Sciencemadness Discussion Board

dissolving gold in nitric/sulfuric acids

AndersHoveland - 31-7-2011 at 23:53

http://books.google.com/books?id=yinmAAAAMAAJ&pg=PA316&a...

Mixtures of manganese dioxide and sulfuric acid can actually dissolve gold!
The reaction is slower at room temperature, but rapid with heating.
Permanganate and sulfuric acid after a few minutes also dissolve gold.
(Allen 1872)


Even 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 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:
(5)Au + (3)NO3[-] + (18)H[+] --> (5)Au[+3] + (3)NO[+] + (6)H3O[+]

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.


But ozone with sulfuric acid did not dissolve gold.

It is already known that aqueous solutions of bromine or chlorine dissolve gold.


[Edited on 1-8-2011 by AndersHoveland]

watson.fawkes - 1-8-2011 at 03:50

A New Twist on Aqua Regia
Powerful organic-based systems dissolve noble metals under mild conditions
http://pubs.acs.org/cen/news/89/i14/8914notw4.html

Sample: "a 1:20 SOCl2-dimethylformamide mixture dissolves gold quickly, but it leaves palladium and platinum untouched"

There's more in the article itself.

AndersHoveland - 2-8-2011 at 14:07

Edit: Note that the equation should have had (2) atoms of gold, not 5. The correct balanced reaction is

(2)Au + (3)NO3[-] + (18)H+ --> (2)Au[+3] + (3)NO[+] + (6)H3O[+]

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!

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)HNO2aq --> (2)Au + (6)H[+]aq + (3)HNO3aq

(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.

Sorry for all the chemistry information, but some of you will undoubtedly be curious about the specifics of the reaction.
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 an unstable radical which can either react with the oxygen in air to form more nitrogen dioxide, or if left on its own will disproportionate into nitrogen dioxide and nitrous oxide after several minutes.

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

(3)NO --> N2O + NO2


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)2 + HNO3*H2O


[Edited on 2-8-2011 by AndersHoveland]

watson.fawkes - 3-8-2011 at 05:44

I examined the quoted reference in the first post, a review article from 1904 about non-halogen agents for dissolving gold. Nitric acid is mentioned, but only qualitatively. Insofar as I can tell, everything after the line "The reaction is probably:" in the middle of the first post is pure speculation, with neither reference nor experimentation to support any of it. There's certainly nothing in the original article that supports this level of specificity.

Even worse, in the middle of the second post, there's the line "Sorry for all the chemistry information, but some of you will undoubtedly be curious about the specifics of the reaction." I don't count unsubstantiated reactions as "information", personally, certainly not from the present author.

Also, reported for previously-warned behavior.

AndersHoveland - 3-8-2011 at 12:40

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.

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 transfered 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 inevitibly 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 will 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-sleaved 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 seap 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.

Dissolving Gold with Manganese Dioxide
about the reaction with manganese dioxide and sulfuric acid. The reaction is probably:
(2)Au + (3)MnO2 + (3)H2SO4 --> Au2O3 + Mn(SO4)2 + (2)H2O

where Mn(SO4)2 is manganese sulfate, and the gold oxide dissolves in the sulfuric acid. concentrated sulfuric acid still needs to be used, but it probably does not need to be quite so concentrated as required for the other reaction; a 70% concentration should be suitable.

[Edited on 3-8-2011 by AndersHoveland]

Fleaker - 3-8-2011 at 19:35

This does not work at all. I have never tried with permanganate but I use sulfuric/nitric to wet ash gold containing filter papers many many many times and it doesn't dissolve in solution. With what would it complex? There's a reason gold "likes" sulfur, selenium and tellurium (with increasing order) as well as a reason the halogens act on it. You come off too pretentious with your equations and speculation. Please read up on hard/soft acid/base theory Anders, or else stick to smoke bombs!

The only things practical for dissolving gold are: aqua regia, a hydrohalogen + oxidizing agent solution (i.e. NaClO3/HCl, or H2O2/HCl etc.), cyanide, persulfuric acid (too transient), thiosulfate solutions, and other metals like silver, lead, tin, mercury, etc. The first three listed are what's used industrially.

AndersHoveland - 4-8-2011 at 09:43

One of the posters at "http://goldrefiningforum.com" stated that the "wet ash" method did indeed dissolve gold if the acids were concentrated enough, although he wrote that it was not a practical method at all.

The extremely concentrated HNO3/H2SO4 mixture might be useful for dissolving gold-silver alloys, without the need for inquartation, since aqua regia only dissolves such alloys with extreme difficulty.

[Edited on 4-8-2011 by AndersHoveland]

Neil - 4-8-2011 at 17:15

I suspect that nitric/sulfuric would still have problems with any lead, tin or copper on top of the silver - while as being far less aggressive on the gold.

AndersHoveland - 4-8-2011 at 22:40

Quote: Originally posted by Neil  
I suspect that nitric/sulfuric would still have problems with any lead, tin or copper on top of the silver - while as being far less aggressive on the gold.


Hot boiling sulfuric acid can dissolve lead or copper, but not at ordinary temperatures. There is no reason why mixed nitric/sulfuric acids would not attack copper or lead, as concentrated nitric acid alone will attack both metals. (if only moderately concentrated acids are used, the presence of sulfuric acid might be problematic, but lead sulfate is supposedly much more soluble in 80%+ concentrated sulfuric acid)
But dissolving tin is indeed problematic. The procedure has actually been done by myself, the nitric acid and tin has to be boiled under flame, otherwise insoluble metastannic acid apparently forms a protective layer that makes the reaction problematic. The procedure is very messy, and the 40% concentrated nitric acid splashes and flies out everywhere! Hydrochloric acid, however, supposedly dissolves tin at room temperature, but apparently some experimenters have had difficulty getting it to dissolve.

High-silicon cast irons also offer excellent resistance to nitric acid at all temperatures and concentrations, with the exception of dilute hot acid. Aluminum is actually resistant to nitric acid if the acid concentration is over 95%, but if the concentration is below 80%, or if the nitric acid is heated above 40degC, the corrosion rate is much faster. Handbook of corrosion data. Bruce D. Craig, David S. Anderson, ASM International

Lead also dissolves in aqua regia; "Lead reacts slowly with hydrochloric acid and nitric acid, HNO3. In the latter case, nitrogen oxides are formed together with lead(II) nitrate, Pb(NO3)2"
http://www.webelements.com/lead/chemistry.html

A mixture of nitric acid and hydrogen peroxide supposedly will dissolve lead. Solutions of hydrobromic acid and bromine will also dissolve lead. A 9:1 ratio of HBr/Br2 can be used to dissolve tin.

KClO3 has been added to HNO3/HCl to dissolve pyrites.

another interesting read about which acid-mixtures burn which metals/alloys:
http://books.google.com/books?id=ViOMjoLKB1gC&pg=PA77&am...

[Edited on 5-8-2011 by AndersHoveland]

AndersHoveland - 30-11-2011 at 15:30

Quote:

The use of bromine in the extraction of gold was proposed by R. Wagner (Dingler's Journal, 218, p253)

Encyclopaedia Britannica, Volume 4 p633

blogfast25 - 1-12-2011 at 13:41

Quote: Originally posted by AndersHoveland  
But dissolving tin is indeed problematic.


Nope. It dissolves well in a mixture of HNO3 (38 %) and HCl (> 20 %), fast, with much NOx, to SnCl<sub>6</sub><sup>2-</sup>. Done it countless times. With HNO3 alone the attack is vigorous but you get SnO2. And SnO2 obtained in those conditions is very insoluble in just about anything.

Pewter too dissolves easily and quickly in HNO3/HCl. Done that many times too...

Pure tin also dissolves fairly well in hot 37 % HCl. A bit of patience is required but not much.

[Edited on 1-12-2011 by blogfast25]

edited compilation of above posts

AndersHoveland - 31-3-2012 at 19:53

A mix of nitric and hydrochloric acids, known as "aqua regia", is well known for being able to dissolve gold. There is, however, 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:
(3)Au + (3)NO3[-] + (18)H[+] --> (3)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!

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)HNO2aq --> (2)Au + (6)H[+]aq + (3)HNO3aq

(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 an unstable radical which can either react with the oxygen in air to form more nitrogen dioxide, or if left on its own will disproportionate into nitrogen dioxide and nitrous oxide after several minutes.

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

(3)NO --> N2O + NO2


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


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 transfered into the larger bowl of water using a 10ml g-l-a-s-s 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 inevitibly 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 will 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 gldonkey, wearing a clean white long-sleaved 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 seap 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.

Further Information:

A mixture of concentrated sulfuric and nitric acids is sometimes referred as the "wet ash" method by gold refiners, and indeed dissolves gold if the acids are concentrated enough, although this is usually not a practical method at all.

The extremely concentrated HNO3/H2SO4 mixture might be useful for dissolving gold-silver alloys, without the need for inquartation, since aqua regia only dissolves such alloys with extreme difficulty.


Dissolving Gold with Manganese Dioxide

Mixtures of manganese dioxide and sulfuric acid can also dissolve gold.
The reaction is slower at room temperature, but rapid with heating.
Permanganate and sulfuric acid after a few minutes also dissolve gold.
(Allen 1872)

the reaction with manganese dioxide and sulfuric acid is probably:
(2)Au + (3)MnO2 + (3)H2SO4 --> Au2O3 + Mn(SO4)2 + (2)H2O

where Mn(SO4)2 is manganese sulfate, and the gold oxide dissolves in the sulfuric acid. concentrated sulfuric acid still needs to be used, but it probably does not need to be quite so concentrated as required for the other reaction; a 70% concentration should be suitable.

Never mix concentrated sulfuric acid with permanganate solutions. Explosive dimanganese heptoxide can separate out in oily liquid droplets and spontaneously explode, spraying the dangerous acid up.

AndersHoveland - 31-3-2012 at 22:13

Pharmaceutical journal; A weekly record of pharmacy and allied sciences, Volume LXXIV, Great Britain, 1905
Quote:

Action of Iodine on Gold.
At ordinary temperatures pure dry iodine is without action on gold ; between 50° C. and the melting point of iodine combination takes place with the formation of amorphous iodide; above that temperature crystalline aurous iodide is formed. The direct reaction is always limited by the inverse decomposition of the iodide formed, but in the presence of excess of iodide pure aurous iodide may be obtained; this in excess is then best removed by subliming the mixture at a temperature of 30° ... In the presence of water, gold and iodine react in a closed vessel to form aurous iodide, but the reaction is limited, and, at normal temperatures, if the iodine can escape, the iodide is entirely decomposed. — F. Meyer (Comptes rend., 1904, 139, 733).


For more information about the chemistry of iodine and gold, see "Hand-book of chemistry", Leopold Gmelin, Volume 6, p211

[Edited on 1-4-2012 by AndersHoveland]

Zan Divine - 1-4-2012 at 13:46

Wow, that SOCl2-DMF dissolution of Au is new to me!

Dissolving the noble metals is always an interesting proposition.

I once had to dissolve 60 troy ounces of platinum to make a series of 1,2-Diaminocyclohexane Pt (II) dicarboxylates. The metal was a mix of particle sizes from perhaps 2 mm to 13 mm chunks. The dissolution was performed in a boiling solution of about 8 L of the standard 4:1 aqua regia.

It took 90 days!

LanthanumK - 2-4-2012 at 10:02

Manganese heptoxide from permanganate/sulfuric acid may very well dissolve gold, being such a powerful oxidizer.

AndersHoveland - 2-4-2012 at 21:49

Quote: Originally posted by LanthanumK  
Manganese heptoxide from permanganate/sulfuric acid may very well dissolve gold, being such a powerful oxidizer.


I would not be surprised by this. I am not sure what type of gold complex anion would form, but the MnO2+ cation can easily form in mixtures of permanganate with concentrated sulfuric acid. (the MnO2+ would act as the counter cation to the gold complex anion)