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papaya
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Identify an alloy
Hi, recently I was asked to look at some wire, which is made of an unknown alloy and i'm interested now to find out it's composition. The owner has no
idea where the roll came from and what for purpose it's intended.
It's 0.35mm diameter wire , a lustrous alloy which looks like nichrome, but it's MUCH softer (first I thought it could even be platinum). I managed to
measure it's specific resistance and it's around 5×10^−7 Ω·m (not very accurate).
Holding the wire in gas flame made it to blacken readily (oxides layer), the if that area is impregnated into concentrated HCL solution afterwards
the black thing disappears and becomes yellow - looks like copper. The reactivity to conc. HCL is very low, it needs to be heated to show some
bubbles, and the solution then becomes dirty green-yellow, also if left for long time at RT in HCL it's obvious that it gives off some black stuff
that doesn't want to dissolve anymore (not tried to boil).
When put in 50% HNO3 it reacts very fast and completely giving beautiful clear cyan-green colored solution, when ammonia is added to this solution
(diluted) it gives blue color confirming the presence of copper. If basic hypochlorite solution is added it first makes a voluminous whitish
precipitate (or colloid) which after seconds turns totally black. I saw same behavior with Ni salts, and also the shiny look of the wire lets me
suppose that there a Ni in there.
Now there's something I can't understand. When that HNO3 solution is added to KMnO4 water solution by drops, at some point it makes permanganate to
discolor sharply (it's reduced by something), the solution stays clear cyan color as before. I thought it could be Mn2+ reacting with MnO4- to give
MnO2, but if hydrogen peroxide is added afterwards there's no oxygen evolution (no MnO2 as well as KMNO4 left). What else can reduce permanganate with
no color change, precipitation, etc?
If that wire is put as an anode into NaCL solution it erodes giving the same volumnious whitish colloid and after few drops of bleach it's totally
black. However it doesn't give yellow chromate colr to solution dissolving as anode (real nichrome does, giving chromium +6 color to come off from
anode surface), so it contains no chromium.
I didn't try for iron (no any rodanide), but it's not likely I think.
So I found nickel and copper nearly sure, specific resistance is around 5×10^−7 Ω·m and from this table
https://en.wikipedia.org/wiki/Electrical_resistivity
it is close to constantan, however I was told(and shown a sample) that constantan looks very differently (it's more brass color than nickel), also
unusual reaction with KMnO4 abovementioned gives me more doubts.
Help.
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plante1999
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Very high end electrical wire are copper with 10% silver coating on them, as the electricity pass on the exterior of the wire, it is unnecessary to
make it plain.
I never asked for this.
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papaya
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No, you misunderstood, it's not something deposited on copper, it's an alloy.
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hyfalcon
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Sterling silver is my guess, and it's just a guess.
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blogfast25
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Copper yes, nickel I'm not convinced yet.
The symbol for chlorine is Cl, not CL.
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papaya
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hyfalcon, sterling silver contains silver, while I think mine is a nickel alloy.
blogfast, why you doubt on Ni?
OK, if you can suggest some more reliable(but non demanding) qualitative tests for metals (Ni, Mn, Ag, .. Zn?) that I can try it would be excellent.
For example I got an idea now that I really should test for Ag with chloride I guess(hope excess nitric acid doesn't interfere). Also any ideas why it
cause permanganate discoloration?
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AJKOER
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What was the original use of the alloy?
If it melts easily, it could be solder. If so, there are various compositions with (Zn)x(Sn)y(Cu)z and (Pb)x(Sn)y(Cu)z. Also, some solders contain Ag,
In, Sb, Bi, Cd and even Au. See Wikipedia http://en.wikipedia.org/wiki/Solder for details.
In my opinion, the first likely metal to test for is Lead given potential toxicity issues.
If electrical wiring, it could contain various alloys with Copper or even (Cr)x(Ni)y (see http://www.rgalloy.com/en/nickelalloywire.htm ).
More generally, the alloys employed in metal wire are even more various (see http://www.globalspec.com/SpecSearch/SearchForm/materials_ch... ).
[Edited on 17-1-2014 by AJKOER]
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cyanureeves
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sounds like silver solder wire to me too,the kind that comes in a spool and not the reddish rod with phosphorous.btw phosphorous makes silver solder
very difficult to dissolve in nitric acid because it forms a crust of crud around the silver/copper.
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Metacelsus
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https://en.wikipedia.org/wiki/Qualitative_inorganic_analysis
You can test for all sorts of ions. Does the oxide layer dissolve completely in hydrochloric acid?
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papaya
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Chloride test gives negative when a drop from HNO3 solution is put in NaCl or HCl solution - no precipitate or turbidity observed, HNO3 cannot
interfere, isn't it?
Cheddite, as I can remember the oxide layer vanished in seconds in concentrated HCL revealing yellow-red copper color, I let the wire(including the
part not covered by oxides) to sit in HCl + few drops of H2O2 until now and today I discovered that wire disintegrated into small thin
pieces(especially when I shake test tube) - it's slowly dissolving in HCl, but some small amount of black colored stuff is still there (repeating wire
shape), I guess it's remaining copper ? I'd not to add peroxide, now I can't tell, but anyway no insoluble chlorides are present (so no silver is
there).
And no, it's not a solder, it doesn't melt in fire as I described it just blackens.
How do I make sure for Ni, or other things I can try out?
[Edited on 18-1-2014 by papaya]
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blogfast25
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Colourful test for Ni:
http://en.wikipedia.org/wiki/Dimethylglyoxime
Unfortunately DMG is not so easy to get.
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papaya
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Yes, I knew that reagent, however it's hard to obtain. I also thought of hydrazine hydrate, that gives pink complexes with Ni salts (and usually
insoluble), however hydrazine will reduce copper as a side reaction.
Btw., what color cupronickel should be in reality? It's said to be looking like silver (so without a hint of yellow), but how it is that the
constantan containing more %-age of nickel looks like brass (as far as I know) ?
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cyanureeves
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american coins that are nickel looking only have about 25% nickel and about 75% copper and will plate out a bunch of copper at the cathode in
hydrochloric acid solution and electricity.i read cupronickel is also used extensively in heating element wire just like nickel/chromium and the wire
does not look copperish.not to mention the look of 10kt. gold with high copper content.
[Edited on 1-18-2014 by cyanureeves]
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Zyklon-A
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It sounds a lot like lead and silver, maybe some nickel as well.
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papaya
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Zyklon, read carefully, there cannot be lead or silver, chlorides of both are nearly insoluble.
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Metacelsus
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Try gassing it with H2S and see if anything precipitates.
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blogfast25
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That used to be used as a separation technique in wet analytics. Copper sulphide is so insoluble (also HgS) that even gassing with H2S in acidic
conditions (where [S<sup>2-</sup>] is extremely small) yields CuS. The slightly more soluble sulphides of Zn but also Ni (if I recall
well) stay in solution because their solubility product isn't reached.
But it's a lot of faffing and H2S is very toxic.
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papaya
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I may try first add NaOH to precipitate Cu and Ni (*any* metal that gives insoluble hydroxides), separate supernatant liquid and add Na2S solution to
see if Zn or something precipitates. Only thing is Cu(OH)2 is still slightly soluble in alkali.
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blogfast25
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Very high values of pH are needed for significant solubilisation of copper(II) as cuprate.
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Fantasma4500
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im just sitting here wondering how you guys can assume something that forms a colourful complex with ammonia would be silver
wtf guys?! 
i suppose you might have a common alloy: 75:25 nickel
i recall this is used in US nickels, and i dissolved this alloy somewhat from common coins in 32% acetic acid with 12% H2O2 and heat
however i dont really recall it turning black when heated, as in with a strong flame? that is indeed weird
you could try to look up threads on here on how to seperate nickel from copper, there was alot of talking about precipitating the copper as Cu2C2
(which is an primary explosive!!) to leave behind copper in the solution, you could try this on a smaller scale?
then you would most likely be left with nickel and something else (if there is anything else)
its like shiny metals never cease to be interesting..
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forgottenpassword
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Calculate the density. No chemical tests are needed.
http://www.avlandesign.com/density_metal.htm
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Vargouille
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Density isn't the best thing to measure. Not only will multiple alloys have the same or similar densities, within the bounds of error for anything
other than a very accurate apparatus, but in an amateur setting it's difficult to get accurate results.
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DraconicAcid
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Quote: Originally posted by Vargouille  | | Density isn't the best thing to measure. Not only will multiple alloys have the same or similar densities, within the bounds of error for anything
other than a very accurate apparatus, but in an amateur setting it's difficult to get accurate results. |
Even a rough density measurement will help narrow down the possibilities.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Vargouille
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Narrow it down, sure, but there's quite a step between narrowing down and "no chemical tests needed".
[Edited on 20-1-2014 by Vargouille]
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forgottenpassword
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What would be better?
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