# What does HCL dissolve?



## rtl326

I have been recovering gold foils from fingers and PC board fragments by simply soaking them in Muriatic Acid. The acid turns an opaque green after several batches but remains quite active and does the job in a few days when left out in the summer sunshine. I had assumed the green was mostly from copper chloride but reads Hoke tells me that HCL will not dissolve copper.

So....what metal is holding the foils onto the boards and turning the bath green when it is eaten by the HCL? Nickel?


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## RESET

There may be a layer of nickle between the copper and the gold. The preferred method of removing foils from fingers here is to put the in what is refereed to as Acid Peroxide. The HCl and the peroxide combine to dissolve the copper. Peroxide oxidizes the copper and the HCl dissolves the Copper Oxide. There may have been an impurity in your HCl that started the oxidization cycle and dissolved the copper. or, if you HCl was diluted, as Muriatic usually is, maybe the water and sunlight worked to oxidize the copper

I threw a copper buss bar into Muriatic thinking that it would dissolve the steel and tin present and leave the copper and silver. It took a while to get going so I forgot about it. Went back to it a couple months later and everything but the silver was dissolving.


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## rtl326

The Muriatic I use is standard old 32% acid from Ace Hardware and from the local farm supply store that is intended for etching concrete. Maybe it is not very pure - it certainly seems to eat copper despite statements by Hoke and some I have read in this forum that it should not. I used peroxide a few times but it doesn't seem necessary. Straight HCL might not be as fast but then I am in no hurry and it does the job reliably. It would seem that peroxide would need to be replenished and dilutes the HCL as it breaks down. As things are, I have used the same four gallons of HCL to pull out 40 grams of gold foils (including soldermask and other trash).

Thanks for you comment.


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## Palladium

Your right hcl won't dissolve copper without an oxidizer to change the copper to copper oxide. Hcl can dissolve copper oxide. The only thing you need is an oxidizer. That can come from the oxygen in the water or oxygen that is adsorbed from the air into solution. We can also use an air bubbler or H2O2. Anything hcl and oxygen is basically what we call AP or acid peroxide that eats copper.


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## Clneal2003

That being said palladium. What metals won't HCl dissolve without the proper oxides? I'm just wondering if it can dissolve brass as well?


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## butcher

Take a good look at the reactivity series of metals.

Some acids can be strong oxidizers like nitric acid, other acids that are not strong oxidizers like HCl will need an oxidizer to dissolve a metal below hydrogen in the reactivity series.


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## Geo

Chlorine is the second lightest halogen following fluorine. Chlorine has the highest electron affinity and the fourth highest electronegativity of all the reactive elements. For this reason, chlorine is a strong oxidizing agent. what this means is, chlorine (hydrochloric acid) will dissolve almost everything. The next strongest being Fluorine (hydrofluoric acid).


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## Palladium

Brass is usually copper and zinc so yes it will dissolve them. Zinc without an added oxidizer and copper with an oxidizer like normal air.


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## butcher

Chlorine and HCl acid (hydrochloric acid) are both of the halogen family, but they are very different in how they will react with metals.

Chlorine can oxidize metals, even metals below hydrogen in the reactivity series (although it normally requires heat to help in this process), it will react with copper, and it will even react with gold to take electrons from gold.

HCl acid will not react with copper or gold.

HCl acid (hydrochloric acid) is normally considered a non-oxidizing acid.

Chlorine will react with copper (normally heated to help this reaction), chlorine gas can be a strong oxidizer.

Even dissolved in water chlorine gas is an oxidizer, mainly as it forms HCl (Hydrochloric acid) and HClO (Hypochlorous acid) in this solution, the HClO is very unstable and begins to break down to HCl acid and oxygen, in this solution.

This is where Chlorine becomes an oxidizer for our gold when we use it with HCl acid (Note the HCl acid we use, and the bleach we add is mostly water), to dissolve our fine gold powder or foils in the process used by many here on the forum.
(Bleach being basic can help hold the HClO in solution a tiny bit longer as it somewhat lowers, the acids pH, but that is starting to get off topic here).

HCL acid (hydrochloric acid) will not react with copper unless an oxidizing agent is involved).

Note this oxidizer can be several things, if we are discussing copper, air or oxygen which can be in solution, some copper oxide in solution (which basically would form copper II chloride which can be an oxidizer for copper), H2O2… 
This list can go on and on, the point here is; for HCl acid (hydrochloric acid) to dissolve copper the copper atoms must be oxidized (an electron taken from the copper atom first) by an oxidizing agent, HCl is not an oxidizing agent for copper.
HCl acid cannot oxidize copper alone. The chloride ion in HCl acid has a full shell of electrons, and will not take one from the copper, copper is below hydrogen in the reactivity series of metals, so the hydronium ion in HCl acid is happy just the way it is, and the copper (copper being below hydrogen in the reactivity series) will not displace the extra hydrogen from the hydronium ion (H3O), in this acidic solution.

HCl acid will react with metals above hydrogen in the reactivity series, the more reactive metals, but Note that it is not the chloride ions of the HCl acid, which is the oxidizing agent for these more reactive metals (or that is oxidizing these metals).
It is the hydrogen of the acid, or to be clearer, or precise, it is the hydronium ions H3O that reacts with these metals which are higher in series than hydrogen in the reactivity series of metals, or hydrogen here is the oxidizing agent for these more reactive metals, (for example zinc) hydrogen taking electrons from these metals as the Hydrogen is reduced to gas.

I think of the hydronium ion H3O as being acidic water, this is probably an improper way of thinking of it in the real scientific world but that’s how I tend to view it. 

Chlorine Cl2-is a gas, this gas atom in its outer shell has 7 electrons, it really wants to have 8 electrons in its outer shell, it will take that electron from metals, to gt a full shell of 8 electrons, (oxidizing the metal atom to make metal ions) for something to be oxidized (lose of electron), something has to be reduced, in this case the chlorine is reduced (gain of electrons) the chlorine here is reduced to chloride.
So basically chloride, and metals will form a solution of the copper ions and chloride ions, basically a dissolved salt of copper chloride in solution.
Acid + base = salt
Acid + Metal = metal salt, 
(Or a salt of that acid and the metal, which was involved in the reaction)

HCl (hydrogen chloride); (note the chloride here, with ide in its name), (also note also the chlorine here is reduced to chloride now having all of its electrons).

HCl (hydrogen chloride is a gas, HCl (g), a colorless gas with a sharp bite to the nose, heavier than air it will settle to the floor in your lab, HCl does not act as an acid, it is not an oxidizer of most metals, it is fairly happy with its hydrogen bond in this case.

HCl (g) (Hydrogen chloride) gas will react with water, in this case the solution becomes Hydrochloric acid, basically the hydrogen here joins with the water H2O to give the H3O hydronium ions in solution, mixed with chloride ions in solution (the extra hydrogen is what gives or makes this solution acidic).

This water can be in your lungs, breathing HCl gas can form hydrochloric acid in your lungs, or the moisture in the air making acid rain…

HCl(g) + H2O --> HCl acid 
or to show this another way
HCl(g) + H2O --> H3O + Cl-

Notice above that the water H2O gained hydrogen, forming hydronium ions (H3O), and we also have that chloride ion in solution running around.

Copper is below hydrogen in the reactivity series of metals (so basically it will not displace hydrogen from the H3O hydronium ion) and we also know the chloride ion is just fat and happy with its full shell of electrons, and does not want, or care to take an electron from the copper atoms..

To say this another way the chloride is not an oxidizing agent for copper, or it will not take electrons from copper...

Basically when it comes to copper metal. Chlorine, and HCl gas, or HCl acid, are completely different, or two completely different animals in this zoo of chemistry, one will eat copper, while one will not.


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## Geo

Thanks Butcher. Very well put, as always. 

I do not want to confuse anyone, and what Richard said is the absolute truth. This has been discussed before and everyone agreed that even though hydrochloric acid will not react to copper, if copper is left in just hcl long enough, it will eventually dissolve. The copper is not inert in hcl but the reaction is very subdued. Without a small amount of copper oxide to start with, it takes a very long time to start but once it does, it moves along at a faster rate. The oxygen comes from the air. There's a big technical name for it that I don't know but oxygen is absorbed at the surface of the liquid. If the oxygen were purged and a inert gas replaced it in the container, it may take even longer. There's a lot of things that happen when it's not suppose to. Even silver is not immune to hcl. It just passivates faster than it can be dissolved.


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