- Structure of the molecule
- Electronic configuration
- Nomenclature
- Properties
- Molecular weight
- Chemical properties
- Obtaining
- Pure mercury hydroxide
- Coprecipitation with iron (III) hydroxide
- Applications
- Recent studies
- Risks
- References
The hydroxide mercury is an inorganic compound in which the metal mercury (Hg) has an oxidation number of 2+. Its chemical formula is Hg (OH) 2. However, this species has not yet been obtained in solid form under normal conditions.
Mercury hydroxide or mercuric hydroxide is a short-lived transient intermediate in the formation of mercuric oxide HgO in alkaline solution. From studies carried out on mercuric oxide HgO solutions, it has been deduced that Hg (OH) 2 is a weak base. Other species that accompany it are HgOH + and Hg 2+.
Chemical formula of mercury (II) hydroxide. Author: Marilú Stea.
Despite not having been able to be precipitated in aqueous solution, Hg (OH) 2 has been obtained by photochemical reaction of mercury with hydrogen and oxygen at very low temperatures. It has also been obtained in the form of coprecipitate together with Fe (OH) 3, where the presence of halide ions influences the pH at which coprecipitation occurs.
Since it has not been easily obtained pure at the laboratory level, it has not been possible to find any use for this compound, nor to determine the risks of its use. However, it can be deduced that it presents the same risks as the other mercury compounds.
Structure of the molecule
The structure of mercury (II) Hg (OH) 2 hydroxide is based on a linear central portion formed by the mercury atom with the two oxygen atoms on the sides.
Hydrogen atoms are attached to this central structure, each next to each oxygen, which freely rotate around each oxygen. It could be represented in a simple way as follows:
Theoretical structure of mercury (II) hydroxide. Author: Marilú Stea
Electronic configuration
The electronic structure of metallic mercury Hg is as follows:
5 d 10 6 s 2
where is the electron configuration of the noble gas xenon.
When observing said electronic structure it is derived that the most stable oxidation state of mercury is the one in which the 2 electrons of the 6 s layer are lost.
In Hg (OH) 2 mercuric hydroxide, the mercury (Hg) atom is in its 2+ oxidation state. Therefore, in Hg (OH) 2, mercury has the following electronic configuration:
5 d 10
Nomenclature
- Mercury (II) hydroxide
- Mercuric hydroxide
- Mercury dihydroxide
Properties
Molecular weight
236.62 g / mol
Chemical properties
According to the information consulted, it is possible that Hg (OH) 2 is a transitory compound in the formation of HgO in alkaline aqueous medium.
The addition of hydroxyl ions (OH -) to an aqueous solution of mercuric ions Hg 2+ leads to the precipitation of a yellow solid of mercury (II) oxide HgO, of which Hg (OH) 2 is a passing agent or temporary.
Mercury (II) oxide. Leiem. Source: Wikipedia Commons.
In aqueous solution, Hg (OH) 2 is a very short-lived intermediate, as it quickly releases a water molecule and solid HgO precipitates.
Although it has not been possible to precipitate mercuric hydroxide Hg (OH) 2, mercuric oxide (II) HgO is somewhat soluble in water, forming a solution of species called “hydroxides”.
These species in water called “hydroxides” are weak bases and, although they sometimes behave like amphoteric, in general Hg (OH) 2 is more basic than acidic.
When HgO is dissolved in HClO 4, studies indicate the presence of mercuric ion Hg 2+, a monohydroxymercuric ion HgOH +, and mercuric hydroxide Hg (OH) 2.
The equilibria that occur in such aqueous solutions are as follows:
Hg 2+ + H 2 O ⇔ HgOH + + H +
HgOH + + H 2 O ⇔ Hg (OH) 2 + H +
In alkaline solutions of NaOH the species Hg (OH) 3 - is formed.
Obtaining
Pure mercury hydroxide
Mercury (II) hydroxide Hg (OH) 2 cannot be obtained in aqueous solution, since when alkali is added to a solution of mercuric ions Hg 2+, the yellow mercuric oxide HgO precipitates.
However, in 2005 some researchers managed to obtain mercuric hydroxide Hg (OH) 2 for the first time in 2005 using a mercury arc lamp, starting from the element mercury Hg, hydrogen H 2 and oxygen O 2.
Mercury lamp. D-Kuru. Source: Wikipedia Commons.
The reaction is photochemical and was carried out in the presence of solid neon, argon or deuterium at very low temperatures (around 5 K = 5 degrees Kelvin). Evidence of compound formation was obtained by IR (infrared) light absorption spectra.
The Hg (OH) 2 prepared in this way is very stable under the conditions of experience. The photochemical reaction is believed to proceed through the O-Hg-O intermediate to the stable HO-Hg-OH molecule.
Coprecipitation with iron (III) hydroxide
If mercury (II) sulfate HgSO 4 and iron (III) sulfate Fe 2 (SO 4) 3 are dissolved in acidic aqueous solution, and the pH begins to increase by adding a solution of sodium hydroxide NaOH, after some time from rest a solid is formed which is inferred to be a coprecipitate of Hg (OH) 2 and Fe (OH) 3.
The formation of Hg (OH) 2 has been found to be a critical step in this coprecipitation with Fe (OH) 3.
The formation of Hg (OH) 2 in the Fe (OH) 3 -Hg (OH) 2 precipitate strongly depends on the presence of ions such as fluoride, chloride or bromide, on their specific concentration and on the pH of the solution.
In the presence of fluoride (F -), at pH greater than 5, the coprecipitation of Hg (OH) 2 with Fe (OH) 3 is not affected. But at a pH of 4, the formation of complexes between Hg 2+ and F - interferes with the co-precipitation of Hg (OH) 2.
In the case of the presence of chloride (Cl -), the coprecipitation of Hg (OH) 2 occurs at a pH of 7 or higher, that is, preferably in an alkaline medium.
When bromide (Br -) is present, coprecipitation of Hg (OH) 2 occurs at even higher pH, that is, pH above 8.5, or more alkaline than with chloride.
Applications
From the review of the available sources of information it is deduced that mercury (II) Hg (OH) 2 hydroxide, being a compound not yet prepared at a commercial level, has no known uses.
Recent studies
Using computational simulation techniques in 2013, the structural and energetic characteristics related to the hydration of Hg (OH) 2 in the gaseous state were studied.
Metal-ligand coordination and solvation energies were calculated and compared by varying the degree of hydration of Hg (OH) 2.
Among other things, it was found that apparently the theoretical oxidation state is 1+ instead of the presumed 2+ usually assigned for Hg (OH) 2.
Risks
Although Hg (OH) 2 as such has not been isolated in sufficient quantity and therefore has not been used commercially, its specific risks have not been determined, but it can be inferred that it presents the same risks as the rest of the salts of mercury.
It can be toxic to the nervous system, digestive system, skin, eyes, respiratory system and kidneys.
Inhalation, ingestion or contact with the skin of mercury compounds can cause damage ranging from eye and skin irritation, insomnia, headaches, tremors, damage to the intestinal tract, memory loss, to kidney failure, among other symptoms.
Mercury has been recognized internationally as a pollutant. Most mercury compounds that come into contact with the environment are methylated by bacteria present in soils and sediments, forming methylmercury.
Methylmercury halide. Author: uploaded by User: Rifleman 82. Source: Unknown. Source: Wikipedia Commons.
This compound bioaccumulates in living organisms, passing from the soil to plants and from there to animals. In the aquatic environment, the transfer is even faster, going from very small to large species in a short time.
Methylmercury has a toxic effect for living beings and in particular for humans, who ingest it through the food chain.
When ingested with food, it is especially harmful for young children and fetuses in pregnant women, since being a neurotoxin it can cause damage to the brain and the nervous system in formation and growth.
References
- Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
- Wang, Xuefeng and Andrews, Lester (2005). Infrared Spectrum of Hg (OH) 2 in Solid Neon and Argon. Inorganic Chemistry, 2005, 44, 108-113. Recovered from pubs.acs.org.
- Amaro-Estrada, JI, et al. (2013). Aqueous Solvation of Hg (OH) 2: Energetic and Dynamical Density Functional Theory Studies of the Hg (OH) 2 - (H 2 O) n (n = 1-24) Structures. J. Phys. Chem. A 2013, 117, 9069-9075. Recovered from pubs.acs.org.
- Inoue, Yoshikazu and Munemori, Makoto. (1979). Coprecipitation of Mercury (II) with Iron (III) Hydroxide. Environmental Science & Technology. Volume 13, Number 4, April 1979. Recovered from pubs.acs.org.
- Chang, LW, et al. (2010). Nervous System and Behavioral Toxicology. In Comprehensive Toxicology. Recovered from sciencedirect.com.
- Haney, Alan and Lipsey, Richard L. (1973). Accumulation and effects of methyl mercury hydroxide in a terrestrial food chain under laboratory conditions. Environ. Pollut. (5) (1973) pp. 305-316. Recovered from sciencedirect.com.