce Remove Tarnish From Pennies?


I recently showed my 10-year-old daughter my grandfather's trick of making old copper pennies shiny bright by immersing them in soy sauce. Naturally, she wants to know how it works-- and I don't have the answer! (My only chemistry class was in high school back in 1966-67.) She also wants to make it part of her Spring Science Fair project. We tried researching this on the web, which is how we found this site. But in all the discussions of ketchup and vinegar [in bulk on eBay or Amazon (adv.)] and toothpaste, etc., as de-tarnishers, no one seems to know about soy sauce. The best I can guess is that soy sauce is both salty and acidic (from the fermentation of the beans?)...PLEASE HELP WITH A RESPONSE AS SOON AS CONVENIENTLY POSSIBLE, AND MANY THANKS IN ADVANCE!

Debra S
- Englewood, Florida, USA

The very high salt content of soy sauce would make it work similarly to ketchup or lemon juice plus salt. Ketchup and lemon juice have citric acid; what acid soy sauce has I don't know, but perhaps acetic acid? It is fermented isn't it? The acid plus the salt allows Cl- ions in solution, which are highly corrosive toward copper oxide, ionizing it without any effect on the copper base metal, leaving a shiny finish.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

"Science Fair projects "

on AbeBooks

or Amazon

(affil links)

As a followup, I tested ketchup vs. soy sauce vs. lemon juice plus salt. I decided to do the practical work of cleaning our copper bottomed pots rather than cleaning pennies though.

My conclusion was that all three performed almost identically. Leave out the salt, though, and the lemon juice is totally ineffective. Apparently, as long as you have salt and a weak acid you'll so quickly dissolve the copper oxides that any differences between the three are minor.

As a practical matter, the lemon juice plus salt rinses most freely, and one could experiment with the lemon juice to salt ratio if desired. The ketchup makes an ungodly mess. Sorry Heinz; sorry Kikkoman.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Oh,by the way do you know how I can make sure all the pennies are equally dirty so you can conduct a fair test? you've done the test before right? How did you know the test was fair?


tiffany h [last name deleted for privacy by Editor]
- NY

Yes, I've done it a number of times. I would suggest that you select pennies that look about equally dirty, then pick about 5 pennies at random for each of the cleaning solutions.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey


I read in a book that white vinegar and salt mixed together can clean pennies. I've tried it myself and it really works well. The chemical name for white vinegar is acetic acid and the chemical name for salt is sodium chloride. When those are mixed together, they form "Hydrochloric Acid," and that's what cleans pennies. (Don't over do it, it took the head right off lincoln when I did it twice on one penny.)

Kristen H [last name deleted for privacy by Editor]
- Manila, Manila, Philippines

Mixing salt and vinegar will not produce a solution containing hydrochoric acid. The solution will, however, have both positive and negative ions, (Na+, Cl-, and H+ and CH3COO-). I suspect the presence of both negative and positive charges is the key. The acid reacts with the metal oxide (or metal, or dirt) to produce water and positive 'stuff', which must be then either dispersed into the solution or reduced (made negative) by electrons (e-), otherwise the buildup of positive charge on the object you want to clean would act as a shield preventing the H+ (acid bit) from getting near. (remember positive charges repel each other)

If you hooked up the penny to a small positive voltage (6V lantern battery, with the negative (cathode) attached to a bit of steel or something, I bet it would would go much faster. This is what is called electropolishing.

Richard Murdey
- Vancouver, BC, Canada

Although mixing salt and vinegar may not exactly make hydrochloric acid, Richard, I think that saying that they don't is equally misleading. Say you've mixed the salt and vinegar into solution and, as you say, you now have ions of Na+, Cl-, H+, and CH3C00- in solution.

How are these ions supposed to "know" that they came from dissolving NaCl and HCH3COO into solution, rather than from dissolving HCl and NaCH3COO into solution? I would say they don't know and they won't act differently.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

I used vinegar and salt to clean a penny in science class and it was truly amazing to see the difference. I had no idea why this happened until I came here. Thanks for clearing it up!

Katie W [last name deleted for privacy by Editor]
- SLC, Utah

simultaneous replies

For my science fair project I picked "what will turn pennies green?" I also picked three substances that I think Will turn pennies green (lemon juice, vinegar, and sugared water). But, when I was searching for my topic and trying to see what contents are in the lemon juice, vinegar, sugared water, and on pennies to form my hypothesis, I couldn't find any information on them. So, I was wondering if any of you guys are willing to help me find some information on my science fair project.


Chenin B [last name deleted for privacy by Editor]
- Fort Walton Beach, Florida, U.S.A

I did the penny science project where the vinegar cleans better than the orange juice and I don't know how to explain my conclusion.

Irma [last name deleted for privacy by Editor]
- Brownsville, Texas

Chenin, you have almost completed your hypothesis. You said "I also picked three substances that I think will turn pennies green (lemon juice, vinegar, and sugared water)". You may want to add in how long a time period and some other qualifications, but there's your hypothesis. Now you must prove it or disprove it BEFORE you try to explain the results. After you have the results, then it's time to try to figure out why.

Hi, Irma: You don't say whether you are in elementary school or high school, so it is a bit hard to answer you in a way that is both informative and understandable. But you have exactly one data point or finding (vinegar cleans pennies better than orange juice). An infinite number of causes can be conjectured. You can assert nothing except that vinegar cleans pennies better than orange juice. That is your entire conclusion at this point. Put two or three more juices or liquids into the experiment and you will have a number of data points, and then you can start conjecturing about causes. Good luck.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

I recently tested 14 different household items and although a vinegar and salt solution works very well, "Louisianna Hot Sauce" IS THE BOMB! It cleaned the penny in about 15 seconds and rinsed effort or scrubbing at all! Spread a thin coating evenly on the coin and wait 10-15 seconds. Then rub for a few seconds if it is particularly soiled or oxidized badly and/or rinse with water. If you use a soft cloth to polish it afterwards, you will find it shining like a brand new penny! Try won't believe the awesome results.

Chynna C
- Macon, Georgia, USA

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