Chemistry - What metals aren't dissolved in/attacked by aqua regia?
Solution 1:
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.$^{[1]\ [2]}$
$$\ce{HNO3 + 3HCl -> Cl2 + NOCl + H2O\\ NOCl -> Cl + NO}$$
- 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.$^{[1]\ [2]}$
$$\ce{Ru + HNO3 + HCl $\kern.6em\not\kern -.6em \longrightarrow$}$$
$$\ce{\underset{powder}{Os} + $\underbrace{\mathrm{HNO_3}}_{\text{boiling}}$ -> OsO4 + N_xO_y + H2O \\ OsO4 + 2H2O <=> H2[OsO4(OH)2] \\ OsO4 + HCl ->OsO2Cl2 + Cl2 + 2H2O\\ OsO2Cl2 + HCl ->OsCl4 + Cl2 + 2H2O\\ 2OsO2Cl2 + H2O <=> OsO2 + H2[OsO2Cl4] \\ 3OsCl4 + 2H2O <=> OsO2 + 2H2[OsCl6]\\ OsO2 + 6HCl <=> H2[OsCl6] + 2H2O}$$
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.
- 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)
$[3]$ Skoog, West, Holler, Crouch. Fundamentals of Analytical Chemistry. 9th edition. (2014)
Solution 2:
- What metals (elemental, forget alloys) are neither attacked by nor dissolved in (freshly prepared) aqua regia?
Wikipedia article of aqua regia gives the answer (thanks to @Ian Bush):
However, aqua regia does not dissolve or corrode silver, titanium, iridium, ruthenium, rhenium, tantalum, niobium, hafnium, osmium, or rhodium.
But this fact is not true. Although, they are resistant to aqua regia at room temperature, if the conditions are changed like temperature, pressure, concentration of acid or state of metal, they are no longer resistant.
The above metals falls in the category of noble metals.They are the metals that resists corrosion and oxidation in moist air and most of the acids including aqua regia. However some of the noble metals are not resistant to aqua regia. Gold, platinum falls in the category of noble metal but still it is attacked by aqua regia. 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.
Above information taken from wikipedia article of noble metals.
This handbook also 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, ruthenium and tantalum are the metals not attacked by aqua regia even in elevated temperatures.
- What makes those metals that don't dissolve or corrode in aqua regia so impervious to the acid?
Maybe there is some sort of formation of protective layer maybe oxide layer or chloride layer that protects the metal from further attack. Actual explanation about its resistance can be found by studying their potentials. Most of the noble metals sit right at the bottom of electrochemical series due to their high positive potential value. The electrode potential of noble metals can be found in its wikipedia article.
(Will expand this part later with evidence)
- 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. This handbook (see table) says that attack by aqua regia will only occur when the silver chloride layer is ruptured.
I recommend to read the handbook(ref. 3) and ref. 4 and 5. It contains enough information on this topic.
References
- https://en.wikipedia.org/wiki/Noble_metal
- https://en.wikipedia.org/wiki/Aqua_regia
- https://books.google.co.in/books?id=KXwgAZJBWb0C&pg=RA1-PT119&lpg=RA1-PT119&dq=silver%20%2B%20aqua%20regia&source=bl&ots=RgNWAqyt0j&sig=5xfGzJlsxlZJkpW9TGJKTvmKDEE&hl=en&sa=X&ved=0ahUKEwjehJKd0OLRAhXLKY8KHTtuA5UQ6AEIRTAJ#v=onepage&q=silver%20%2B%20aqua%20regia&f=false
- https://books.google.co.in/booksid=WCYSBQAAQBAJ&pg=PA4&dq=noble+metals+aqua+regia&hl=en&sa=X&ved=0ahUKEwjK2OzgsTRAhVKrY8KHTQfC7kQ6AEIIjAA#v=onepage&q=noble%20metals%20aqua%20regia&f=false
- https://books.google.co.in/booksid=sE3krPsMoVYC&pg=PA232&dq=noble+metals+aqua+regia&hl=en&sa=X&ved=0ahUKEwjK2OzgsTRAhVKrY8KHTQfC7kQ6AEIKDAB#v=onepage&q=noble%20metals%20aqua%20regia&f=false