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chornedsnorkack
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[*] posted on 12-2-2021 at 13:32
Acid mixtures that dissolve gold


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?
  1. HCl/HNO3
  2. HCl/HClO3
  3. HCl/HClO4
  4. HCl/H2O2
  5. HCl/H2Cr2O7
  6. HCl/HMnO4

?
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Maurice VD 37
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[*] posted on 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.
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[*] posted on 12-2-2021 at 16:02


4



[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 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).



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clearly_not_atara
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[*] posted on 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 :P



[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 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.




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[*] posted on 13-2-2021 at 12:58


Don't forget about selenic acid



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[*] posted on 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.




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[*] posted on 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




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[*] posted on 13-2-2021 at 14:42


Quote: Originally posted by Bedlasky  


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


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!".




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[*] posted on 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




Nuclear physics is neat. It's a shame it's so regulated...

Now that I think about it, that's probably a good thing. Still annoying though.
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[*] posted on 13-2-2021 at 18:53


Quote: Originally posted by itsallgoodjames  

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

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]




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[*] posted on 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 :D).

[Edited on 14-2-2021 by Bedlasky]




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[*] posted on 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.




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[*] posted on 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?




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[*] posted on 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]




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[*] posted on 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?
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[*] posted on 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]
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[*] posted on 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.
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[*] posted on 22-2-2021 at 18:56


Doesnt elemental iodine dissolve gold. It certainly ready combines with platinum...
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[*] posted on 22-2-2021 at 19:46


Quote: Originally posted by DraconicAcid  

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!".

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




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 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.
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[*] posted on 23-2-2021 at 11:04


A concentrated solution of I2 and KI in water will certainly dissolve gold.
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[*] posted on 23-2-2021 at 14:30


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



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[*] posted on 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).




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