Barium Sulfate
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Barium sulfate (or sulphate) is the
inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...
with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
. It occurs in nature as the mineral barite, which is the main commercial source of barium and materials prepared from it. Its opaque white appearance and its high density are exploited in its main applications.Holleman, A. F. and Wiberg, E. (2001) ''Inorganic Chemistry'', San Diego, CA. Academic Press, .


Uses


Drilling fluids

About 80% of the world's barium sulfate production, mostly purified mineral, is consumed as a component of oil well drilling fluid. It increases the density of the fluid, increasing the hydrostatic pressure in the well and reducing the chance of a blowout.


Radiocontrast agent

Barium sulfate in suspension is often used medically as a radiocontrast agent for
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
imaging and other diagnostic procedures. It is most often used in imaging of the GI tract during what is colloquially known as a " barium meal". It is administered orally, or by enema, as a suspension of fine particles in a thick milk-like solution (often with sweetening and flavoring agents added). Although barium is a heavy metal, and its water-soluble compounds are often highly toxic, the low solubility of barium sulfate protects the patient from absorbing harmful amounts of the metal. Barium sulfate is also readily removed from the body, unlike Thorotrast, which it replaced. Due to the relatively high
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (''n''p) or the number of pro ...
(''Z'' = 56) of barium, its compounds absorb X-rays more strongly than compounds derived from lighter nuclei.


Pigment

The majority of synthetic barium sulfate is used as a component of white pigment for paints. In oil paint, barium sulfate is almost transparent, and is used as a filler or to modify consistency. One major manufacturer of artists' oil paint sells "permanent white" that contains a mixture of titanium white pigment ( TiO2) and barium sulfate. The combination of barium sulfate and zinc sulfide (ZnS) is the inorganic pigment called lithopone. In photography it is used as a coating for certain photographic papers. It is also used as a coating to diffuse light evenly.


Light-reflecting paint for cooling

Barium sulfate is highly reflective, of both visible and ultraviolet light. Researchers used it as an ingredient in paint that reflects 98.1% of
solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...
, allowing surfaces to which it has been applied to stay cooler in sunlit conditions. Commercially available white paints only reflect 80 - 90% of solar radiation. By using hexagonal nanoplatelet boron nitride, the thickness of a coat of this type of paint was reduced to 0.15 mm.


Paper brightener

A thin layer of barium sulfate called baryta is first coated on the base surface of most
photographic paper Photographic paper is a coated paper, paper coated with a light-sensitive chemical, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then Photographic developer, developed to form ...
to increase the reflectiveness of the image, with the first such paper introduced in 1884 in
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
.The Getty Conservation Institute, ''Silver Gelatin. The Atlas of Analytical Signatures of Photographic Processes''. J. Paul Getty Trust, 2013. The light-sensitive silver halide emulsion is then coated over the baryta layer. The baryta coating limits the penetration of the emulsion into the fibers of the paper and makes the emulsion more even, resulting in more uniform blacks. Further coatings may then be present for fixing and protection of the image. Baryta has also been used to brighten papers intended for ink-jet printing.


Plastics filler

Barium sulfate is commonly used as a filler for plastics to increase the density of the polymer in vibrational mass damping applications. In polypropylene and
polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It i ...
plastics, it is used as a filler in proportions up to 70%. It has an effect of increasing acid and alkali resistance and opacity. Such composites are also used as X-ray shielding materials due to their enhanced radio-opacity. In cases where machinability and weight are a concern, composites with high mass fraction (70–80%) of barium sulfate may be preferred to the more commonly used steel shields. Barium sulfate can also be used to enhance the material properties of HDPE, although typically in relatively low concentrations, and often in combination with other fillers like calcium carbonate or titanium oxide.


Niche uses

Barium sulfate is used in soil testing. Tests for soil pH and other qualities of soil use colored indicators, and small particles (usually clay) from the soil can cloud the test mixture and make it hard to see the color of the indicator. Barium sulfate added to the mixture binds with these particles, making them heavier so they fall to the bottom, leaving a clearer solution. In colorimetry, barium sulfate is used as a near-perfect diffuser when measuring light sources. In metal casting, the moulds used are often coated with barium sulfate in order to prevent the molten metal from bonding with the mould. It is also used in brake linings, anacoustic foams, powder coatings, and root canal filling. Barium sulfate is an ingredient in the "rubber" pellets used by Chilean police. This together with silica helps the pellet attain a 96.5 Shore A hardness.


Catalyst support

Barium sulfate is used as a
catalyst support In chemistry, a catalyst support or carrier is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the ma ...
when selectively hydrogenating functional groups that are sensitive to overreduction. With a low surface area, the contact time of the substrate with the catalyst is shorter and thus selectivity is achieved. Palladium on barium sulfate is also used as a catalyst in the Rosenmund reduction.


Pyrotechnics

As barium compounds emit a characteristic green light when heated at high temperature, barium salts are often used in green pyrotechnic formulas, although
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
and chlorate salts are more common. Barium sulfate is commonly used as a component of "strobe" pyrotechnic compositions.


Copper industry

As barium sulfate has a high
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
and is insoluble in water, it is used as a release material in casting of copper anode plates. The
anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...
plates are cast in copper molds, so to avoid the direct contact of the liquid copper with the solid copper mold, a suspension of fine barium sulfate powder in water is used as a coating on the mold surface. Thus, when the molten copper solidifies in form of an anode plate it can be easily released from its mold.


Radiometric measurements

Barium sulfate is sometimes used, besides
polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a corporate spin-of ...
(PTFE), to coat the interior of integrating spheres due to the high reflectance of the material and near Lambertian characteristics.


3D printing of firearms

Barium sulfate is listed among the materials acceptable to the
Bureau of Alcohol, Tobacco, Firearms and Explosives The Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE), commonly referred to as ATF, is a domestic law enforcement agency within the United States Department of Justice. Its responsibilities include the investigation and prevention ...
(BATFE) for the manufacturing of firearms and/or components that are made of plastic, to achieve compliance with the U.S. federal requirement that an X-ray machine must be able to accurately depict the shape of the plastic firearm or component.


Production

Almost all of the barium consumed commercially is obtained from barite, which is often highly impure. Barite is processed by thermo-chemical sulfate reduction (TSR), also known as carbothermal reduction (heating with coke) to give barium sulfide: : BaSO4 + 4 C → BaS + 4 CO In contrast to barium sulfate, barium sulfide is soluble in water and readily converted to the oxide, carbonate, and halides. To produce highly pure barium sulfate, the sulfide or chloride is treated with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
or sulfate salts: : BaS + H2SO4 → BaSO4 + H2S Barium sulfate produced in this way is often called , which is French for "permanent white". Blanc fixe is the form of barium encountered in consumer products, such as paints. In the laboratory barium sulfate is generated by combining solutions of barium ions and sulfate salts. Because barium sulfate is the least toxic salt of barium due to its insolubility, wastes containing barium salts are sometimes treated with
sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 mill ...
to immobilize (detoxify) the barium. Barium sulfate is one of the most insoluble salts of sulfate. Its low solubility is exploited in qualitative inorganic analysis as a test for Ba2+ ions, as well as for sulfate. Untreated raw materials such as natural baryte formed under hydrothermal conditions may contain many impurities, a.o.,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
, or even amorphous
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
.


History

Barium sulfate is reduced to barium sulfide by carbon. The accidental discovery of this conversion many centuries ago led to the discovery of the first synthetic
phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
. The sulfide, unlike the sulfate, is water-soluble. During the early part of the 20th century, during the Japanese colonization period, hokutolite was found to exist naturally in the Beitou hot-springs area near Taipei City, Taiwan. Hokutolite is a radioactive mineral composed mostly of PbSO4 and BaSO4, but also containing traces of uranium, thorium and radium. The Japanese harvested these elements for industrial uses, and also developed dozens of “ therapeutic hot-spring baths” in the area.


Safety aspects

Although soluble salts of barium are moderately toxic to humans, barium sulfate is nontoxic due to its insolubility. The most common means of inadvertent barium poisoning arises from the consumption of soluble barium salts mislabeled as BaSO4. In the Celobar incident (Brazil, 2003), nine patients died from improperly prepared radiocontrast agent. In regards to occupational exposures, the Occupational Safety and Health Administration set a permissible exposure limit at 15 mg/m3, while the National Institute for Occupational Safety and Health has a recommended exposure limit at 10 mg/m3. For respiratory exposures, both agencies have set an occupational exposure limit at 5 mg/m3.


See also

* Baryte * List of inorganic pigments


References

{{Authority control Barium compounds Sulfates Inorganic pigments Radiocontrast agents