Sciencemadness Discussion Board - Acid mixtures that dissolve gold

Acid mixtures that dissolve gold

chornedsnorkack - 12-2-2021 at 13:32

The classical solvent for Au is aqua regia.
But this is because alchemists discovered HNO₃ before other strong oxidants.
What is needed is a complexing agent, such as chloride, and oxidant.
Which hydrogen chloride/oxidant mixtures are better to dissolve gold?

HCl/HNO₃
HCl/HClO₃
HCl/HClO₄
HCl/H₂O₂
HCl/H₂Cr₂O₇
HCl/HMnO₄

Maurice VD 37 - 12-2-2021 at 13:49

What do you mean by "being better to dissolve gold" ? All these mixtures will produce chlorine gas. And Chlorine gas dissolves gold the same way, whatever its origin.

clearly_not_atara - 12-2-2021 at 16:02

4

Bedlasky - 12-2-2021 at 22:44

HCl/HClO4 don't work. HClO4 is often sold as azeotrope, which don't have oxidizing properties (these have just pure HClO4).

clearly_not_atara - 13-2-2021 at 09:06

^indeed, and HClO4 azeotrope is IIRC distilled from HCl/NaClO4 metathesis (with excess NaClO4 to maximize NaCl precipitation) which would be problematic if it caused the perchlorate to decompose

woelen - 13-2-2021 at 09:46

A mix of HClO4 and HCl is nonoxidizing, even at quite high concentration. No Cl2 will be produced from that.

I would use either mix 2 or mix 4. Mixes 5 and 6 are not good, a lot of other metal ions are introduced in the mix and you'll have to put effort in separating the gold from that.

symboom - 13-2-2021 at 12:58

Don't forget about selenic acid

DraconicAcid - 13-2-2021 at 13:52

Quote: Originally posted by Maurice VD 37

What do you mean by "being better to dissolve gold" ? All these mixtures will produce chlorine gas. And Chlorine gas dissolves gold the same way, whatever its origin.

Aqua regia doesn't produce chlorine gas. The nitric acid is able to oxidize gold in the presence of chloride ion because the chloride ion stabilizes the gold as a complex.

Bedlasky - 13-2-2021 at 14:33

Quote: Originally posted by DraconicAcid

Quote: Originally posted by Maurice VD 37

What do you mean by "being better to dissolve gold" ? All these mixtures will produce chlorine gas. And Chlorine gas dissolves gold the same way, whatever its origin.

Aqua regia doesn't produce chlorine gas. The nitric acid is able to oxidize gold in the presence of chloride ion because the chloride ion stabilizes the gold as a complex.

Nitric acid isn't capable of oxidizing gold even in the presence of the chloride, look on E° values. Aqua regia does produce Cl2 and NOCl (and Cl2 is active oxidizing agent).

https://en.wikipedia.org/wiki/Aqua_regia

DraconicAcid - 13-2-2021 at 14:42

Quote: Originally posted by Bedlasky

And yet you think it's a strong enough oxidizing agent to produce chlorine from HCl? That's about 0.3 V harder. It's just not going to happen.

I've seen it hypothesized that aqua regia forms chlorine, but I've never seen any evidence beyond "and that's why it dissolves gold!".

itsallgoodjames - 13-2-2021 at 17:03

Quote:

HCl/HClO3

HClO3 isn't stable, at least not in any significant concentration. From what I understand, it likes to decompose unprovoked, especially in anything resembling a concentrated solution. Tdep made a video on it

DraconicAcid - 13-2-2021 at 18:53

Quote: Originally posted by itsallgoodjames

Quote:

HCl/HClO3

True, but one could try adding NaClO3 to HCl to make such a mixture, and either the HClO3 or the ClO2 produced would happily react with the gold. According to the tables, hypochlorite and chlorite should also work.

[Edited on 14-2-2021 by DraconicAcid]

Bedlasky - 13-2-2021 at 23:04

HClO3 is stable up to 30% concentration. Problem is when you mix it with HCl, because ClO2 is evolved, which is explosive since 10% concentration in air. So you need to add HClO3 (or some chlorate solution) from burette with good stiring and heating.

Btw. also HBr based mixtures can be used for dissolving gold. Or chlorinated HCl (I saw video when someone dissolve gold using bleach/HCl, bleach was added from burette). There are also some rare substances, which can dissolve gold, like chloryl perchlorate (but this isn't very practical

).

[Edited on 14-2-2021 by Bedlasky]

DraconicAcid - 14-2-2021 at 08:44

Quote: Originally posted by Bedlasky

Btw. also HBr based mixtures can be used for dissolving gold.

Just judging from electrode potentials, I would expect iodine to dissolve gold. And cyanide in the presence of air, of course.

Bedlasky - 14-2-2021 at 09:04

Quote: Originally posted by DraconicAcid

Quote: Originally posted by Bedlasky

Btw. also HBr based mixtures can be used for dissolving gold.

Just judging from electrode potentials, I would expect iodine to dissolve gold. And cyanide in the presence of air, of course.

Isn't iodide capable of reducing Au(III) in to metallic gold?

DraconicAcid - 14-2-2021 at 09:24

Quote: Originally posted by Bedlasky

Isn't iodide capable of reducing Au(III) in to metallic gold?

I think the [AuI4]- complex ion is stable.

ETA: Page 7 of this:
https://library.lanl.gov/cgi-bin/getfile?rc000062.pdf

[Edited on 14-2-2021 by DraconicAcid]

RogueRose - 14-2-2021 at 23:30

I have to say I was kind of surprised when I had some gold dissolve from using only HCl + H2O2. Both were around 32% and there are a couple other metals mixed in (it was parts of circuit boards). I had thousands of the little gold flakes from the connectors on ISA/PCI/PCIe cards in computers and after awhile there was only a few of them left and I never knew how it happened. I think they only disappeared when I had a lid on the bottle so maybe some Cl2 gas had built up?

Panache - 20-2-2021 at 19:25

I note that mixtures of 10% Hcl and 3% peroxide are promoted as 'deplating mixtures' for circuit boards and the like. The logic being the milder solution will slowly will attack/dissolves first the copper surface between the gold and the pin/pcb/whatever, thereby allowing the, as yet undissolved gold, to flake off. The problem is once it runs out of copper the gold is attacked or the gold dust that reforms from inadvertently dissolved gold ions (from local concentration deficits) plating back out as dust when encountering copper ions. This dust is notoriously difficult to filter/accumulate.
Large scale ewaste recycling utilizes plasmas...the economics of scale offsetting the energy inputs...widespread local solar should shortly see the hobbyist negate this impediment provided he/she/they run their plasma/arc furnace whilst the sun shines or the wind blows.
/cf with chemical means of recovery in which you must make a trade off between a tedious lengthy labour intensive picking process or a tedious (slightly) less lengthy labour intensive picking process and a more expensive tedious refining process......the pleasure of banging the whole shebang into a furnace wherein your noble metals sink to the bottom and everything else is smelted away or floats... well i guess people are going to get better at iridium/gold/pt separations in due course.

edit..replaced the he with a he/she/they!

[Edited on 21-2-2021 by Panache]

Lion850 - 21-2-2021 at 02:39

Concentrated HCl with I2 dissolved in it dissolves gallium (fast!) and also tin (slower, but faster than HCl on its own). If I had a bit of gold to spare I would try, but I don't have.

Panache - 22-2-2021 at 18:56

Doesnt elemental iodine dissolve gold. It certainly ready combines with platinum...

clearly_not_atara - 22-2-2021 at 19:46

Quote: Originally posted by DraconicAcid

Chlorine definitely forms in aqua regia. This is endothermic and driven by the entropy of Cl2, which does not exist for Ag(NO3)3.

However, the quantity of molecular chlorine produced is not sufficient to explain the dissolution of gold, which instead is generally believed to occur as you described by complex stabilization. See particularly:
https://pubs.acs.org/doi/pdf/10.1021/ja02220a009

Panache - 22-2-2021 at 23:44

Quote: Originally posted by Lion850

Concentrated HCl with I2 dissolved in it dissolves gallium (fast!) and also tin (slower, but faster than HCl on its own). If I had a bit of gold to spare I would try, but I don't have.

Wait i didnt think through this question of yours.
In those two examples of yours, its not cl2 being formed and acting as oxidant rather, its just the iodine acting directly upon the metal and the hcl is the hydrogen ion/acid source and solvent for the iodine
So i just tried straight i2 ar rt with gold...no reaction. Sealed the tube and heated no reaction. I reacon addinghcl will do sweet fa.
Illletyou know

unionised - 23-2-2021 at 11:04

A concentrated solution of I2 and KI in water will certainly dissolve gold.

ChemichaelRXN - 23-2-2021 at 14:30

I believe HCl/FeCl3 or FeCl3 alone works too, but never tried it.

Bedlasky - 24-2-2021 at 05:34

Quote: Originally posted by unionised

A concentrated solution of I2 and KI in water will certainly dissolve gold.

But Au isn't trivalent, but monovalent in this case:

https://en.wikipedia.org/wiki/Gold_monoiodide

Remy says that Au(III) reacts with iodide to form Au(I).

zed - 26-2-2021 at 18:08

Well, HCl/H2O2 will sure dissolve Platinum, but the reaction doesn't seem to be very robust.

The H2O2 liberates Chlorine, but the H2O2 needs constant replacement, as Platinum decomposes it.

Folks sometimes use the process to dissolve finely divided spent catalysts, or to leech the Platinum out of Platinized Carbon.

For dissolving Chunks of Platinum, perhaps messy Aqua Regia is the was to go. It's something I have to do soon, and I lack enthusiasm. Clouds of toxic red gas.... Yuk! Still, it does produce the product I need, straightaway, and in its pure form. Chloroplatinic acid.

Thought Electrolysis with HCl might also work. Or, reaction with elemental Chlorine.... But, each has its drawbacks, and Aqua Regia is tried and true. Additionally, Nurdrage has provided a decent Youtube video.

Oh yeah, Gold? Gold is easier. Some of the More Arcane Precious Metals, aren't touched by any reagent we have discussed. You can fuse them with Molten Alkali Salts though.

Well, I can't provide a link. This odd Southern guy, does interesting work, but perhaps he has rubbed Youtube, the wrong way. One day, you are a star.... The next day, you are gone.

[Edited on 27-2-2021 by zed]

Panache - 28-2-2021 at 04:47

I find aqua regia dissolution of precious metals are best performed using a behive type setup. Basically your metal and aqua regia are in a beaker. This has a larger inverted beaker on top. Both sitting in a tray with water...

[Edited on 28-2-2021 by Panache]

ChemTalk - 28-2-2021 at 07:45

>> This odd Southern guy, does interesting work, but perhaps he has rubbed Youtube, the wrong way. One day, you are a star.... The next day, you are gone

Who was that?

Panache - 2-3-2021 at 04:33

Quote: Originally posted by Panache

Quote: Originally posted by Lion850

Concentrated HCl with I2 dissolved in it dissolves gallium (fast!) and also tin (slower, but faster than HCl on its own). If I had a bit of gold to spare I would try, but I don't have.

So i just tried straight i2 ar rt with gold...no reaction. Sealed the tube and heated no reaction. I reacon addinghcl will do sweet fa.
Illletyou know

Ahh yeah i was completely wrong...no metallic gold.
So iodine and hcl most definitely dissolves gold.

zed - 2-3-2021 at 09:24

So, Panache. You are suggesting to place this project in a shallow pan of water... and to heat the whole thing? One large beaker, inverted over a smaller beaker containing Aqua Regia and Metal?

Thereby, trapping the noxious fumes, in the water bath?

I like the simplicity.

Lion850 - 2-3-2021 at 11:02

Quoting Panache:
Ahh yeah i was completely wrong...no metallic gold.
So iodine and hcl most definitely dissolves gold.
Unquote

Hi Panache very interesting, can you please give more details? Did the gold go into solution with no ppt? Gold monoiodide is yellow-greenish according to Wikipedia; did you see these colors? Do you know the weights of reagents involved?

zed - 2-3-2021 at 12:17

Alright, that Precious Metals poster, is still on Youtube. Not excluded.... yet. And, maybe he isn't a Southern Guy.

Not Gold, but a very interesting procedure. Fusion of Platinum Group metals, in molten Alkali. Precipitation via Barium Salt.

https://www.youtube.com/watch?v=gV2N2Jqpo6w

[Edited on 2-3-2021 by zed]

Panache - 2-3-2021 at 16:33

Firstly zed, yes it's fantastic, you can even use your kitchen sink as the pan of water and run a constant water stream down the drain. I once did a larger one in a 20L pp bucket, over the bucket i had a shallow wider bucket and then over this a towel and the entire thing ran under the shower, once they get bigger they really don't need any heating.
And lion 850, ahh you're australian.
So i grabbed i guess enough gold plated pins to cover the bottom of a 50ml beaker, i then added i guess about quarter this volume in iodine. The pins were very fine. i then sealed the top with some silicon film and warmed the underside melting the iodine. not much happen. I let it cool and added some hcl conc. It began working upon the copper (the pins were gold plated over copper) but i saw no attack of any gold surfac. After about 30minutes i got bored left it for three days snd then when i looked again there was no gold left on any of the pins but srangely there were pins left, well partly.
I rinsed them well to ensure that the observation was correct.
And yes absolutely a ppt had formed and it was yellowish green. i haven't filterred it yet, prtobably wont. as it would also contain copper nickel al compounds also.
With this Au(!) iodide, does it dissolve other metals liberating gold netal?

pneumatician - 2-3-2021 at 16:42

if you know how, with only HCL, Gold CAN BE dissolved. PERIOD ;-)

Panache - 6-3-2021 at 16:32

Quote: Originally posted by pneumatician

if you know how, with only HCL, Gold CAN BE dissolved. PERIOD ;-)

ok, ill bite, what do you know? I find it difficult to believe you need only hcl, i mean one would need at least a beaker and water (because HCL is a gas at stp.....)

pneumatician - 9-3-2021 at 07:41

Quote: Originally posted by Panache

Quote: Originally posted by pneumatician

if you know how, with only HCL, Gold CAN BE dissolved. PERIOD ;-)

ok, ill bite, what do you know? I find it difficult to believe you need only hcl, i mean one would need at least a beaker and water (because HCL is a gas at stp.....)

oh yes, period, I asume you are right... of course you need a table, a pair of hands, a bottle to contain the hcl, a company producing hcl if you not make your own and etc etc etc ad infinitum, ok? right? but the point here is Gold dissolved with only hcl, in water or gas, in a beaker and etc etc etc, ok? right? :-D

ok, after all this ironic mumbo-jumbo... what is Gold?

ok, right, a metal, produced in the big bang, star dust and more and more mumbo-jumbo...

Stop! Gold is a COMPACT BODY, right?

so, et dividant dissolvere, I need to say ironic free something more??

in some thread here a guy "dissolved" Gold with chlorine only.

Sapere aude!

clearly_not_atara - 9-3-2021 at 07:55

possibility #979831: gold in hydrochloric acid, ozone generator bubbles (slowest possible method)

pneumatician - 9-3-2021 at 09:27

With ONLY HCL, no more, no less!

https://www.youtube.com/watch?v=oevJf2aYHjo

clearly_not_atara - 9-3-2021 at 11:12

^my post wasn't responding to you so much as providing a humorous way to dissolve gold, but, what I would suppose is, you can dissolve Au in "pure" hydrochloric acid (HCl@H2O) using a membrane-cell setup (no membrane likely = gold all over cathode), but this isn't actually simple to set up in practice

densest - 13-3-2021 at 22:45

(4) works quite well. Here, 35% H2O2 is (relatively) inexpensive and is used in gallons for swimming pool treatment. Swimming pool chlorinator tablets Trichloro-S-Triazinetrione are a good chlorine source.The reaction seems to work at near neutral pH.

I tried various methods without HNO3 in order to simplify subsequent reduction. Also 35%H2O2 was -much- easier to obtain OTC locally with no hazmat shipping.

Want to be Auric Goldfinger? (Ian Fleming reference) Spill a beaker of aqua regia + gold on your hands. Instant Purple of Cassius! I got to the sink before the acid did any significant damage, but the gold precipitated in my skin -instantly-. Lasted for weeks.

Another reaction with H2O2 and gold plated copper: H2O2 and CH3COOH happily dissolve copper and leave gold behind. 3% H2O2 and 5% acetic acid work albeit slowly leaving pretty blue-green cupric acetate and little gold flakes.