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What metals aren't dissolved in/attacked by aqua regia?
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Mike Jonas
What metals aren't dissolved in/attacked by aqua regia?
Keep in mind
The answer will depend upon the reaction conditions. Most importantly,
physical state of the metal: porosity, degree of comminution;
temperature;
mechanical aggravation of metal surface during reaction.
Often times a chemistry text mentions that no reaction occurs. The reaction might still happen. It is just that for the specified parameters the process is meaningless and negligible.
Short overview of aqua regia
Aqua regia is the $3:1$ volumetric mixture of $\ce{HCl}$ and $\ce{HNO3}$. Its additional reactive power draws from monochlorine created in situ.$^{[1]\ [2]}$
Which metals are impervious to $3:1\ \ce{HCl/HNO3}$?
Almost every metal will react with aqua regia provided certain criteria are met.$^{[1]\ [2]}$ The closest you will probably get is ruthenium $\ce{Ru}$, and perhaps osmium $\ce{Os}$. To the best of my knowledge, $\ce{Ru}$ will not react with aqua regia in a meaningful way even if aqua regia is boiling.$^{[2]}$ The difference with $\ce{Os}$ is that powdered osmium is attacked by boiling aqua regia.$^{[1]\ [2]}$
Rhenium $\ce{Re}$ reacts slowly at room temperature $\ce{->HReO4}$. This will further react with $\ce{HCl -> ReCl4 + Cl2}$.$^{[2]}$
Hafnium $\ce{Hf}$ does react at room temperature. The reaction is slower than with titanium; overall equation is identical.$^{[2]}$
Tantalum $\ce{Ta}$ reacts when aqua regia is heated to $150\ ^{\circ}\mathrm{C}$. Rhodium $\ce{Rh}$ reacts in a grinded state. As a large compact piece, iridium $\ce{Ir}$ is affected over temperatures of $100\ ^{\circ}\mathrm{C}$. Niobium $\ce{Nb}$ is inert at room temperatures.$^{[2]}$
Summary: ruthenium $\ce{Ru}$ is your best bet.
What makes metals $\ce{Ru}$ and $\ce{Os}$ so stable in aqua regia?
The nobility of these metals is not the best explanation. As you correctly pointed out, $\ce{Pt}$ and $\ce{Au}$ react fine. This is direct evidence that for other metals a protective layer should form. The layer varies from metal to metal, but usually is either an oxide (or oxide hydrate), or a chloride.
Effectiveness of mechanical aggravation also points to stable, non-reactive compound formation on the metal's surface.
For ruthenium, as of now I am unsure what this precipitate could be. If anyone has a reference, please edit or leave a comment.$^\text{[reference needed]}$
What happens with silver?
Silver and aqua regia react very poorly, and for a short amount of time.$^{[2]}$ The culprit is $\ce{AgCl}$ ($K_s = 1.8 \cdot 10^{-10}$)$^{[2]}$. A slow reaction might still take place due to complexation.$^{[2]}$
Surprisingly, silver reacts with $\ce{HBr}$!$^{[2]}$ Its solubility product is even worse, $K_s = 5.0 \cdot 10^{-13}$.$^{[3]}$ My guess is that this layer is not as dense as $\ce{AgCl}$ but this still needs verifying.$^\text{[citation needed]}$
References
(In progress)
$[1]$ N. N. Ahmetov. Anorgaaniline keemia. (1974)
$[2]$ H. Karik, Kalle Truus. Elementide keemia. (2003)
The answer will depend upon the reaction conditions. Most importantly,
physical state of the metal: porosity, degree of comminution;
temperature;
mechanical aggravation of metal surface during reaction.
Often times a chemistry text mentions that no reaction occurs. The reaction might still happen. It is just that for the specified parameters the process is meaningless and negligible.
Short overview of aqua regia
Aqua regia is the $3:1$ volumetric mixture of $\ce{HCl}$ and $\ce{HNO3}$. Its additional reactive power draws from monochlorine created in situ.$^{[1]\ [2]}$
Which metals are impervious to $3:1\ \ce{HCl/HNO3}$?
Almost every metal will react with aqua regia provided certain criteria are met.$^{[1]\ [2]}$ The closest you will probably get is ruthenium $\ce{Ru}$, and perhaps osmium $\ce{Os}$. To the best of my knowledge, $\ce{Ru}$ will not react with aqua regia in a meaningful way even if aqua regia is boiling.$^{[2]}$ The difference with $\ce{Os}$ is that powdered osmium is attacked by boiling aqua regia.$^{[1]\ [2]}$
Rhenium $\ce{Re}$ reacts slowly at room temperature $\ce{->HReO4}$. This will further react with $\ce{HCl -> ReCl4 + Cl2}$.$^{[2]}$
Hafnium $\ce{Hf}$ does react at room temperature. The reaction is slower than with titanium; overall equation is identical.$^{[2]}$
Tantalum $\ce{Ta}$ reacts when aqua regia is heated to $150\ ^{\circ}\mathrm{C}$. Rhodium $\ce{Rh}$ reacts in a grinded state. As a large compact piece, iridium $\ce{Ir}$ is affected over temperatures of $100\ ^{\circ}\mathrm{C}$. Niobium $\ce{Nb}$ is inert at room temperatures.$^{[2]}$
Summary: ruthenium $\ce{Ru}$ is your best bet.
What makes metals $\ce{Ru}$ and $\ce{Os}$ so stable in aqua regia?
The nobility of these metals is not the best explanation. As you correctly pointed out, $\ce{Pt}$ and $\ce{Au}$ react fine. This is direct evidence that for other metals a protective layer should form. The layer varies from metal to metal, but usually is either an oxide (or oxide hydrate), or a chloride.
Effectiveness of mechanical aggravation also points to stable, non-reactive compound formation on the metal's surface.
For ruthenium, as of now I am unsure what this precipitate could be. If anyone has a reference, please edit or leave a comment.$^\text{[reference needed]}$
What happens with silver?
Silver and aqua regia react very poorly, and for a short amount of time.$^{[2]}$ The culprit is $\ce{AgCl}$ ($K_s = 1.8 \cdot 10^{-10}$)$^{[2]}$. A slow reaction might still take place due to complexation.$^{[2]}$
Surprisingly, silver reacts with $\ce{HBr}$!$^{[2]}$ Its solubility product is even worse, $K_s = 5.0 \cdot 10^{-13}$.$^{[3]}$ My guess is that this layer is not as dense as $\ce{AgCl}$ but this still needs verifying.$^\text{[citation needed]}$
References
(In progress)
$[1]$ N. N. Ahmetov. Anorgaaniline keemia. (1974)
$[2]$ H. Karik, Kalle Truus. Elementide keemia. (2003)
Possibly bromide ion forms a soluble complex with silver, more easily than chloride ion?More
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What metals (elemental, forget alloys) are neither attacked by nor dissolved in (freshly prepared) aqua regia?
At normal condition, most of the metals are impervious to aqua regia but if the conditions are changed like temperature, pressure, acid concentration or state of metal, they can be prone to attack.
The metals discussed falls in the category of noble metals. These metals resists corrosion and oxidation in moist air and most of the acids including aqua regia. But there are exceptions. More specifically:
Platinum, gold and mercury can be dissolved in aqua regia but iridium
and silver cannot. Palladium and silver are, however, soluble in
nitric acid. Ruthenium can be dissolved in aqua regia only when in the
presence of oxygen (Why? will be asked in a separate question), while rhodium must be in a fine pulverized form.
Niobium and tantalum are resistant to all acids, including aqua regia.
This handbook1 gives an overview of which noble metals are attacked by aqua regia:
Niobium is reported a corrosion rate of 0.025mm/yr in aqua regia at 328.15 K.
Gold is attacked by aqua regia.
Iridium resist both cold and hot aqua regia but dissolves under pressure at 523-623 K.
Platinum is attacked by aqua regia.
Rhodium unattacked by aqua regia in either wrought or cast form.
Ruthenium is by far the most chemically resistant of the noble metal, unattacked either in hot or cold aqua regia.
Tantalum also not attacked by aqua regia even in elevated temperatures.
Zirconium is attacked by aqua regia.
Hafnium is dissolved in aqua regia only in the addition of some soluble fluoride salts.
So, the winners are ruthenium and tantalum.
What makes those metals that don't dissolve or corrode in aqua regia so impervious to the acid?
Maybe there is some sort of protective layer like oxide or chloride layer that protects the metal from further attack. It can be explained by studying their potentials. Most of the noble metals sit right at the bottom of electrochemical series due to their high positive potential value. So, they are quite resistant to attacks.
Does silver metal actually develop a silver chloride layer on exposure to aqua regia? If so, would that mean the Wikipedia article is incorrect?
No, silver actually forms a silver chloride layer that protects the metal from further attack. Aqua regia can only dissolve silver if the chloride layer is ruptured.
What metals (elemental, forget alloys) are neither attacked by nor dissolved in (freshly prepared) aqua regia?
At normal condition, most of the metals are impervious to aqua regia but if the conditions are changed like temperature, pressure, acid concentration or state of metal, they can be prone to attack.
The metals discussed falls in the category of noble metals. These metals resists corrosion and oxidation in moist air and most of the acids including aqua regia. But there are exceptions. More specifically:
Platinum, gold and mercury can be dissolved in aqua regia but iridiumand silver cannot. Palladium and silver are, however, soluble innitric acid. Ruthenium can be dissolved in aqua regia only when in thepresence of oxygen (Why? will be asked in a separate question), while rhodium must be in a fine pulverized form.Niobium and tantalum are resistant to all acids, including aqua regia.
This handbook1 gives an overview of which noble metals are attacked by aqua regia:
Niobium is reported a corrosion rate of 0.025mm/yr in aqua regia at 328.15 K.
Gold is attacked by aqua regia.
Iridium resist both cold and hot aqua regia but dissolves under pressure at 523-623 K.
Platinum is attacked by aqua regia.
Rhodium unattacked by aqua regia in either wrought or cast form.
Ruthenium is by far the most chemically resistant of the noble metal, unattacked either in hot or cold aqua regia.
Tantalum also not attacked by aqua regia even in elevated temperatures.
Zirconium is attacked by aqua regia.
Hafnium is dissolved in aqua regia only in the addition of some soluble fluoride salts.
So, the winners are ruthenium and tantalum.
What makes those metals that don't dissolve or corrode in aqua regia so impervious to the acid?
Maybe there is some sort of protective layer like oxide or chloride layer that protects the metal from further attack. It can be explained by studying their potentials. Most of the noble metals sit right at the bottom of electrochemical series due to their high positive potential value. So, they are quite resistant to attacks.
Does silver metal actually develop a silver chloride layer on exposure to aqua regia? If so, would that mean the Wikipedia article is incorrect?
No, silver actually forms a silver chloride layer that protects the metal from further attack. Aqua regia can only dissolve silver if the chloride layer is ruptured.
Regarding "No [the Wikipedia article is not incorrect], silver actually forms a silver chloride layer that protects the metal from further attack", I rather took @paracresols meaning to be that the formation of a protective chloride layer is, in itself, a form of corrosion (albeit minimal, and self-limiting). Perhaps p-cresol could confirm/deny/clarify?More
Definitions that I can find online for "corrosion" all seem to reference "gradual destruction". So perhaps we could argue that it needs to be an ongoing process to constitute "corrosion". Coming at it from a more intuitive, empirical, chemists perspective, I dont imagine that any of us would argue that oxygen in the air More
Keep in mind
The answer will depend upon the reaction conditions. Most importantly,
Often times a chemistry text mentions that no reaction occurs. The reaction might still happen. It is just that for the specified parameters the process is meaningless and negligible.
Short overview of aqua regia
Aqua regia is the $3:1$ volumetric mixture of $\ce{HCl}$ and $\ce{HNO3}$. Its additional reactive power draws from monochlorine created in situ.$^{[1]\ [2]}$
$$\ce{HNO3 + 3HCl -> Cl2 + NOCl + H2O\\ NOCl -> Cl + NO}$$
Almost every metal will react with aqua regia provided certain criteria are met.$^{[1]\ [2]}$ The closest you will probably get is ruthenium $\ce{Ru}$, and perhaps osmium $\ce{Os}$. To the best of my knowledge, $\ce{Ru}$ will not react with aqua regia in a meaningful way even if aqua regia is boiling.$^{[2]}$ The difference with $\ce{Os}$ is that powdered osmium is attacked by boiling aqua regia.$^{[1]\ [2]}$
$$\ce{Ru + HNO3 + HCl $\kern.6em\not\kern -.6em \longrightarrow$}$$
$$\ce{\underset{powder}{Os} + $\underbrace{\mathrm{HNO_3}}_{\text{boiling}}$ -> OsO4 + N_xO_y + H2O \\ OsO4 + 2H2O <=> H2[OsO4(OH)2] \\ OsO4 + HCl ->OsO2Cl2 + Cl2 + 2H2O\\ OsO2Cl2 + HCl ->OsCl4 + Cl2 + 2H2O\\ 2OsO2Cl2 + H2O <=> OsO2 + H2[OsO2Cl4] \\ 3OsCl4 + 2H2O <=> OsO2 + 2H2[OsCl6]\\ OsO2 + 6HCl <=> H2[OsCl6] + 2H2O}$$
Brief discussion about the list provided in the comments
Titanium $\ce{Ti}$ does react, and does so at room temperature.
$$\ce{3Ti + $\underbrace{\mathrm{12HCl + 4HNO_3}}_{\text{room temperature}}$ -> 3TiCl4 + 4NO + 8H2O}$$
Rhenium $\ce{Re}$ reacts slowly at room temperature $\ce{->HReO4}$. This will further react with $\ce{HCl -> ReCl4 + Cl2}$.$^{[2]}$
Hafnium $\ce{Hf}$ does react at room temperature. The reaction is slower than with titanium; overall equation is identical.$^{[2]}$
Tantalum $\ce{Ta}$ reacts when aqua regia is heated to $150\ ^{\circ}\mathrm{C}$. Rhodium $\ce{Rh}$ reacts in a grinded state. As a large compact piece, iridium $\ce{Ir}$ is affected over temperatures of $100\ ^{\circ}\mathrm{C}$. Niobium $\ce{Nb}$ is inert at room temperatures.$^{[2]}$
Summary: ruthenium $\ce{Ru}$ is your best bet.
The nobility of these metals is not the best explanation. As you correctly pointed out, $\ce{Pt}$ and $\ce{Au}$ react fine. This is direct evidence that for other metals a protective layer should form. The layer varies from metal to metal, but usually is either an oxide (or oxide hydrate), or a chloride.
Effectiveness of mechanical aggravation also points to stable, non-reactive compound formation on the metal's surface.
For ruthenium, as of now I am unsure what this precipitate could be. If anyone has a reference, please edit or leave a comment.$^\text{[reference needed]}$
Silver and aqua regia react very poorly, and for a short amount of time.$^{[2]}$ The culprit is $\ce{AgCl}$ ($K_s = 1.8 \cdot 10^{-10}$)$^{[2]}$. A slow reaction might still take place due to complexation.$^{[2]}$
Surprisingly, silver reacts with $\ce{HBr}$!$^{[2]}$ Its solubility product is even worse, $K_s = 5.0 \cdot 10^{-13}$.$^{[3]}$ My guess is that this layer is not as dense as $\ce{AgCl}$ but this still needs verifying.$^\text{[citation needed]}$
References
(In progress)
$[1]$ N. N. Ahmetov. Anorgaaniline keemia. (1974)
$[2]$ H. Karik, Kalle Truus. Elementide keemia. (2003)
$[3]$ Skoog, West, Holler, Crouch. Fundamentals of Analytical Chemistry. 9th edition. (2014)
Keep in mind
The answer will depend upon the reaction conditions. Most importantly,
Often times a chemistry text mentions that no reaction occurs. The reaction might still happen. It is just that for the specified parameters the process is meaningless and negligible.
Short overview of aqua regia
Aqua regia is the $3:1$ volumetric mixture of $\ce{HCl}$ and $\ce{HNO3}$. Its additional reactive power draws from monochlorine created in situ.$^{[1]\ [2]}$
$$\ce{HNO3 + 3HCl -> Cl2 + NOCl + H2O\\ NOCl -> Cl + NO}$$
Almost every metal will react with aqua regia provided certain criteria are met.$^{[1]\ [2]}$ The closest you will probably get is ruthenium $\ce{Ru}$, and perhaps osmium $\ce{Os}$. To the best of my knowledge, $\ce{Ru}$ will not react with aqua regia in a meaningful way even if aqua regia is boiling.$^{[2]}$ The difference with $\ce{Os}$ is that powdered osmium is attacked by boiling aqua regia.$^{[1]\ [2]}$
$$\ce{Ru + HNO3 + HCl $\kern.6em\not\kern -.6em \longrightarrow$}$$
$$\ce{\underset{powder}{Os} + $\underbrace{\mathrm{HNO_3}}_{\text{boiling}}$ -> OsO4 + N_xO_y + H2O \\ OsO4 + 2H2O <=> H2[OsO4(OH)2] \\ OsO4 + HCl ->OsO2Cl2 + Cl2 + 2H2O\\ OsO2Cl2 + HCl ->OsCl4 + Cl2 + 2H2O\\ 2OsO2Cl2 + H2O <=> OsO2 + H2[OsO2Cl4] \\ 3OsCl4 + 2H2O <=> OsO2 + 2H2[OsCl6]\\ OsO2 + 6HCl <=> H2[OsCl6] + 2H2O}$$
Brief discussion about the list provided in the comments
Titanium $\ce{Ti}$ does react, and does so at room temperature.
$$\ce{3Ti + $\underbrace{\mathrm{12HCl + 4HNO_3}}_{\text{room temperature}}$ -> 3TiCl4 + 4NO + 8H2O}$$
Rhenium $\ce{Re}$ reacts slowly at room temperature $\ce{->HReO4}$. This will further react with $\ce{HCl -> ReCl4 + Cl2}$.$^{[2]}$
Hafnium $\ce{Hf}$ does react at room temperature. The reaction is slower than with titanium; overall equation is identical.$^{[2]}$
Tantalum $\ce{Ta}$ reacts when aqua regia is heated to $150\ ^{\circ}\mathrm{C}$. Rhodium $\ce{Rh}$ reacts in a grinded state. As a large compact piece, iridium $\ce{Ir}$ is affected over temperatures of $100\ ^{\circ}\mathrm{C}$. Niobium $\ce{Nb}$ is inert at room temperatures.$^{[2]}$
Summary: ruthenium $\ce{Ru}$ is your best bet.
The nobility of these metals is not the best explanation. As you correctly pointed out, $\ce{Pt}$ and $\ce{Au}$ react fine. This is direct evidence that for other metals a protective layer should form. The layer varies from metal to metal, but usually is either an oxide (or oxide hydrate), or a chloride.
Effectiveness of mechanical aggravation also points to stable, non-reactive compound formation on the metal's surface.
For ruthenium, as of now I am unsure what this precipitate could be. If anyone has a reference, please edit or leave a comment.$^\text{[reference needed]}$
Silver and aqua regia react very poorly, and for a short amount of time.$^{[2]}$ The culprit is $\ce{AgCl}$ ($K_s = 1.8 \cdot 10^{-10}$)$^{[2]}$. A slow reaction might still take place due to complexation.$^{[2]}$
Surprisingly, silver reacts with $\ce{HBr}$!$^{[2]}$ Its solubility product is even worse, $K_s = 5.0 \cdot 10^{-13}$.$^{[3]}$ My guess is that this layer is not as dense as $\ce{AgCl}$ but this still needs verifying.$^\text{[citation needed]}$
References
(In progress)
$[1]$ N. N. Ahmetov. Anorgaaniline keemia. (1974)
$[2]$ H. Karik, Kalle Truus. Elementide keemia. (2003)
$[3]$ Skoog, West, Holler, Crouch. Fundamentals of Analytical Chemistry. 9th edition. (2014)
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At normal condition, most of the metals are impervious to aqua regia but if the conditions are changed like temperature, pressure, acid concentration or state of metal, they can be prone to attack.
The metals discussed falls in the category of noble metals. These metals resists corrosion and oxidation in moist air and most of the acids including aqua regia. But there are exceptions. More specifically:
This handbook1 gives an overview of which noble metals are attacked by aqua regia:
So, the winners are ruthenium and tantalum.
Maybe there is some sort of protective layer like oxide or chloride layer that protects the metal from further attack. It can be explained by studying their potentials. Most of the noble metals sit right at the bottom of electrochemical series due to their high positive potential value. So, they are quite resistant to attacks.
No, silver actually forms a silver chloride layer that protects the metal from further attack. Aqua regia can only dissolve silver if the chloride layer is ruptured.
References
At normal condition, most of the metals are impervious to aqua regia but if the conditions are changed like temperature, pressure, acid concentration or state of metal, they can be prone to attack.
The metals discussed falls in the category of noble metals. These metals resists corrosion and oxidation in moist air and most of the acids including aqua regia. But there are exceptions. More specifically:
This handbook1 gives an overview of which noble metals are attacked by aqua regia:
So, the winners are ruthenium and tantalum.
Maybe there is some sort of protective layer like oxide or chloride layer that protects the metal from further attack. It can be explained by studying their potentials. Most of the noble metals sit right at the bottom of electrochemical series due to their high positive potential value. So, they are quite resistant to attacks.
No, silver actually forms a silver chloride layer that protects the metal from further attack. Aqua regia can only dissolve silver if the chloride layer is ruptured.
References
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