Sciencemadness Discussion Board

Dissolving unreactive metals in diluted sulfuric acid

azpcpXX - 4-3-2016 at 00:53

My chemistry teacher said that copper and another unreactive metal can be dissolved in diluted sulfuric acid. She says that if you pass oxygen thrоugh diluted sulfuric acid it will become very reactive and can react with copper without heating. I looked for more information about that but i wasnt able to find anything. Do anyone knows more?

gsd - 4-3-2016 at 00:57

I don't know about reaction happening without heating, but it will certainly happen.

In fact even dilute acetic acid reacts with copper in presence of air but it needs reflux temperature. (about 100 Deg C)

gsd

azpcpXX - 4-3-2016 at 01:13

Do you now about the reaction mechanism, how does oxygen interacts with the hydrogen ions?

gsd - 4-3-2016 at 01:50

I guess oxygen converts surface layer of copper into cupric oxide which then reacts with acid to form the salt. The salt gets leached into the solution exposing fresh surface for further oxidation and so on.

gsd



[Edited on 4-3-2016 by gsd]

woelen - 4-3-2016 at 02:34

This indeed works, but the reaction is slow.

As a test, put some copper piece or wire in a thin layer (a few mm) of dilute sulphuric acid in a petri dish and set aside. Do not cover the dish. Slowly, over the course of days, the copper will dissolve and you get a blue solution. It may be necessary to add water every other day or so, because of evaporation.

Fulmen - 4-3-2016 at 03:24

Another reaction route is Cu0 + Cu2+ = 2Cu+, oxygen can in turn oxidize the Cu+ back to Cu2+.

This reaction is used to leach copper with ammonia and ammonium carbonate to form cupric ammonium carbonate. It's initially very slow, but as the copper concentration increases it speeds up.

woelen - 4-3-2016 at 04:09

This route only works in the presence of suitable coordinating agents for Cu(+). The ion Cu(+) cannot exist in water without suitable coordinating agents. It works well in the presence of chloride ions and in the presence of ammonia. Both easily form complexes with Cu(+).

Copper etching with CuCl2 and HCl is based on this reaction. Cu(2+) and chloride ion oxidize copper(0) to copper(I), which dissolves as CuCl2(-). Oxygen from the air oxidizes the copper(I) to copper(II), and in this process, acid is used as well, hence the need for HCl. The concentration of copper(II) increases in this situation. Acid needs to be replenished every now and then.

ave369 - 4-3-2016 at 04:53

Not only oxygen can do this. Any oxidizer mixed with a non-oxidizing acid lets it react with unreactive metals. Usually, hydrogen peroxide is used, mixed with either dilute sulfuric or hydrochloric acid.

pesco - 10-12-2024 at 11:02

Quote: Originally posted by Fulmen  
Another reaction route is Cu0 + Cu2+ = 2Cu+, oxygen can in turn oxidize the Cu+ back to Cu2+.

This reaction is used to leach copper with ammonia and ammonium carbonate to form cupric ammonium carbonate. It's initially very slow, but as the copper concentration increases it speeds up.



This reaction is also the basis for PCB copper chloride etchant. I always preferred it over other etchants. First reason is, that it is very "precise", much easier to etch PCB's with very thin tracks. Second reason is that you will never run out of this etchant. The more PCB's you etch the more etching solution you get. Just add a splash of HCl every now and then and O2 diffuses into the solution on its own and can be sped up by bubbling air through it or agitating surface.
It is also easy to dispose of once you have too much.

pesco - 10-12-2024 at 11:11

Quote: Originally posted by azpcpXX  
My chemistry teacher said that copper and another unreactive metal can be dissolved in diluted sulfuric acid. She says that if you pass oxygen thrоugh diluted sulfuric acid it will become very reactive and can react with copper without heating. I looked for more information about that but i wasnt able to find anything. Do anyone knows more?


She is dead right.
I have a 3L jar with dilute sulfuric solution and when I get dead fuse it goes into the jar. I get quite a bit of them at work. Fuses are ceramic body with copper/brass caps plated with silver. After couple of weeks only the ceramic body remains and all the metals are dissolved.
Once or twice a year I empty it and have a bit of fun separating, purifying and converting the metals into different salts for later use.

chornedsnorkack - 13-12-2024 at 03:26

Quote: Originally posted by woelen  
This indeed works, but the reaction is slow.

As a test, put some copper piece or wire in a thin layer (a few mm) of dilute sulphuric acid in a petri dish and set aside. Do not cover the dish. Slowly, over the course of days, the copper will dissolve and you get a blue solution. It may be necessary to add water every other day or so, because of evaporation.

Is it necessary?
A table for equilibrium humidities of sulphuric acid:
https://bio.groups.et.byu.net/Equilibrium_Humidity_of_Sulfur...
You would like room air humidity between 30% and 60%. Which means that any sulphuric acid open to air would stabilize at concentrations between 40% and 55%. Maybe a bit more than 55% now in winter with central heating.
Is that a concentration where Cu is efficiently dissolved with air oxygen for oxydant?
Which substances are stable to prolonged exposure of 40...55% sulphuric acid plus air (which is what any sulphuric acid spill will wind up as)? Pottery is. I presume Au is (no complexants like halides or cyanides). What will Ag do? (Ag is much more inert than Cu, and Ag2SO4 poorly soluble in water).

teodor - 13-12-2024 at 05:22

Quote: Originally posted by chornedsnorkack  
Quote: Originally posted by woelen  
This indeed works, but the reaction is slow.

As a test, put some copper piece or wire in a thin layer (a few mm) of dilute sulphuric acid in a petri dish and set aside. Do not cover the dish. Slowly, over the course of days, the copper will dissolve and you get a blue solution. It may be necessary to add water every other day or so, because of evaporation.

Is it necessary?
A table for equilibrium humidities of sulphuric acid:
https://bio.groups.et.byu.net/Equilibrium_Humidity_of_Sulfur...
You would like room air humidity between 30% and 60%. Which means that any sulphuric acid open to air would stabilize at concentrations between 40% and 55%. Maybe a bit more than 55% now in winter with central heating.


I don't believe H2SO4 in concentration less than 80-83% can adsorb water from air with sufficient speed to reach this equilibrium during the time of the experiments. Also, there is no information about temperature and heat radiation/light, wind speed etc, so looks like this equilibrium lacks a lot of parameters for practical usage and requires some experiment to proof its applicability for the experimental conditions.

I have interesting results from my experiments of dissolving Cu in K2S2O8 solution. There are several grains of Cu metal from eBay supplier "Snauke elements". During the experiment oxidation of different grains went to different paths, I think due to some electric potential difference in the solution (I have no other explanation yet). Most of grains are dissolving and have shiny surface but one has a black coat of oxides. So, it should not be taken as granted that the reaction with Cu and acid can go only to one way at the same conditions IF THE METAL IS NOT ELECTRICALLY CONNECTED. As woelen mentioned, the actual chemistry of oxidation is complex and probably also can be dependent on some other effects. (As most prominent are different forms of galvanic corrosion).

[Edited on 13-12-2024 by teodor]

https://iopscience.iop.org/article/10.1088/0959-5309/40/1/33...
It mentions that Wilson's data is innacurate.

(For my general observation any data published in a book with the world "industrial" in its tittle is innacureate at least. Especially this is correct regarding synthetic pathways).


[Edited on 13-12-2024 by teodor]

pesco - 14-12-2024 at 22:58

Quote: Originally posted by teodor  


I have interesting results from my experiments of dissolving Cu in K2S2O8 solution. There are several grains of Cu metal from eBay supplier "Snauke elements". During the experiment oxidation of different grains went to different paths, I think due to some electric potential difference in the solution (I have no other explanation yet). Most of grains are dissolving and have shiny surface but one has a black coat of oxides.



I would double check the purity of these Cu "ingots".

For Cu of high purity best to use electrical wires/busbars. Its electrolytic copper with purity of at least 99.95%. It has to be as any contaminants, especially As, dramatically increase electrical resistance.
In my opinion electrical wires are more reliable than copper bars on eBay.
With that said, I have heard of poor quality chinese wiring. Haven't had any of these inferior wires in my hand, but the rumour is out there.

teodor - 15-12-2024 at 04:21



The problem is that any experiment should be repeatable. Getting wire from the same batch again is more problematic than relaying on a seller who claims certain purity of his material. Either his claim is false (and this is also a scientific result) or experiment really shows something. With just wires it could be anything. But they could be used as e reference. The point is that the type of surface changes an electrode potential and having different shapes (as in my case) is interesting.


[Edited on 15-12-2024 by teodor]

pesco - 15-12-2024 at 11:04

Quote: Originally posted by teodor  


The problem is that any experiment should be repeatable. Getting wire from the same batch again is more problematic than relaying on a seller who claims certain purity of his material. Either his claim is false (and this is also a scientific result) or experiment really shows something. With just wires it could be anything. But they could be used as e reference. The point is that the type of surface changes an electrode potential and having different shapes (as in my case) is interesting.


[Edited on 15-12-2024 by teodor]



As mentioned, wiring is made out of electrolytic copper, at least 99.95% copper. Same goes with busbars.

Also it is much easier to chop up some wire than cutting some tiny "ingots" into smaller bits.

Copper pipe is not as pure. Standards state 99.5%. Doesn't have to be pure and sometimes it is better if its not. Sometimes small amount of zinc is added for better solderability, but there is also whole host of impurities. It is not electrolytic copper. Electrolytic copper is more expensive and, unlike for wiring, it is not required for plumbing use.

Regarding eBay stuff, from experience, more often than not you get not really what was advertised. It is really not much work to buy some copper pipe, melt it, cast ingots and stump on them 99.999% Cu.

I bought some Pt anodes eBay and all of them didn't have any Pt on them.
Maybe I was just unlucky. Several times.

bnull - 15-12-2024 at 16:03

Want tiny ingots of high purity copper? RJ45 cables. An old office undergoing reform is probably the best source.

teodor - 16-12-2024 at 01:44

Quote: Originally posted by pesco  

Also it is much easier to chop up some wire than cutting some tiny "ingots" into smaller bits.


I never mentioned that I use "ingots".

If somebody sells 99.9% Cu for element collectors on ebay which is not 99.9% Cu I can contact him/her and ask for the explanations / make a review for my own benefit and benefit of element collectors. Otherwise I would take the responsibility to find proper wires by myself.

If the market of metals with high purity for element collectors exists there is a lot of people who are interested that the market is not a fake one.

[Edited on 16-12-2024 by teodor]