MadSci Network: Chemistry

Re: Why do iron and platinum not form amalgams?

Date: Fri Apr 29 16:10:17 2011
Posted By: Narayan Variankaval, Process Chemistry, Merck Research Labs
Area of science: Chemistry
ID: 1299845259.Ch

Though it appears to common knowledge that iron and platinum do not form
alloys with mercury - which is what an amalgam is - it seems like there is
evidence in the literature on the contrary.

Iron-Mercury amalgam:

Stability and magnetic properties of an iron-mercury alloy
Journal of Physics Condensed Matter, Volume 4, Number 44, 8627 (1992).
Authors: S. Linderoth, S. Morup

The abstract of the article reads thus:
Iron amalgams have been studied by Mossbauer spectroscopy and magnetization
measurements between 7 and 425 K. The Mossbauer spectra at 12 K show that
the iron is present in at least two different sites. After heat treatments
the Mossbauer spectra change into the typical spectrum of alpha -Fe, and
the magnetic moment per iron atom, at 275 K increases. It is suggested that
the iron forms a metastable alloy with mercury, which decomposes at about
360 K. The Fe-Hg alloy is found to be ferromagnetically ordered with a
corresponding Curie temperature of about 445 K. 

It is possible that since the alloy decomposes at 360K (87C) that it has
not found use in many applications and hence is not very popular.

Platinum-mercury amalgam:
Found this in the book... 
Dental chemistry and metallurgy - Clifford Mitchell
Platinum amalgam: Metallic platinum does not unite with mercury. Spongy
platinum unites with mercury when triturated in a warm mortar with the
latter or with acetic acid. Or Sodium amalgam if introduced into a solution
of platinic chloride, will form an amalgam of silvery appearance. The
amalgam containing 100 parts mercury and 15.48 parts platinum, has a
specific gravity of 14.29 and has metallic lustre when rubbed.  100 mercury
to 21.6 platinum is a dark gray solid; 100 mercury to 34.76 platinum is of
14.69 specific gravity dark gray but of no lustre etc.
The solid amalgam containing the most mercury is PtHg2. Mercury exposed to
platinic chloride for some time forms a thick pasty amalgam.

The book also says the amalgam of platinum and mercury along does not
harden well. So it is possible that this is a reason why it has not found
use and much more is maybe now known about it.

Having said this it appears that both iron and platinum dont necessarily
form "strong alloys" with mercury. An alloy is a solid solution of two or
more elements. At least one is usually a metal if not all components. What
determines a solid solution is whether atoms of each component are able to
occupy the others' position or fill in interstices of one another's
lattices. It is possible - though I dont know this for sure - that iron and
platinum do not have a strong interaction with mercury - and hence are not
too compatible with mercury - this may result in alloys with mercury being
formed only at low temperatures and these amalgams falling apart at higher

Hope this helps.

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