urenthesage
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Mystery metal
I bought a ball of metal from an element dealer at a great discount as we both had no idea what it was. I thought I might be getting a decent price
for a cadmium ball. The problem came up when I tested it in acid (weak muriatic acid (probably 10%)) I got a black chloride salt on the surface and I
cant seem to find a chart on what chlorides may be black. I still think it may be cadmium but Id like the board to see if it can help me here.
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Texium
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Thread Moved
30-8-2016 at 06:26 |
Sulaiman
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Lead and silver come to mind, but they are visually identifiable ....
A measurement of density would probably help in your quest,
a simple method https://www.youtube.com/watch?v=N1HCrNMnb9I
make sure that there are no air bubbles when weighing submerged
and no water when weighing dry.
[Edited on 30-8-2016 by Sulaiman]
P.S. Cadmium chloride is soluble https://en.wikipedia.org/wiki/Cadmium_chloride
so black precipitate is unlikely
[Edited on 30-8-2016 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Texium
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Well, if you thought it was cadmium then you couldn't really have "no idea what it was." What makes you think it's supposed to be cadmium? There are
other tests you can do too, without resorting to chemical reactions. Assuming that it's solid metal, the density should be able to tell you a lot, or
at least eliminate some possibilities.
Also, try adding the HCl you tried to react with it to a soluble source of sulfide (H2S or Na2S). If it is cadmium, you should
immediately see a yellow precipitate of CdS, even if the concentration of Cd is very low.
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MrHomeScientist
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A black coating suggests an oxide to me. I agree though, your first course of action should be to measure density. That will eliminate many options. A
picture here would be nice too!
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unionised
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A black coating suggests to me that there are two (or more) metals present.
For example, perhaps it is mainly Cd with a little Cu.
You put it in the acid and the Cd dissolves leaving a finely divided powder of Cu on the surface.
Another possibility is that it is a pure(ish) metal and the acid is etching the surface to give a very finely pitted surface which will look very
dark.
For the record, AgCl is white (and not made by the direct reaction of HCl with Ag and PbCl2 is also white- but at least it is possible to get it from
the direct reaction of the metal and the acid.
However that reaction is very slow so not much PbCl2 is formed.
And it's slightly soluble so you wouldn't expect to see a ppt with lead either.
There are very few insoluble chlorides and I don't think that any of them is black. Anhydrous CrCl3 is dark and only dissolves slowly- but you
wouldn't get that from reaction with a solution of HCl.
[Edited on 30-8-16 by unionised]
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phlogiston
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Uranium metal behaves in this way.... the black stuff would then be a mixture of uranium hydrides.
But you would probably have quickly noticed that the ball was unusually dense if it really was U.
Still, it would help if you could establish the density. I remember there was a thread once of a guy that found some solid bars of unknown metals and
one of them turned out to be a very large and heavy bar of platinum... the density was a helpful starting point in that case too. (Edit: found that thread)
A picture might help too. There are sometimes slight differences in hue, the way it is scratched, a slight patina that provide useful hints to the
trained eye.
Are there any notable markings, dents, ridges or indications of wear that may provide an indication of its prior use?
How hard is it? Can you scratch it? With your nail? A tool?
If at some point you are confident that is not very toxic or radioactive (cadmium, uranium) you could also hold it against a grinder to see if you get
sparks and what color/shape they have.
[Edited on 30-8-2016 by phlogiston]
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Melgar
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Quote: Originally posted by unionised  | A black coating suggests to me that there are two (or more) metals present.
For example, perhaps it is mainly Cd with a little Cu.
You put it in the acid and the Cd dissolves leaving a finely divided powder of Cu on the surface.
Another possibility is that it is a pure(ish) metal and the acid is etching the surface to give a very finely pitted surface which will look very
dark.
For the record, AgCl is white (and not made by the direct reaction of HCl with Ag and PbCl2 is also white- but at least it is possible to get it from
the direct reaction of the metal and the acid.
However that reaction is very slow so not much PbCl2 is formed.
And it's slightly soluble so you wouldn't expect to see a ppt with lead either.
There are very few insoluble chlorides and I don't think that any of them is black. Anhydrous CrCl3 is dark and only dissolves slowly- but you
wouldn't get that from reaction with a solution of HCl.
[Edited on 30-8-16 by unionised] |
This guy gets my vote for what's probably happening here. Just about any metal is black when finely divided, and you'd get that type of dissolving
and plating from an alloy. Most metal is in alloy form, so use Occam's razor, guys. If you still have the acid you used on it (and if there was any
suspicion of cadmium present you SHOULD), evaporate that and see if it leaves a white salt. Lots of tests you can do on the salt, if there is one.
(the salt would be one of the metals in the alloy, the black stuff would be another)
[Edited on 8/31/16 by Melgar]
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Sulaiman
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when I left a lump of 99.9% tin ingot in cold dilute HCl it also developed a black coating,
when given a good swirl around the black layer comes off as a powdery deposit,
then dissolves.
probably common for many metals ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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