Sciencemadness Discussion Board

Identify an alloy

papaya - 17-1-2014 at 13:13

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.

plante1999 - 17-1-2014 at 13:48

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.

papaya - 17-1-2014 at 13:53

No, you misunderstood, it's not something deposited on copper, it's an alloy.

hyfalcon - 17-1-2014 at 14:01

Sterling silver is my guess, and it's just a guess.

blogfast25 - 17-1-2014 at 14:23

Copper yes, nickel I'm not convinced yet.

The symbol for chlorine is Cl, not CL.

papaya - 17-1-2014 at 14:31

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?

AJKOER - 17-1-2014 at 15:06

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]

cyanureeves - 17-1-2014 at 15:40

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.

Metacelsus - 17-1-2014 at 20:32

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?

papaya - 18-1-2014 at 02:14

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]

blogfast25 - 18-1-2014 at 09:33

Colourful test for Ni:

http://en.wikipedia.org/wiki/Dimethylglyoxime

Unfortunately DMG is not so easy to get.

papaya - 18-1-2014 at 09:46

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) ?

cyanureeves - 18-1-2014 at 09:57

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]

Zyklon-A - 18-1-2014 at 09:59

It sounds a lot like lead and silver, maybe some nickel as well.

papaya - 18-1-2014 at 10:12

Zyklon, read carefully, there cannot be lead or silver, chlorides of both are nearly insoluble.

Metacelsus - 18-1-2014 at 10:34

Try gassing it with H2S and see if anything precipitates.

blogfast25 - 18-1-2014 at 13:16

Quote: Originally posted by Cheddite Cheese  
Try gassing it with H2S and see if anything precipitates.


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.

papaya - 18-1-2014 at 13:38

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.

blogfast25 - 18-1-2014 at 13:45

Quote: Originally posted by papaya  
Only thing is Cu(OH)2 is still slightly soluble in alkali.


Very high values of pH are needed for significant solubilisation of copper(II) as cuprate.

Fantasma4500 - 19-1-2014 at 06:37

im just sitting here wondering how you guys can assume something that forms a colourful complex with ammonia would be silver
wtf guys?! :D

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..

forgottenpassword - 19-1-2014 at 07:14

Calculate the density. No chemical tests are needed.
http://www.avlandesign.com/density_metal.htm

Vargouille - 19-1-2014 at 09:46

Quote: Originally posted by forgottenpassword  
Calculate the density. No chemical tests are needed.
http://www.avlandesign.com/density_metal.htm


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.

DraconicAcid - 19-1-2014 at 09:58

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.

Vargouille - 20-1-2014 at 10:03

Quote: Originally posted by DraconicAcid  
Even a rough density measurement will help narrow down the possibilities.


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]

forgottenpassword - 20-1-2014 at 10:28

Quote: Originally posted by Vargouille  
Quote: Originally posted by forgottenpassword  
Calculate the density. No chemical tests are needed.
http://www.avlandesign.com/density_metal.htm


Density isn't the best thing to measure.
What would be better?

Vargouille - 21-1-2014 at 06:56

Quote: Originally posted by forgottenpassword  
What would be better?


There's a pathway for qualitative chemical analysis that is nicely summarized on this Wiki page. It'll be a monster if you have too many metals included from too many groups, or if more exotic metals are included in the sample, but it's a good place to start.