I am asking here another beginner question. I thought I could dissolve the zinc out of a brass coin with an acid, but it did not work. I put the coin
into 28% muriatic acid and there was no reaction at all. Then I put it into sulphuric acid (about 40%), but there was no reaction either, even when I
boiled it.
It did react with copper sulphate, though, being covered with a thin dark layer supposedly of copper; so I suppose it must be the zinc which reduced
the copper ions. I wonder why it does not react with acids? and is there a feasible way to selectively dissolve either the zinc or the copper from
brass? not_important - 3-4-2009 at 07:36
with a low zinc brass the zinc on the surface will dissolve, leaving just copper exposed to the HCl. The amount of zinc dissolved is tiny, it's not
like there are big crystals of zinc but rather it is in solid solution with the copper. bquirky - 3-4-2009 at 08:33
the process of desolving zinc from brass in an acid is called dealloying and results in a pourus structure of copper it is however a very slow process
and no immediate signs of a reaction is visible. after a few days you might find that the HCL starts turning purple as the zinc starts to be leached
out but it takes a long time mutch longer if the brass is a solid chunk instead of a sheet or a tube ! woelen - 3-4-2009 at 11:46
The HCl turning purple??? Are you sure you are talking about zinc? Zinc ions are colorless! Titanium(III) ions are purple, so you may confuse the zinc
with titanium.Sedit - 3-4-2009 at 12:54
Iv done this before and it leaves a bright copper layer exposed and the HCl will take on a slight green tinge assuming from trace copper dissolving.
Having it progress to the point of dealloying doesn't seem to reasonable unless the brass was finely ground to begin with.
~Seditbquirky - 4-4-2009 at 02:23
Hmm Every time i have dissolved zinc or brass or anything galvanized in HCL it has turned quite purple.
I must have some interesting contaiminants in my HCL i guess. itd be nice if id found a source of titanium !
I think the ability to dealloy the whole sheet of brass is dependent on the percentage of zinc present.
Ive gotten interested now i might try chucking my peice of formally brass tube under a microscope and see what the surface looks like ill see if i can
get a picture to post.
regardsDaddy - 6-4-2009 at 12:58
Thank you all for the information ... I will have to read some more. As far as I saw, the amount of zinc in the brass is crucial, it will probably not
dissolve if there is less than 15% zinc... how can I find out how much zinc is in my brass?
Maybe I will sand some of the brass to get brass powder and try if that works.Sedit - 6-4-2009 at 13:08
Sanding will introduce Aluminum oxide contamination... I guess that could be filtered out but a faster way to get some smaller grains would be to take
a hacksaw to it.kclo4 - 6-4-2009 at 16:39
Daddy, it sounds like you are wanting zinc, try the "newer" US pennies, they are almost pure zinc, wrapped in a layer of copper and they will happily
dissolve in acid. markgollum - 6-4-2009 at 16:54
The galvanic series is valid even on the atomic scale. But interestingly, there are many galvanic series, since changing the conditions can affect the
ordering of the series quite a bit. You can get de-zincification of brass but I think it is slow since it would be limited by solid state diffusion. http://www.corrosionsource.com/technicallibrary/corrdoctors/...Daddy - 8-4-2009 at 07:41
Quote:
try the "newer" US pennies, they are almost pure zinc,
That would be great, but no way to get US coins here in Peru. Our coins are all steel or brass.kclo4 - 8-4-2009 at 15:40
I see, well that is unfortunate! I guess you might have to extract the zinc and copper with some nitric acid maybe?not_important - 8-4-2009 at 16:52
Try electroplating, it should be possible to plate copper from a brass anode to a cathode that becomes covered with the copper, while the zinc remains
in solution. An acid electrolyte would be used, the zinc appearing as the salt of the acid, and a rather low voltage applied between the electrodes -
often no greater than 1 volt. In effect you're transporting the metal with the lowest activity from anode to cathode, as the more active metal - zinc
- is also the minor component.
If the majority metal is more active than the minor one, and the minor metal is not too active, then the minor one remains behind as a sludge below
the anode.
Strange result
Daddy - 11-4-2009 at 07:14
Nitric acid... cannot get that, I would have to make it by myself, once I learn how to do it.
Electroplating sounds interesting, I will have to try that.
Meanwhile I tried the suggestion with powdered brass. I put some brass powder into a cup, poured some HCl over it and let it sit there for some days.
I got a strange result. The liquid had turned into a dark brownish green, and what was left of the powder had turned white. I will try to attach a
photo below.
So this looks as if the copper had dissolved (does powdered copper react with HCl?); but copper chloride would be blue. As for the green color, my
guess would be iron chloride; I had produced that once and it had the same color, and it attacked stainless steel, as the liquid I got now also does.
(Is it common for brass to contain considerable amounts of iron?) - If there is copper chloride too, its color would of course mix into the green.
As for the white powder, I am clueless. I do not know any white copper salt. If it were zinc chloride, it would have dissolved.
There are also some black stains, but I would guess that is contamination from the sandpaper, as was mentioned earlier. (At the moment I have no other
means to produce brass powder.)
So here goes the photo:
(Clarification: The brass powder was NOT in contact with air during the experiment; I only inclined the cup in order to take the photo.)
not_important - 11-4-2009 at 18:21
What you have is a mix, inlcuding copper(I) - cuprous - chloride. It is a white solid, and will dissolve in concentrated HCl or solutions of
chlorides to form a yellowish liquid; plus copper(II) - cupric - chloride, green to blue green in moderately dilute solutions, going green then
yellow-brown as the concentration is increased and/or more chloride ion is added as with an excess of strong HCl.
If you took about half the powdered brass and dropped in an excess of hydrochloride acid, and occasionally shook it to expose it to fresh air
(oxygen), you'd get Cu(II) as the chloride complex. If you then add the remaing half of the metal powder and stoppered it up to keep out air, the
copper would mostly be converted to Cu(I). With excess HCl it would all go into solution, pouring into cold boiled (no O2) water dilutes the
solution, drops the chloride concentrate, and precipitates most of the CuCl as a white powder that easily oxidises and turns green when damp and
exposed to air. kclo4 - 11-4-2009 at 19:54
I thought of another easy source for zinc if you are interested. The shell on a battery I believe is made out of zinc. .. sorry bit off topic but I
thought I'd let you know. Daddy - 14-4-2009 at 12:53
Thank you, not_important, that was quite detailed. So I have now both copper and zinc dissolved... but no way to dissolve selectively either only zinc
or only copper, I am afraid.
Quote:
I thought of another easy source for zinc if you are interested. The shell on a battery I believe is made out of zinc. .. sorry bit off topic but I
thought I'd let you know.
Not so much off topic, I will try if I find such a battery. Most of those I have seem to be made from steel (they attract a magnet), but maybe I find
a brand which is made from zinc. (Peru is a large producer of iron, so there is probably more iron and steel used here in places where other countries
use other metals.)
There is certainly zinc or zinc oxide *inside* a battery, but mixed with other substances, so there is again the problem of separating them.
S.C. Wack - 14-4-2009 at 14:11
You would want the older type of battery, for zinc, if they are still sold there. These are the ones that are cheaper than alkaline batteries, don't
last as long, and have a carbon electrode inside. I have seen these with an outer steel can right on top of the Zn can.
Surely Zn sacrificial anodes could be found.Sedit - 14-4-2009 at 14:25
Yes zinc can be found in abundance as anodes for underground water pipe and in a nice large brick as the camp plate for boats to prevent all the
underwater pieces from corroding. A cheeper source would be zinc based metallic paint which an acetone wash should remove any petroleum products
present and leave on with zinc dust. Sheet metal thats used for duct work is coated in a layer of zinc crystals which HCl will quickly remove.chloric1 - 14-4-2009 at 16:04
Brass is definately not the idea source for zinc. Copper sulfate is actually a catalyst to assist in dissolving zinc in acid. This is becuase very
pure zinc is sluggishly soluble in acid! The copper sulfate provides free cupric ions to establish a galvanic affect. I would try your 28% HCl or 40%
Sulfuric solution with 2-4% copper sulfate to see if reaction commenses.Paddywhacker - 15-4-2009 at 05:00
Aren't many carburettors made from zinc? Ask a scrap merchant. You can use a drill to make lots of zinc shavings.
About your mixed zinc/copper solution ... a little NaOH will precipitate both. They can be filtered to concentrate your metals, then a little more
NaOH will dissolve the zinc as soluble sodium zincate. Filter off the copper and you have them separated.
An alternative is to acidify and pass in H2S. CuS will precipitate from acid solution, but ZnS will not.
