[jasongnome]

I have some sheets of very very thin leaf metal which I'm fairly certain are either silver or aluminium. They're protected by paper sheets between the sheets of leaf and are still very shiny despite being left in a cupboard for over ten years.

Density isn't helpful because the metal is so thin so I really need a reliable chemical test. It doesn't react with any 2M acids, but I wouldn't expect either metal to so all that's really done is confirm that it probably is one of the two.

Aluminium foil can be made to displace the copper from 1M copper sulfate by adding a little sodium chloride. You get loads of copper made, it heats up significantly and hydrogen is given off (I guess that the aluminium exposed by the NaCl reacts with the hot water as well at the CuSO₄). I tried this and it was inconclusive. The metal did disintegrate, but there was no heat, no hydrogen and because it's so thin the tiny amount of metal particles left was too small to see if it was copper of not.

Any ideas?

[Arkcon]

Re-review the chemical properties of aluminum and silver, they are very distinct with regard to common mineral acids.  Also, its aluminum foil.  Why?  Simple.  Because other metal foils: silver, platinum, tin, or antimony are for specialized applications and are very expensive, and no one "loses track" and forgets them somewhere.

[jasongnome]

Re-review the chemical properties of aluminum and silver, they are very distinct with regard to common mineral acids.  Also, its aluminum foil.  Why?  Simple.  Because other metal foils: silver, platinum, tin, or antimony are for specialized applications and are very expensive, and no one "loses track" and forgets them somewhere.

No, it isn't foil, it's leaf. It's much much thinner than foil, it's so thin it has a level of transparency when you hold it up to the light. Whether it's aluminium or silver it's only for specialised applications. TBH, I have no use for it, I guess I could use it instead of gold leaf in the electroscopes, but as I have plenty of gold leaf that's not an issue. The fact that it's leaf complicates the issue, being so thin, I'm not 100% sure it will react identically to normal foil.

As to "losing track and forgetting them somewhere", I'm the head of science in an international school. My predecessor (well two predecessors ago) bought all sorts of stuff and forgot about it, this was found in the back of a cupboard I've finally got round to clearing out in a room that has, over the last 20 years been used at various times as a physics, technology and engineering room. I've found stuff, unopened, that's a damn site more expensive than silver leaf. Indeed, silver leaf isn't even all that expensive, one of it's less specialised uses is cake decorating...

[Enthalpy]

Transparency appears well under 1µm. To be made so thin, aluminium would be very pure, and then its excellent oxide layer makes chemical tests difficult.

Melting point? Oxidation in hot air?

Make a battery with a second electrode of aluminium foil, a mild electrolyte, and measure the voltage? Compare with graphite and aluminium electrodes to double-check that the oxide layer doesn't play a trick.

Taste the unknown metal, compare with known aluminium and silver? Or compare the reflection colour with known pure aluminium (kitchen foil suffices) and silver, as silver is whiter.

If you have optical apparatus, silver transmits blue light better than aluminium does. Tables are in the Crc Handbook of Chemistry and Physics. Instead of measuring the absolute attenuation, you could compare several wavelengths.

[jasongnome]

Make a battery with a second electrode of aluminium foil, a mild electrolyte, and measure the voltage? Compare with graphite and aluminium electrodes to double-check that the oxide layer doesn't play a trick.

This seems like the most reliable way, I should have thought of it myself. I could try it with a silver electrode as the second one as well.

If you have optical apparatus, silver transmits blue light better than aluminium does. Tables are in the Crc Handbook of Chemistry and Physics. Instead of measuring the absolute attenuation, you could compare several wavelengths.

I do have a colourimeter that may be sensitive enough for that. I'll try that as well.

Thanks for your help.

(I'm not going for the taste test as the reason I was clearing out this particular cupboard was that a pigeon had got in and crapped everywhere!)

[Corribus]

Possible beyond what's available to you, but: Digest s small amount in hot acid, then AAS, ICP-OES, or ICP-MS. For an identity check, wouldn't even need standards. If you don't have this equipment, a lot of universities do- with some sweet talking maybe you could get someone to take a quick look gratis.

[jasongnome]

Thanks for the idea, but I doubt the Uni's here would be able to do it.

[Arkcon]

The fact that it's leaf complicates the issue, being so thin, I'm not 100% sure it will react identically to normal foil.

Do you have some sort of citation that explains how the chemical properties of aluminum and silver are altered by being as leaf instead of foil, or wire, or lump?

[jasongnome]

The fact that it's leaf complicates the issue, being so thin, I'm not 100% sure it will react identically to normal foil.

Do you have some sort of citation that explains how the chemical properties of aluminum and silver are altered by being as leaf instead of foil, or wire, or lump?

No. If I had then I wouldn't be so vague as "not 100% sure". We all know that nanoparticles behave differently chemically, and in this case, although not a nanoparticle, the thickness of the material is on a microscopic scale.

The main problem with chemical tests on it, as I found with the copper sulfate test I tried is that the actual amount of material is tiny even if you tear off what looks like quite a large chunk so it very difficult to see if anything's happening or not. It's most likely, as you suggested, aluminium, however, the experiment with copper sulfate was inconclusive because there just wasn't enough material to get visible bubbles, measurable temperature increase, or a product that was actually visible. In that reaction I would expect no reaction with silver and copper displaced if it was aluminium. What I actually saw was the metal disintegrate as I stirred it and was left with tiny metal particles that I couldn't even see properly.

Do the math, according to Enthalpy (the user, not energy per mole!), the leaf must be less than 1μm thick to be displaying transparency. So if I take a piece 2cm on each side, my maximum total volume (using 1μm) is 4x10^-10m³. This is probably a significantly high estimate.

Given a density of aluminium of approx 2700 kg/m³ that gives me under 1mg of metal if we accept that it is LESS than 1μm. Given the amount I have, 2x2cm is quite a hefty chunk to take for a reaction that might not even be visible.

[Arkcon]

No. If I had then I wouldn't be so vague as "not 100% sure". We all know that nanoparticles behave differently chemically, and in this case, although not a nanoparticle, the thickness of the material is on a microscopic scale.

Speaking briefly, and in the most general sense, because I have to get to work, I think you're inflating this problem, in your own mind, to your own detriment.

The main problem with chemical tests on it, as I found with the copper sulfate test I tried is that the actual amount of material is tiny even if you tear off what looks like quite a large chunk so it very difficult to see if anything's happening or not.

OK.  Aluminum reacts very rapidly with most mineral acids, and NaOH.  Silver only reacts with nitric acid.  If the leaf can handle a drop of water without disintegrating, then just about anything, even house hold vinegar will eat a hole in it, and not harm silver.

It's most likely, as you suggested, aluminium, however, the experiment with copper sulfate was inconclusive because there just wasn't enough material to get visible bubbles, measurable temperature increase, or a product that was actually visible. In that reaction I would expect no reaction with silver and copper displaced if it was aluminium. What I actually saw was the metal disintegrate as I stirred it and was left with tiny metal particles that I couldn't even see properly.

Yes.

Do the math, according to Enthalpy (the user, not energy per mole!), the leaf must be less than 1μm thick to be displaying transparency. So if I take a piece 2cm on each side, my maximum total volume (using 1μm) is 4x10^-10m³. This is probably a significantly high estimate.

Given a density of aluminium of approx 2700 kg/m³ that gives me under 1mg of metal if we accept that it is LESS than 1μm. Given the amount I have, 2x2cm is quite a hefty chunk to take for a reaction that might not even be visible.

No.