[chris (OP)]

I have found that dumping old tarnished (brown) copper coins in vinegar (with a little added salt (NaCl)), results in the coins becoming pink and shiny within a few minutes.

What is the nature of the chemical reaction that is obviously taking place, and why do the coins go brown in the first place? By contrast, a copper roof goes green yet must be exposed to similar chemistry to a coin.

Chris

[Karen W.]

I have used vinegar for years to clean metal objects, tools that were exposed to the weather ect. it removes old  oxidizing surfaces from metal also. I always thought that copper oxidized over time, Tarnished etc! If you ever have like an old pair of pliers that have been left out and begun to rust, soak them in vinegar overnight and clean with a wire of stiff brush!. I then thouroughy dry with a grease rag which has been lubed with Wd 40 or some other penetrating oil to prevent rust again!

I wonder if it's ozidation on pennies or jus the build up of oil from our hands and dirt etc. Coins ar so filthy dirty and a grat source of germs. Well you know about hepatitus and money handling... I bet the oils tend to attract more dirt and then just builds up over time until coins become stinky or tarnished!

[lightarrow]

I have found that dumping old tarnished (brown) copper coins in vinegar (with a little added salt (NaCl)), results in the coins becoming pink and shiny within a few minutes.

What is the nature of the chemical reaction that is obviously taking place, and why do the coins go brown in the first place? By contrast, a copper roof goes green yet must be exposed to similar chemistry to a coin.

Chris

Black copper oxide CuO reacts with acetic acid CH₃COOH (inside vinegar) forming copper acetate Cu(CH₃COO)₂ and water:

CuO + 2CH₃COOH  --> Cu(CH₃COO)₂ + H₂O

A similar reaction happens with Cu(OH)₂, CuCO₃,  Cu₂CO₃(OH)₂ ecc.

The copper acetate is water soluble and dissolves away, leaving a clean copper metal surface.

By the way: copper acetate forms very beautiful dark emerald green cristals when dried.

[Karen W.]

Hey Alberto, Is the copper acetate, the old copper tarnish? Are the beautiful crystals hard like Chrystal's? How would I do this for an experiment for 3 to 5 year olds? How long do the Chrystal's take to form, or to dry? You said it is water soluable, Meaning No vinegar??

[chris (OP)]

Thanks Alberto for that very clear and comprehensive answer. Can you explain why the coins form black copper oxide (CuO), whilst the copper roofs around here are all green? Why doesn't a copper-topped house end up looking all tarnished like my loose change?

Thanks, Chris

[lightarrow]

Hey Alberto, Is the copper acetate, the old copper tarnish? Are the beautiful crystals hard like Chrystal's? How would I do this for an experiment for 3 to 5 year olds? How long do the Chrystal's take to form, or to dry? You said it is water soluable, Meaning No vinegar??

Hello Karen!
1.I sincerely don't know what old copper tarnish could be; consider that, especially in a kitchen, there are hundreds of volatile compounds that can react with copper in a long time; however it can't be (simply) copper acetate, because this is water soluble and it doesn't form a resistant layer on the metal surface. A tipical chemical that copper forms in the kitchen is copper sulphide (black tarnish).

2.The beautiful crystals are not hard, unfortunately, they are quite soft, instead. As a general rule, the harder a crystalline form of a compound, the more difficult it is to make big crystals of it!

3. You need: CuO (copper oxide) or Cu(OH)₂ (copper idroxide) or CuCO₃ (copper carbonate) and acetic acid (if you use vinegar you probably obtain a mixture of copper acetate with others compounds and I can't guarantee the results, I used acetic acid).
 
I made Cu(OH)₂ from common CuSO₄ (those blue crystals used against grape mildew in agricolture, it's commonly sold here) dissolving CuSO₄ in water and then adding NaOH (sodium idroxide). Cu(OH)₂ is not water soluble so you can filter it (and wash with water). Then you add acetic acid and Cu(OH)₂ dissoves forming copper acetate. Let the solution be a little acidic adding a little more amount of acetic aced and then let the solution evaporate under a hood (to avoid inspiring acetic acid vapours) heating it in water bath (for example). You will have tiny crystals of copper acetate.

You can use Na₂CO₃ (soda) instead of NaOH; you will make CuCO₃ but it's the same (just it will "fizz" adding the acid!)

4.If you want bigger crystals (3-4 millimeters) you have to make the solution dry very slowly at room temperature. It could need some days. Put something over the container, without completely covering it (or absorbing paper) to avoid powder from air going inside.

5.It's vinegar soluble (actually, vinegar is mostly water!).

[lightarrow]

Thanks Alberto for that very clear and comprehensive answer. Can you explain why the coins form black copper oxide (CuO), whilst the copper roofs around here are all green? Why doesn't a copper-topped house end up looking all tarnished like my loose change?

I wrote CuO to semplify the concept, but, actually, it's probably a mixture of many compounds, including CuO, CuS and fat acids esters. Consider that a copper-topped house and your loose change "lives" different chemical and physical conditions. Think about the idrolipydic film on human's skin, for example (lipidic acids ecc.)

[Karen W.]

 THanks Alberto That is very interesting indeed! Where does one get the compounds and are they safe as far as poisen control for wee ones?

[neilep]

I have found that dumping old tarnished (brown) copper coins in vinegar (with a little added salt (NaCl)), results in the coins becoming pink and shiny within a few minutes.

What is the nature of the chemical reaction that is obviously taking place, and why do the coins go brown in the first place? By contrast, a copper roof goes green yet must be exposed to similar chemistry to a coin.

Chris

As a firm believer in empirical study  I've just done this ....they look amazing !

What will happen should I leave my coins in the solution overnight ?...it's a lot of money !!..about 7 pence !!!

[Karen W.]

I have left mine overnight.. They are just really clean looking. Its nice to do to pennies as they get icky to handle from so much handling. I have soaked so much rust off tools, that the rusted rivets that held them together just disintegrated! LOL The rust was actually all that was holding them together! LOL

[lightarrow]

THanks Alberto That is very interesting indeed! Where does one get the compounds and are they safe as far as poisen control for wee ones?

All copper salts, (e.g., CuSO₄) are toxic if eaten. You can make Cu(OH)₂ or CuCO₃ as I explained. About acetic acid, you can try to look for it in shops who sell solvents for paintings, or you can try to use white concentrated vinegar. (BTW, don't breath concentrated acetic acid directly, the smell is very strong).

You can also try to dissolve the very copper metal with vinegar and oxigenated water, but it would take much more time and I don't know if the reaction also produces other chemicals; you can try with little metal chips and heating (avoiding brathing the fumes) to speed the reaction.

Maybe you can even find copper acetate itself:
http://en.wikipedia.org/wiki/Verdigris

[lightarrow]

What will happen should I leave my coins in the solution overnight ?...it's a lot of money !!..about 7 pence !!!

You want to become poor!
Nothing happens in 1 day, but in a long time (I don't know how much) air oxygen together with the acid will dissolve your "treasure".

[Karen W.]

THanks Alberto That is very interesting indeed! Where does one get the compounds and are they safe as far as poisen control for wee ones?

All copper salts, (e.g., CuSO₄) are toxic if eaten. You can make Cu(OH)₂ or CuCO₃ as I explained. About acetic acid, you can try to look for it in shops who sell solvents for paintings, or you can try to use white concentrated vinegar. (BTW, don't breath concentrated acetic acid directly, the smell is very strong).

You can also try to dissolve the very copper metal with vinegar and oxigenated water, but it would take much more time and I don't know if the reaction also produces other chemicals; you can try with little metal chips and heating (avoiding brathing the fumes) to speed the reaction.

Maybe you can even find copper acetate itself:
http://en.wikipedia.org/wiki/Verdigris

Thanks alberto, I will investigate to see what I can come up with!

[paul.fr]

But what do you do with the "solution", or rather the liquid that is left in your recepticle now you have taken your lovely shiney coins out?

You can plate some aluminium nales....Just pop some nails into the "solution" and a few hours later you have copper plated nails, a good one for the kids to try...quite cool realy.

Now, can you do this the other way around?
Leave dirty 5 or 10 pences in a cup of vinegar, wait a few hours and then "plat" your 2 pences to look like 10 pences?

Paul

[paul.fr]

oh, and does lemon juice not work better than vinegar?

Paul

[lightarrow]

But what do you do with the "solution", or rather the liquid that is left in your recepticle now you have taken your lovely shiney coins out?
You can plate some aluminium nales....Just pop some nails into the "solution" and a few hours later you have copper plated nails, a good one for the kids to try...quite cool realy.

The Cu⁺⁺ solution should be quite dilute, or it dissolve aluminum.

Now, can you do this the other way around?
Leave dirty 5 or 10 pences in a cup of vinegar, wait a few hours and then "plat" your 2 pences to look like 10 pences?

What 2 pences are made of?

[lightarrow]

oh, and does lemon juice not work better than vinegar?

It could be. Never tried. You have already done the experiment and it works better?

[paul.fr]

Yes, lemon juice is better and smells a whole lot nicer too.

[paul.fr]

But what do you do with the "solution", or rather the liquid that is left in your recepticle now you have taken your lovely shiney coins out?
You can plate some aluminium nales....Just pop some nails into the "solution" and a few hours later you have copper plated nails, a good one for the kids to try...quite cool realy.

The Cu⁺⁺ solution should be quite dilute, or it dissolve aluminum.

Now, can you do this the other way around?
Leave dirty 5 or 10 pences in a cup of vinegar, wait a few hours and then "plat" your 2 pences to look like 10 pences?

What 2 pences are made of?

But what do you do with the "solution", or rather the liquid that is left in your recepticle now you have taken your lovely shiney coins out?

You can plate some aluminium nales....Just pop some nails into the "solution" and a few hours later you have copper plated nails, a good one for the kids to try...quite cool realy.

Now, can you do this the other way around?
Leave dirty 5 or 10 pences in a cup of vinegar, wait a few hours and then "plat" your 2 pences to look like 10 pences?

Paul

But what do you do with the "solution", or rather the liquid that is left in your recepticle now you have taken your lovely shiney coins out?
You can plate some aluminium nales....Just pop some nails into the "solution" and a few hours later you have copper plated nails, a good one for the kids to try...quite cool realy.

The Cu⁺⁺ solution should be quite dilute, or it dissolve aluminum.

Now, can you do this the other way around?
Leave dirty 5 or 10 pences in a cup of vinegar, wait a few hours and then "plat" your 2 pences to look like 10 pences?

What 2 pences are made of?

Quite right, and why did i put aluminium nails in the first place? I should have said bog standard steel nails.

[chris (OP)]

Paul - have you actually tried this?