# unusual reaction for dissolving gold-silver alloys



## AHposter (Nov 23, 2014)

The following information might not be practical for anyone in this forum to use, but it is an interesting read.
I just wanted to bring this potential process to everyone's attention.


A mix of nitric and hydrochloric acids, known as "aqua regia", is well known for being able to dissolve gold. However, _aqua regia_ can only dissolve gold-silver alloys with extreme difficulty. Normally, in these situations, refiners have to resort to inquartation, where the gold-silver alloy is first dissolved in a greater quantity of molten silver to decrease the gold content, then the resulting new alloy is dissolved with plain nitric acid. The process of inquartation can be cumbersome, and of course one must have a large quantity of expensive silver on hand for the process. All this additional silver that has to be alloyed in means additional acid required and more waste.

Besides _aqua regia_, there is a different acid combination that can be used to dissolve gold, and the interesting thing about this reaction is that the gold will reappear after the dissolved solution of gold is diluted in water! This reaction, however, involves much more concentrated acids, and the procedure is much more dangerous. So I do not suggest you try this reaction unless you have a good chemistry background and know about the proper safety precautions. This post is more for information purposes.


Dissolving Gold with Concentrated Nitric and Sulfuric Acids

A hot mixture of concentrated nitric and sulfuric acids can dissolve gold, with lower oxides of nitrogen forming. Addition of water caused the gold to precipitate back out in metallic form, but if a solution of permanganate is used instead, the gold remain dissolved.
_Reynolds, later by Spiller
Chemical engineering, Volume 2, p316_ 

The text also mentions that even concentrated mixtures of nitric and phosphoric acid attacks gold at room temperature, although the reaction is very slow unless heated.

The reaction is probably:
 (2)Au + (3)NO3- + (18)H+ --> (2)Au+3 + (3)NO+ + (6)H3O+

The nitrosyl ion, NO[+], exists in the form of nitrosyl sulfuric acid, ONOSO3H.

The nitronium cations, NO2[+], which form in equilibrium in concentrated nitric acid solutions, probably initially attack the gold, creating nitrogen dioxide. Basically,
 Au + (3)NO2+ --> Au+3 + (3)NO2

The nitrogen dioxide produced would likely remain in the concentrated acid,
 (2)NO2 + (3)H2SO4 --> NO+HSO4- + NO2+HSO4- + H3O+HSO4-

and the nitronium ions formed from the NO2 would then attack more gold.


Excess sulfuric acid needs to be used. This is an equilibrium reaction, and the gold is not going to dissolve easily. The mixture needs to be extremely acidic. Even a 1:1 ratio of 70% HNO3 to 95% H2SO4 is not going to be concentrated enough. For good results, use a 1:10 rato of 70% nitric acid to 98.5% concentrated sulfuric. Essentially, there can be no water in the reaction!

While this mixture of sulfuric and nitric acids can slowly corrode gold cold, the reaction is very slow unless heated.
Even in the hot boiling mixed acids, the gold takes several minutes to dissolve.

The NO[+] ion hydrolyzes (reacts with) water to form nitrous acid.

NO+ + (2)H2O --> HNO2 + H3O+

Nitrous acid is fairly reactive, and can act as either a reducing or an oxidizing agent. It will reduce the dissolved gold (Au+3) to elemental form (Au). This explains why the gold precipitates back out when the reaction is diluted with water.

(2)Au+3 + (3)H2O + (3)HNO2(_aq_) --> (2)Au + (6)H+(_aq_) + (3)HNO3(_aq_)

(note that "(_aq_)", which stands for "aqueous", means it is dissolved in water)

If fuming nitric acid is added to the reaction containing the dissolved gold, the gold will solidify out as a purple solid. The gold is probably still in its elemental form, but small particle sizes of gold are known to exhibit strong colorations, from red to purple.

Nitrous acid is unstable, and only exists in the form of solutions which gradually degrade after several minutes. Solutions of nitrous acid exist in equilibrium with nitrogen dioxide and nitric oxide, the latter of which is a reactive radical which, if exposed to air, will immediately react with the oxygen to form more nitrogen dioxide.

 (2)HNO2 <==> H2O + NO2 + NO



In the reaction,
 (2)Au + (3)NO3- + (18)H+ --> (2)Au+3 + (3)NO+ + (6)H3O+
sulfate ions are not shown because they do not directly take place in the reaction. The literature even states that phosphoric acid can be used in place of the sulfuric acid.

The above reaction is in ionic form. Some of you may prefer to see it in the form:
 (2)Au + (3)HNO3 + (15)H2SO4 --> (2)Au(SO4H)3 + (3)NOSO4H + (6)H2SO4*H2O

Note that the "Au(SO4H)3" only exists in the solution, it cannot be isolated. Gold trinitrate, if it even exists, would also be nearly impossible to obtain as a pure solid. Gold trinitrate only exists in highly concentrated solutions of nitric acid. When these solutions are diluted with water, auric oxide precipitates out. Similarly, auric oxide only only dissolves in very concentrated acids, since it is only very weakly basic.

Au2O3 + (9)HNO3 <==> (2)Au(NO3)3 + (3)HNO3*H2O 


Again, the reaction is more interesting from a chemical perspective than a practical way to refine out gold. Nevertheless, the reaction may be useful to directly dissolve gold-silver alloys, without having to go to the trouble of inquartation, since aqua regia only dissolves such alloys with extreme difficulty.


Procedure and Precautions:

Yes, it is extremely dangerous. The dangers of using concentrated mixed acids are commonly taken for granted among those that frequently perform nitrations. Obviously those unfamiliar with such procedures should think twice before handling such high concentrations of acid.

More details about the reaction. The concentrated acid mix that contains the dissolved gold should be gradually transferred into the larger bowl of water using a 10ml glass transfer pipette. You will also need a rubber pipette suction bulb. For those of you unfamiliar with this tool, it is basically like a turkey baster that is used to suck up a small quantity of liquid, then move it to another container. The pipette can be bought here:
http://www.pelletlab.com/pipette

Using the pipette to slowly add the acid mixture to the water is important for two reasons. First, safety. Water should never be added to concentrated acid, since this can result in the acid spraying up. Neither should the acid be poured into the water, because of the possibility of an accidental spill or splashing, and because it can be hard to control the rate that the liquid is poured in. Adding the acid in too fast can lead to overheating, which could result in boiling/splashing in the water. Second, it is important that each small portion of the acid quickly be diluted with as much excess water as possible. This will help prevent the gaseous nitrogen oxides (NO and NO2) from escaping. Although nitrosylsulfuric acid reacts with excess water to form a solution of nitrous acid, if not enough water is used nitrogen oxides will bubble out instead.

There will inevitably be some loses of nitrogen oxides, in the form of some bubbling and some brown gas being given off. Unfortunately, when some of the nitrogen oxides escape, there will not be enough nitrous acid to completely reduce the gold. After neutralizing, all the gold will still precipitate out, but a small portion of it will be in the form of hydrated gold oxide, Au2O3. If the gold is going to later be melted, the gold oxide should not pose any problems, as the compound decomposes to the pure metal at 160°C, giving off oxygen gas.

One other note of warning, unless the gold oxide has been completely reduced, it should not be reacted with ammonia, as this could potentially form the dangerous sensitive explosive known as "fulminating gold". In the event that the acid solution was previously boiled with ammonium sulfate to prevent precipitation of the gold, fulminating gold can result upon neutralization if too much ammonium sulfate was added.


More safety information: 

Only use small quantities of mixed acids at a time. Be aware that with concentrated acids, even tiny drops can splash out and result in painful burns on exposed skin. To get some understanding of these dangers, try pouring cranberry into a glass, wearing a clean white long-sleeved shirt. Even with cautious pouring, you are likely to find one or two tiny little red stains on the sleeves afterwards, even though you were not aware of any splashing while the juice was being poured. If this was concentrated acid, painful burns would have been felt.

You may desire to cover your shoes with a plastic bags and a rubber band, so that if any of the acid spills onto the floor, it will not seep into your shoes. Protective shoe coverings can also be purchased:
http://www.labsafety.com/search/shoe%2Bcovers/
If you choose to wear rubber boots instead, it is advised that the top of the rubber be tied tight around your legs, so that if any of the acid is spilled on you, it will not drip down into the boots and collect in a puddle. If the acid is in contact with your skin for more than a few seconds, the burns will be much more severe. http://www.amazon.com/b?ie=UTF8&node=393294011

A boiling mixture of concentrated nitric and sulfuric acids is extremely dangerous, much more so than 70% concentrated sulfuric acid, for example. The chemistry of this mixture presents several unique hazards. Extremely concentrated sulfuric is a strong dehydrating agent, that will turn anything organic, such as a strip of paper or your skin, into black char immediately on contact. A note about treating concentrated nitric acid burns, after you immediately rinse the affected area with plenty of water, and neutralize with sodium bicarbonate solution, there is special recommendation for concentrated nitric acid burns. Use a swab dipped in chlorine bleach to gently scrub the affected area. Some of the yellow color from the burn should be absorbed onto the cotton swab. Continue to scrubbing with fresh swabs until no more yellow can be absorbed onto the cotton. Then rinse well in soapy water. Doing this will help remove some of the nitro compounds which have formed. These compounds act as allergens and greatly slow the healing process. In fact nitric acid burns take much longer to heal than sulfuric acid of the same concentration. The unique effects of concentrated nitric acid are due to the formation of nitronium ions, NO2[+], in equilibrium in the solution. The addition of highly concentrated sulfuric acid greatly enhances this equilibrium, and so the special burn effect of nitric acid will be greatly exaggerated by the acid mixture. In other words, it would be very important to treat the burns in the way described above, and the healing time is likely to be much longer.


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## butcher (Nov 24, 2014)

The reason you may not have heard much about this process on the forum is because of its dangers, and because of the fact there are much safer ways to refine gold that will actually do a better job without such extreme dangers.

Normally I would not like to see this type of a post on the forum because of the dangers involved, yet you have done a good job of explaining many of the dangers.

I just hope members here are smart enough to know what they are doing if they even think of this process as an option, but then if they were that smart, they would already know a safer option to use in most cases.

This method will not work well for separating many of the metals that are normally associated with gold, silver would also dissolve...

I do not see this as a good recovery method or even as a refining method.
Just a dangerous way to try and dissolve gold.

I see too many dangers, no benefits, it is an interesting read, but that is about it.

Just my opinion.


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## solar_plasma (Nov 24, 2014)

After experimenting since I was a child more lucky than educated and after nearly twenty years as a teacher of chemistry, educated enough to have been lucky always, I like the sheep more than the wolves. Just for the theoretical knowledge interesting though.


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## AHposter (Nov 24, 2014)

butcher said:


> for separating many of the metals that are normally associated with gold, silver would also dissolve...


But afterwards, when diluted with water, the gold would reappear as metal, while the silver would remain dissolved.



butcher said:


> I see no benefits


One of the really interesting things about this reaction process is that it does not require a separate reducing agent to recover the gold. As soon as the reaction mixture is diluted with water, the gold will turn back into its elemental form.

One can look at it this way: Gold has an equilibrium with nitric acid. Under normal conditions, this equilibrium is extremely small, essentially negligible. But in an extremely super-acidic environment in concentrated sulfuric acid, the equilibrium can be pushed forward. Remove the super-acidic environment and the equilibrium goes back.


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## solar_plasma (Nov 24, 2014)

I am quite sure everyone has understood that, but if (better when) it is as dangerous as Butcher mentioned, there is no benefit.


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## AHposter (Nov 24, 2014)

Maybe one day one of the members in this forum may happen across some rare gold-rhodium or gold-iridium alloy, and nothing else will work to dissolve it.


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## g_axelsson (Nov 24, 2014)

Any such rare beast could be simple to tame by inquarting with silver, then take the porous gold sponge and attack with AR to dissolve the gold.
The leftover rhodium can be fused with sodium bisulfate to create a water-soluble salt.
Iridium is then dissolved by fusing with sodium peroxide and sodium hydroxide at 500 C forming iridium dioxide which could be dissolved in AR.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=6638

That feels a *lot* safer than boiling sulfuric and nitric acid mixture.

Göran

Edit : it's not piranha solution as I erroneously wrote earlier.


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## Lino1406 (Nov 29, 2014)

A problem which was not stressed out. There precipitates Ag2SO4.
So, after all again a mixture of gold with salt


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## butcher (Nov 29, 2014)

This is just a dangerous curiosity nothing to play with.
Dangers of working with this mix produces no benefit.
Safer and easier methods to recover or refine gold, are used for a reason, they work, and can be relatively safer, at least a lot less dangerous than the above method described, which is just an accident waiting to happen.

We can melt the snow off of the streets with an atomic bomb but I believe the snow plow would be a better option, or even couple of workers with shovels sounds like a better option.

Just because something may work does not make it a good idea or even an option that should be considered.


This will not work as a refining method, several metals would precipitate back out with the gold.

I also believe the assumptions of how and why (and even some of the formulas are incorrect) the gold dissolve in the reaction is incorrect, it is most likely the formation of persulfate that dissolves the gold, which can be done safer with the concentrated sulfuric acid cell discussed on the form, where at least your not dealing with hot deadly acids, and your not working with acids that can form explosive nitrations out of organic material's if spilled in your lab, not only speaking of the obvious risk danger to yourself, which will surely get someone hurt bad.

This was some of what I have found on this method plastered all over the Net.

http://www.sciencemadness.org/talk/viewthread.php?tid=17164
https://sites.google.com/site/unusualchemistry/aqua-regia
http://www.chemicalforums.com/index.php?topic=63832.0
http://goldrefiningforum.com/~goldrefi/phpBB3/posting.php?mode=quote&f=37&p=106842
http://www.scienceforums.net/topic/19457-changing-aqua-regia-composition/
http://www.scienceforums.net/topic/60327-different-type-of-aqua-regia/
https://sites.google.com/site/ecpreparation/other
http://www.forum.alchemyforums.com/showthread.php?2353-Dissolving-Gold-with-Nitric-and-Sulfuric-Acids
Lists of other sites
https://search.yahoo.com/yhs/search;_ylt=AwrTceBQ73lUnMQA.zQPxQt.?p=%282%29Au+%2B+%283%29HNO3+%2B+%2815%29H2SO4+--%3E+%282%29Au%28SO4H%293+%2B+%283%29NOSO4H+%2B+%286%29H2SO4%2AH2O&type=W3i_SP%2C204%2C0_0%2CStartPage%2C20141145%2C20029%2C0%2C31%2C6944&fr=sfp&fr2=sb-top-search&hsimp=yhs-syctransfer&hspart=w3i&xargs=0&b=9&pz=8&xa=yuvLFnRf93qHif3USmbSvg--,1417363664
https://search.yahoo.com/yhs/search;_ylt=AwrTccLI83lUIKMAiZUPxQt.?p=%282%29Au+%2B+%283%29HNO3+%2B+%2815%29H2SO4+--%3E+%282%29Au%28SO4H%293+%2B+%283%29NOSO4H+%2B+%286%29H2SO4%2AH2O&type=W3i_SP%2C204%2C0_0%2CStartPage%2C20141145%2C20029%2C0%2C31%2C6944&fr=sfp&fr2=sb-top-search&hsimp=yhs-syctransfer&hspart=w3i&xargs=0&b=17&pz=8&xa=ZayAyqKmmR94XOFpHycezw--,1417364808
...

Note Harolds previous comment on this subject, which I completely agree with!



Harold_V said:


> Anders Hoveland said:
> 
> 
> > Excess sulfuric acid needs to be used. This is an *equilibrium* reaction, and the gold is not going to dissolve easily.
> ...



I agree with Harold, and others on the forum, who say this is a stupid idea, stop promoting this STUPID IDEA, spend time learning how to recover and refine gold safely


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## butcher (Nov 29, 2014)

AH Poster ?

Anders Hoveland ???

Banned from many chemical forums, for promoting dangerous chemical formulas, and ideas, sounds to me to be an arm chair chemist prowling the internet, promoting stupid ideas that will hurt people, he most likely read in some old book, a curiosity that likely found no practical use.

I believe he has struck again here on our forum again.

AH poster will not have a chance to respond under this identity, he has been banned from the forum, (probably again) for promoting a stupid and dangerous idea that will surely get someone hurt.


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## solar_plasma (Nov 29, 2014)

Just remembering, when I was a child, maybe 13, I did an experiment, I had read in a school book, with this mixture. All went out good. The second time I did the same experiment some days later, the reaction was strongly exothermic (no fume hood!), the acids heating up, forming NOx gasses. I held my breath, brought the smoking mixture out of the house (never go for a walk with open concentrated acids!) and then I covered it with sand (beside the goggles, gloves and the apron, this might be the only thing I did correct).

More luck, than understanding. Horrible what could have happened.


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## Lou (Dec 1, 2014)

It's a procedure that must be routinely done in many analytical labs "wet ashing". 
I don't want us turning into the thought police here but I would say this suggestion is rather inappropriate.


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