Why doesn't tantalum and tungsten form amalgam?

Question

User @Poutnik has recently answered a question in which he quoted this statement from Wikipedia:

Almost all metals can form amalgams with mercury, the notable exceptions being iron, platinum, tungsten, and tantalum.

I am not sure about the validity of this statement as I found contradicting results regarding iron and platinum amalgams. There has been discussion around iron amalgams on internet. I found this reddit post and on further research, I came to know that iron indeed form amalgam albeit unstable (See here). This has also been discussed in an another question. User @DrMoishePippik has pointed out that iron can form mercury-based alloys(amalgams?) at specific conditions. So, there is a contradiction regarding formation of iron amalgam.

Some searching also proved that platinum can also form amalgams:

Platinum Amalgam is obtained by trituration of platinum sponge with mercury in a warm mortar; it cannot be obtained by direct union of platinum foil and mercury.

The amalgam has a silvery appearance, and with 12 per cent, of platinum is soft and greasy to the touch, but higher percentages of platinum increase its stiffness. When heated strongly the mercury is volatilised and platinum remains as a grey residue.[...]

I searched the same site for tantalum alloys and gave me following information:

Sodium, potassium, mercury and silver do not alloy with tantalum even at high temperatures; attempts to prepare alloys with arsenic, antimony, lead, zinc and tellurium have also failed, but the formation of an alloy with silver, copper and tin for making a dental amalgam with mercury has recently been claimed.

No information about tungsten amalgam were given on that site. Other sources is giving me irrelevant information although the only other thing I found out is that since they doesn't form amalgams, they can be separated from a mixture of other metals.

So, the questions are:

• Does tantalum and tungsten really doesn't form amalgams?
• If yes, what is the reason?
• If no, is there any evidence/scientific papers to prove it?

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I just saw a comment User @Mithoron has left in the question "Why does mercury not form amalgam with iron?"

Pt, W and Ta also resist dissolution with Hg

Can anyone elaborate on this statement?

Answer 1

Imagine a lump of a metallic element in close contact with an equal amount of liquid mercury. Amalgamation might proceed to some degree if the metallic element has an even higher surface tension than mercury. A surface layer of adsorbed mercury will be the first step in amalgamation.

This first step, wetting, but not involving extensive solution of, or into, the metal, is well illustrated by this image of a mercury switch (Ref 1). The electrodes are well wetted, but must remain integral for many years thru electric currents, temperature changes and sparking.

Mercury can penetrate some metals, like aluminum, probably faster thru grain boundaries, thereby causing the metal to disintegrate. There is an added complexity that aluminum has an impermeable oxide layer that must be breached first.

Atoms may disperse thru mercury until a solubility limit, or more accurately, a viscosity increase is reached that effectively immobilizes the mercury. The term amalgam commonly means a fluid or semisolid material like a dental amalgam, which, although it finally becomes hard enough to bite on, is usually encountered in a semisolid state (see the image below, Ref 2). The time required for complete dispersal of the mercury in the liquid into the solid particles of the silver-tin alloy determines the alloy’s usefulness.

The phase diagrams of other mercury alloys shows the effect of melting point of the non-mercury element. The thallium (mp = 304 C) alloy is liquid at room temperature to about 45% thallium (Ref 3).

Uranium, melting at 1132 C), can stiffen a 1% alloy at room temperature, and 3% uranium will solidify the alloy as the mercury boils away at 357 C! (Ref 4)

Another way amalgams can form is by addition of the other metallic element, atom by atom, one at a time. Such is the usual method of preparing sodium amalgam electrolytically. A pool of mercury in contact with a solution of a sodium salt is made negative enough to attract sodium ions, where they will discharge and enter the mercury mass before reacting with water. This can be continued until the mercury is stiffened, which amounts to reaching a solubility limit, and at that point, any more sodium formed will react with water.

Aluminum and sodium have relatively low melting points, suggesting that they have low cohesive strength. “Low melting point” also includes silver and gold, with melting points around 1000 C. These metals can be expected to alloy, or amalgamate, easily with mercury.

But high melting point (for Pt, 1768 C) is not a guarantee of complete non-amalgamatability. The OP mentioned a way of making what appears to be a soft solid at 12% Pt from Pt sponge, which can be stiffened with more Pt, but this cannot be done with Pt foil. Pt foil can be obtained from Aldrich as thin as 0.025 mm; Pt sponge is composed of particles less than 10 microns thick - i.e., with 2500 times the surface area (Wikipedia). A surface reaction product Pt(Hg)4 has been found, but no penetration into the Pt (Ref 5) until the temperature is raised to 250 C (Ref 6), when the solubility reaches 15.5%, but forms another solid, not liquid, phase.

X-ray analysis showed that V, Nb, Ta, Ti, Zr, Cr, Mo, W, Fe, Ni, Co, Re, Ru, Rh, Os, Ir, Al, Th, and U dissolve only negligible amounts of mercury (Ref 6). So melting point can be a good indicator of the ability of a metal to alloy with mercury. Higher melting point suggests greater difficulty in atomizing the metal. There could be a correlation with surface tension of the metal. And one more quantitative measurement should be the heat of vaporization (not fusion, since the heats of fusion are quite a bit smaller). The heats of vaporization of tungsten and tantalum are higher than all the other elements I have checked by a factor of about two! (WebElements)

Temperature of preparation is important because some mercury-metal alloys may form only at higher temperatures. Whether the amalgam is liquid or solid will depend on the concentration of the metal in the mercury and the observation temperature. And time may affect properties like fluidity and stability (e.g., note the instability of ammonium amalgam, although it is easily prepared).

Ref 1. https://en.wikipedia.org/wiki/Mercury_(element)

Ref 2. https://pocketdentistry.com/14-silver-amalgam/

Ref 3. https://www.researchgate.net/figure/Phase-diagram-of-binary-system-Hg-Tl_fig3_47393888

Ref 4. https://www.researchgate.net/publication/238894897_The_HgU_Mercury-Uranium_System

Ref 5. S. K. Lahiri and D. Gupta, Journal of Applied Physics 51, 5555 (1980); https://doi.org/10.1063/1.327440

Ref 6. G. Jangg & E. Lugscheider, Monatshefte für Chemie / Chemical Monthly 104, 1269–1275 (1973)

Answer 2

Does tantalum or tungsten really form amalgam(s) (with mercury)?

By X-ray analysis, G. Jangg and E. Lugscheider in 1973 found that tantalum $(\ce{_{73}Ta})$ or tungsten $(\ce{_{74}W})$ does not form amalgams with mercury $(\ce{_{80}Hg})$ based on their analytical measures. The authors describe that these metals dissolve only negligible amounts of mercury. They also found out that not only $(\ce{_{73}Ta})$ and $(\ce{_{74}W})$, but also other metals such as aluminum $(\ce{_{13}Al})$, titanium $(\ce{_{22}Ti})$, vanadium $(\ce{_{23}V})$, chromium $(\ce{_{24}Cr})$, iron $(\ce{_{26}Fe})$, cobalt $(\ce{_{27}Co})$, nickel $(\ce{_{28}Ni})$, zirconium $(\ce{_{40}Zr})$, niobium $(\ce{_{41}Nb})$, molybdenum $(\ce{_{42}Mo})$, ruthenium $(\ce{_{44}Ru})$, rhodium $(\ce{_{45}Rh})$, rhenium $(\ce{_{75}Re})$, osmium $(\ce{_{76}Os})$, iridium $(\ce{_{77}Ir})$, thorium $(\ce{_{90}Th})$, and uranium $(\ce{_{92}U})$ dissolve only negligible amounts of mercury under the conditions used. The abstract states following:

Investigations by X-ray analysis proved that $\ce{V, Nb, Ta, Ti, Zr, Cr, Mo, W, Fe, Ni, Co, Re,}$ $\ce{Ru, Rh, Os, Ir, Al, Th,}$ and $\ce{U}$ dissolve only negligible amounts of mercury. The solubility of mercury in platinum up to $\pu{250 ^\circ C}$ is small; at $\pu{250 ^\circ C}$ the solubility increases abruptly to ca. 15.5% and alters little with further temperature increases. The jump at $\pu{250 ^\circ C}$ corresponds to a peritectic reaction in the system $\ce{Pt−Hg}$. The data obtained are in good agreement with data in the literature. Manganese dissolves at $\pu{500 ^\circ C}$ ca. 0.3–0.5% $\ce{Hg}$; the temperature dependence of the solubility was not examined. Rhenium forms no compounds with Hg, at least at $100$–$\pu{500 ^\circ C}$. The metals of the Va and VIa-group of the periodic table do not dissolve any measurable amounts of $\ce{Zn}$.

Following are the conditions used: $$ \begin{array}{lcr} \text{Atomic number} &\text{Metal} & \text{Heat Treatment}/\pu{^\circ C} \\\hline 13 & \text{Aluminum} & 260 \\ 22 & \text{Titanium} & 280, 400-500 \\ 23 & \text{Vanadium} & 800 \\ 24 & \text{Chromium} & 400 \\ 26 & \text{Iron} & 800 \\ 27 & \text{Cobalt} & 800 \\ 28 & \text{Nickel} & 260 \\ 40 & \text{Zirconium} & 280, 400-500 \\ 41 & \text{Niobium} & 800 \\ 42 & \text{Molybdenum} & 800 \\ 44 & \text{Ruthenium} & 800 \\ 45 & \text{Rhodium} & 800 \\ 73 & \text{Tantalum} & 800 \\ 74 & \text{Tungsten} & 800 \\ 75 & \text{Rhenium} & 100-500^a \\ 76 & \text{Osmium} & 800 \\ 77 & \text{Iridium} & 800 \\ 90 & \text{Thorium} & 800 \\ 92 & \text{Uranium} & 800 \\ \hline\\ \end{array}\\ ^a\text{No evidence of compound formation with mercury over the temperature interval indicated}$$

It is interesting to see, according to the table (except for $\ce{Th}$ and $\ce{U}$) that most of $\mathrm{d}$-block elements does not make amalgams with mercury. Seemingly, only $\ce{_{25}Mn}$ $(\chi_\ce{Hg} = 0.004)$, and $\ce{_{30}Zn}$ $(\chi_\ce{Hg} = 0.020)$, amongst 3d-elements significantly makes an amalgam with $\ce{Hg}$ at $\pu{773 K}$ and $\pu{293 K}$, respectively $(\chi_\ce{Hg} \text{ represent the mole fraction of mercury in the mixture})$.

The solubility of $\mathrm{3d}$-block elements in mercury has also been checked in 1932 with similar results (Ref.2). The percentage solubility determined in this work is $0.0010\%$ $\ce{Mn}$ and $0.0020\%$ $\ce{Cu}$ amongst $\mathrm{3d}$-block elements tested. The other percentages obtained were: titanium, $ \lt 1 \times 10^{-5}$; vanadium, $ \lt 5 \times 10^{-5}$; chromium, $ \lt 5 \times 10^{-5}$; iron, $ \lt 1 \times 10^{-5}$; cobalt, $ \lt 8 \times 10^{-5}$; and nickel, $ \lt 2 \times 10^{-5}$. The authors have also checked a $\mathrm{4d}$-block element, molybdenum $( \lt 2 \times 10^{-5})$ and a $\mathrm{5d}$-block element, tungsten $ (\lt 1 \times 10^{-5})$ with similar results.

Similar to $\ce{Cu}$, $\ce{Ag}$ has shown best solubility $(\chi_\ce{Hg} = 0.364)$ amongst $\mathrm{4d}$-block elements at $\pu{373 K}$ and $\ce{Au}$ has shown best solubility $(\chi_\ce{Hg} = 0.092)$ amongst $\mathrm{5d}$-block elements at $\pu{663 K}$ (Ref.3).

References:

1. G. Jangg and E. Lugscheider, "Die Löslichkeit von Quecksilber in verschiedenen Metallen (Solubility of mercury in various metals)," Monatshefte für Chemie 1973, 104, 1269-1275 (DOI: https://doi.org/10.1007/BF00910041).
2. Nevill Maxsted Irvin and Alexander Smith Russell, "117. The solubilities of copper, manganese, and some sparingly soluble metals in mercury," J. Chem. Soc. 1932, 891-898 (DOI: https://doi.org/10.1039/JR9320000891).
3. H. M. Day and C. H. Mathewson, "Solid Solubility of Mercury in Silver and in Gold," Transactions of the American Institute of Mining and Metallurgical Engineers 1938, 128, 261-281 (DOI: https://doi.org/10.1039/JR9320000891).