Anders Hoveland
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- Joined
- Aug 1, 2011
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- 7
Gold and most other metals will dissolve in aqua regia; silver, however, will form a silver chloride precipitate. Because of this reaction of silver, it is usually necessary to restrict the amount of silver in the start material to be refined to a maximum of 10%; any more than this will run the risk of the silver chloride preventing complete dissolution of the gold by masking it off.
Some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in aqua-regia as an insoluble silver chloride film is formed which prevents further action by the aqua-regia.
Chloride ions act as a sort of inhibitor, preventing corrosion of metallic silver by forming a surface layer of insoluble AgCl. That a mix of H2O2 and HCl will not dissolve silver is not at all surprising. Neither is silver attacked by aqueous chlorine.
Silver will react with hot concentrated H2SO4, with HNO3, and with aqua regia, although much more slowly in the latter
One researcher encountered problems trying to dissolve away elemental gold, while leaving silver chloride in a photographic plate intact. “I could not employ aqua regia of the usual strength because the concentrated hydrochloric acid dissolved the silver halide at once. A success was obtained with dilute aqua regia.”
"The remainder of the silver was placed in another beaker and covered with concentrated HNO3. To this beaker was added an equal volume of concentrated HCl, forming a mixture that lacked the proportions of true aqua regia (3 HCl : 1 HNO3). As the acid attacked the silver, AgCl precipitated out as white clumps. The amount of precipitate steadily increased, peaking at about four hours. It redissolved after prolonged standing, however.
The method of using mixed HCl-HNO3 was done in this experiment to illustrate a curious situation in which Ag seems to go into solution via the nitric acid and comes right back out again as AgCl because of the hydrochloric. If we could make a diagram to show what we think is happening, it would show newly-mobilized Ag+ ions coming off the metal surface, only to form insoluble AgCl as soon as they meet up with nearby Cl- ions. It is well-known that silver chloride is highly insoluble, even in nitric acid; however, beginning chemistry texts often neglect to mention that silver chloride will in fact form a soluble complex ion (AgCl2-) in the presence of excess chloride. AgCl is therefore soluble in excess NaCl or concentrated HCl, especially when some AgNO3 is present (Merck Index, 1983).
In this experiment, much of the silver came right out of solution as AgCl. However, some remained in solution as AgCl2-. Dilution of this liquid caused more AgCl to precipitate. This tends to confirm that [Cl-]aq must be high in order to allow significant [AgCl2-]aq. Dilution favors solid AgCl over dissolved AgCl2- ions."
The ability of aqua regia to dissolve silver in gold is due both to the oxidizing power of the nitrate ion and the ability of chloride ion to form highly stable, partially covalent polyatomic ions with the metal ions once they are oxidized.
"solvent action of strong nitric acid upon silver chloride. A large quantity of the strongest nitric acid, apparently free from..."
http://pubs.rsc.org/en/content/articlelanding/1872/js/js8722500452 (perhaps someone could share with us what is in the paper)
Apparently, silver chloride does have some equilibrium with concentrated nitric acid, thus the AgCl can be dissolved, although only very slowly)
http://pubs.acs.org/doi/abs/10.1021/ja01333a009
If trying to more rapidly dissolve elemental silver with aqua regia, I think it would be important to use a much larger excess of nitric acid than hydrochloric, and use as high a concentration of nitric acid as possible (preferably over 90%, but possibly 99-100% would be required).
If the acids are concentrated enough, and more nitric acid is used than hydrochloric in a sufficient ratio, the reaction will be:
(3)AgCl + (4)HNO3 --> (3)AgNO3 + (2)H2O + NOCl + Cl2
the above reaction only works with the nitric acid (which itself must be >60%) in the pressence of concentrated (95%+) sulfuric acid.
Basically, to answer the original question, aqua regia can dissolve pure silver, but only with great difficulty.
Some low carat white golds contain considerable silver and are very difficult or impossible to dissolve in aqua-regia as an insoluble silver chloride film is formed which prevents further action by the aqua-regia.
Chloride ions act as a sort of inhibitor, preventing corrosion of metallic silver by forming a surface layer of insoluble AgCl. That a mix of H2O2 and HCl will not dissolve silver is not at all surprising. Neither is silver attacked by aqueous chlorine.
Silver will react with hot concentrated H2SO4, with HNO3, and with aqua regia, although much more slowly in the latter
One researcher encountered problems trying to dissolve away elemental gold, while leaving silver chloride in a photographic plate intact. “I could not employ aqua regia of the usual strength because the concentrated hydrochloric acid dissolved the silver halide at once. A success was obtained with dilute aqua regia.”
"The remainder of the silver was placed in another beaker and covered with concentrated HNO3. To this beaker was added an equal volume of concentrated HCl, forming a mixture that lacked the proportions of true aqua regia (3 HCl : 1 HNO3). As the acid attacked the silver, AgCl precipitated out as white clumps. The amount of precipitate steadily increased, peaking at about four hours. It redissolved after prolonged standing, however.
The method of using mixed HCl-HNO3 was done in this experiment to illustrate a curious situation in which Ag seems to go into solution via the nitric acid and comes right back out again as AgCl because of the hydrochloric. If we could make a diagram to show what we think is happening, it would show newly-mobilized Ag+ ions coming off the metal surface, only to form insoluble AgCl as soon as they meet up with nearby Cl- ions. It is well-known that silver chloride is highly insoluble, even in nitric acid; however, beginning chemistry texts often neglect to mention that silver chloride will in fact form a soluble complex ion (AgCl2-) in the presence of excess chloride. AgCl is therefore soluble in excess NaCl or concentrated HCl, especially when some AgNO3 is present (Merck Index, 1983).
In this experiment, much of the silver came right out of solution as AgCl. However, some remained in solution as AgCl2-. Dilution of this liquid caused more AgCl to precipitate. This tends to confirm that [Cl-]aq must be high in order to allow significant [AgCl2-]aq. Dilution favors solid AgCl over dissolved AgCl2- ions."
The ability of aqua regia to dissolve silver in gold is due both to the oxidizing power of the nitrate ion and the ability of chloride ion to form highly stable, partially covalent polyatomic ions with the metal ions once they are oxidized.
"solvent action of strong nitric acid upon silver chloride. A large quantity of the strongest nitric acid, apparently free from..."
http://pubs.rsc.org/en/content/articlelanding/1872/js/js8722500452 (perhaps someone could share with us what is in the paper)
Apparently, silver chloride does have some equilibrium with concentrated nitric acid, thus the AgCl can be dissolved, although only very slowly)
http://pubs.acs.org/doi/abs/10.1021/ja01333a009
If trying to more rapidly dissolve elemental silver with aqua regia, I think it would be important to use a much larger excess of nitric acid than hydrochloric, and use as high a concentration of nitric acid as possible (preferably over 90%, but possibly 99-100% would be required).
If the acids are concentrated enough, and more nitric acid is used than hydrochloric in a sufficient ratio, the reaction will be:
(3)AgCl + (4)HNO3 --> (3)AgNO3 + (2)H2O + NOCl + Cl2
the above reaction only works with the nitric acid (which itself must be >60%) in the pressence of concentrated (95%+) sulfuric acid.
Basically, to answer the original question, aqua regia can dissolve pure silver, but only with great difficulty.
