Sciencemadness Discussion Board - Ammonium Tetrachroloroaurate (Water Soluble Noble Metals)

Ammonium Tetrachroloroaurate (Water Soluble Noble Metals)

weilawei - 29-1-2018 at 09:05

There's a fascinating (and seemingly easy to reproduce) post at In The Pipeline about making a water soluble solution of gold and other noble metals.

Quote:

The group shows that if you take one of these metals (powder, wire, what have you) and mechanically mill it with either potassium chloride or ammonium chloride along with the weak oxidant Oxone (potassium peroxymonosulfate), all as powders, you get a powder at the end that completely dissolves in water. That’s a startling thought to a chemist, grinding up palladium powder or gold wire and having it form just a clear colored solution at the end. That solution contains ammonium tetrachloroaurate (or the corresponding other salts), which is already a known soluble compound, of course – it’s just that in the past, if you wanted your gold in that form, you had to sort of go all the way around the barn to get it. If you add other ligands to the grinding mixture, you can form triphenylphosphine complexes and many others directly.

There's also a bonus round about ball milling cellulose to achieve better yields of glucose.

elementcollector1 - 29-1-2018 at 11:32

So it's a solid-solid reaction? Interesting. Is the yield theoretical, or is there some leftover of any of the compounds at the end?

The Austrian Scientist - 29-1-2018 at 12:14

Will it also work for Pt or Ir?

aga - 29-1-2018 at 12:31

For Gold recovery this would not be as good as cyanide.

Very interesting is the bit about "surface atoms are very different from those in the bulk phase"

Perhaps that difference is why some calcined substances are almost inert.

Fleaker - 30-1-2018 at 13:56

I mean if dry milling ammonium chloride with gadolinium oxide gives some soluble GdCl3 then I don't see why this wouldn't work.

I wouldn't call oxone a weak oxidant. Probably in situ chlorine generation doing the work?

vmelkon - 31-1-2018 at 11:21

Quote: Originally posted by Fleaker

I mean if dry milling ammonium chloride with gadolinium oxide gives some soluble GdCl3 then I don't see why this wouldn't work.

I wouldn't call oxone a weak oxidant. Probably in situ chlorine generation doing the work?

The reaction would be
Gd₂O₃ + 6 NH₄Cl -> GdCl₃ + 6 NH₃ + 3 H₂O

since the NH3 is volatile, the reaction goes forward.

DraconicAcid - 31-1-2018 at 12:13

Quote: Originally posted by Fleaker

I wouldn't call oxone a weak oxidant. Probably in situ chlorine generation doing the work?

You don't need to postulate in-situ generation of chlorine to explain the reactivity of gold towards aqua regia- the chloride ion stabilizes the gold(III) ion via coordination, making it easier to form.

If you argue that the nitrate ion in aqua regia isn't strong enough to oxidize gold, then you also have to accept that it isn't strong enough to oxidize chloride ion to chlorine.

clearly_not_atara - 31-1-2018 at 13:42

I think oxone could certainly oxidize gold on contact. Persulfate in water oxidizes Ag+ to Ag2+ which is a similarly difficult oxidation.

Fleaker - 2-2-2018 at 08:55

Quote: Originally posted by vmelkon

Quote: Originally posted by Fleaker

The reaction would be
Gd₂O₃ + 6 NH₄Cl -> GdCl₃ + 6 NH₃ + 3 H₂O

since the NH3 is volatile, the reaction goes forward.

Yes, this (dry way) reaction is very well known to people working with rare earths. I have done this before in a ball mill with alumina media. When the mill is opened, there is an odor of ammonia that can be smelled. However, in my experience the reaction never goes to completion even with hardened steel media that have much more kinetic energy. In my case, I was working with Gd2Zr2O7, not pure Gd2O3, though I can confirm that it works for Nd, Y, Yb, Gd (produced from the aforementioned) in my experience. To get high yields, I have always had to prepare the rare earth chlorides in porcelain or pyroceram boats, covered over, at about 450 C for 5-6 h. This is generally effective, even on well calcined REO. At that temperature though, a lot of HCl is produced.

Quote: Originally posted by DraconicAcid

Quote: Originally posted by Fleaker

I wouldn't call oxone a weak oxidant. Probably in situ chlorine generation doing the work?

I am confused by this post. Are you talking "you" as in me? Who is talking about aqua regia, let alone arguing about it? Draconic, I'm postulating that my talking apples to oranges by comparing ball milling REOs with ammonium chloride versus a bona fide redox reaction upset you. Nevertheless, I do think some formation of chlorine is happening in this system. I was just asking if other people thought so as well. Do you think this is possible?

There are a lot of things going on with aqua regia mixtures--be it NOCl, NO, NO2, Cl2. But yes, it takes a coordinating ion for stable gold solutions. For instance, dissolved "gold nitrate" stable in red fuming nitric acid immediately hydrolyzes upon dilution. The powder (maybe Au2O3, which isn't exactly the strongest base!) that is obtained melts to gold metal. I never had the opportunity to see how much oxygen is produced from its decomposition.

One of the issues in removing gold from materials that can be passivated by nitric acid (i.e. gold plated stainless steel) with reverse aqua regia is that as the mixture ages, eventually the gold precipitates as the solution loses chlorine. The complex is not forever stable despite how well put together [AuCl4]- is. It takes many months depending on temperature so indeed even the chloride bound in that complex will be oxidized over time.

Odd thing though, if I reflux silver chloride with azeotropic nitric acid for days and days and take a sample to the ICP-OES, I don't have much in the way of silver reporting as its soluble nitrate and I don't end up with an aqua regia solution. Chlorine is effectively trapped in that system as any chlorine formed simply doesn't have time to escape the solution before recombinaton with Ag+. On the other hand, one can surely convert AgCl to soluble Ag2SO4 by fuming with sulfuric--with HCl sent off. Funny thing is that when the same silver chloride is put it in a quartz flask and illuminated with UV, it gives chlorine water and silver metal. Not quite completely, but it does.

Quote: Originally posted by clearly_not_atara

I think oxone could certainly oxidize gold on contact. Persulfate in water oxidizes Ag+ to Ag2+ which is a similarly difficult oxidation.

In my experience, peroxymonosulfuric acid will oxidize gold. It is not a stable situation and the gold is soon thrown down as a blackish brown solid that maybe is a mixture of oxide and metal. Maybe all metal. I never looked to see. Gold plated copper items are often stripped of their gold content by electrolysis in hot, concentrated sulfuric acid.

Whether persulfate salts can do this with no complexing ligand present... I do not know but I think it very unlikely to put Au into solution in quantity. I can simply take a piece of gold, weigh it, and toss it in a strong persulfate solution. Any mass loss wouldn't be easily measured, I'd imagine. Now persulfate with HCl, you are going to have gold dissolve. I can give this a go, if you'd like.

I think in reality, oxone is probably substitutable for nitrate, chlorate, and so on. Even air dissolved in HCl will attack the PGM blacks.

My interest in this process is from a practical standpoint--why use aqua regia or HCl-Cl2 or HBr/Br2 to dissolve something if it can be accomplished simply and cheaply useing KCl and oxone while ball milling the living hell out of it for a couple weeks?