Fluoride is the anion An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. An anion , from the Greek word ἀνω (anο), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively F⁻, the reduced Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the form of fluorine Fluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule. F2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegative of all the elements. For. Both organic An organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic. The distinction between "organic" and " and inorganic Traditionally, inorganic compounds are considered to be of a mineral, not biological origin. Complementarily, most organic compounds are traditionally viewed as being of biological origin. Over the past century, the precise classification of inorganic vs organic compounds has become less important to scientists, primarily because the majority of compounds containing the element A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, fluorine are sometimes called fluorides. Fluoride, like other halides A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides. All Group 1 metals form halides with the halogens and they are white solids, is a monovalent ion (−1 charge). Its compounds often have properties that are distinct relative to other halides. Structurally, and to some extent chemically, the fluoride ion resembles the hydroxide In chemistry, hydroxide is the name for the diatomic anion OH−, consisting of covalently bonded oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest and most pervasive diatomic ions known ion. Fluorine-containing compounds range from potent toxins such as sarin Sarin is an organophosphorus compound with the formula [2CHO]CH3P(O)F. It is a colorless, odorless liquid, which is used as a chemical weapon. It has been classified as a weapon of mass destruction in UN Resolution 687. Production and stockpiling of sarin was outlawed by the Chemical Weapons Convention of 1993 to life-saving pharmaceuticals such as efavirenz Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) and is used as part of highly active antiretroviral therapy (HAART) for the treatment of a human immunodeficiency virus (HIV) type 1, and from inert materials such as calcium fluoride Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride anions and each F− ion is surrounded by four Ca2+ to the highly reactive sulfur tetrafluoride. The range of fluorine-containing compounds is considerable as fluorine is capable of forming compounds with all the elements except helium and neon.^[1][2]

Compounds containing fluoride anions and in many cases those containing covalent bonds A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, and other covalent bonds. In short, the attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding to fluorine are called fluorides.

Occurrence

Solutions of inorganic fluorides in water contain F⁻ and bifluoride HF₂⁻.^[3] Few inorganic fluorides are soluble in water without undergoing significant hydrolysis. Examples of inorganic fluorides include hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and difficult to handle, it is technically a weak acid. Hydrogen fluoride, often in the aqueous form as hydrofluoric acid, is a valued source of fluorine, being the precursor to numerous pharmaceuticals such as fluoxetine (Prozac), diverse polymers such (HF), sodium fluoride Sodium fluoride is the inorganic compound with the formula NaF. This colorless solid is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride (NaF), and uranium hexafluoride Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure (STP), is highly toxic, reacts violently with water and is corrosive to most metals. It (UF₆). In terms of its reactivity, fluoride differs significantly from chloride The chloride ion is formed when the element chlorine picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides and other halides, and is more strongly solvated due to its smaller radius/charge ratio. Its closest chemical relative is hydroxide In chemistry, hydroxide is the name for the diatomic anion OH−, consisting of covalently bonded oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest and most pervasive diatomic ions known. The Si-F linkage is one of the strongest single bonds. In contrast, other silyl halides are easily hydrolyzed.

Natural occurrence

Many fluoride minerals are known, but paramount in commercial importance are fluorite Fluorite is a halide mineral composed of calcium fluoride, Ca and fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca53F (calcium halophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue, violet, or colorless), the pure mineral is colorless as expected for a material lacking transition metals. It is an. Fluoride is found naturally in low concentration in drinking water and foods. Water with underground sources is more likely to have higher levels of fluoride, whereas the concentration in seawater averages 1.3 parts per million Parts-per notation is used, especially in science and engineering, to denote relative proportions in measured quantities; particularly in low-value proportions at the parts-per-million (ppm) 10–6, parts-per-billion (ppb) 10–9, and parts-per-trillion (ppt) 10–12 level. Since parts-per notations are quantity-per-quantity measures, they are (ppm).^[4] Fresh water supplies generally contain between 0.01–0.3 ppm, while the ocean contains between 1.2 and 1.5 ppm.^[5]

Applications

Fluorides are pervasive in modern technology. Hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and difficult to handle, it is technically a weak acid. Hydrogen fluoride, often in the aqueous form as hydrofluoric acid, is a valued source of fluorine, being the precursor to numerous pharmaceuticals such as fluoxetine (Prozac), diverse polymers such is the most important fluoride synthesized. It is principally used in the production of fluorocarbons Fluorocarbons, sometimes referred to as perfluorocarbons, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes, that contain only single bonds, are more chemically and thermally stable than alkanes. However, fluorocarbons with double bonds and especially triple bonds ( and aluminium fluorides. Hydrofluoric acid has a variety of specialized applications, including its ability to dissolve glass.^[6]

Organic synthesis

Fluoride reagents are significant in synthetic Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has developed into one of the most important branches of organic chemistry Organic chemistry is a discipline within chemistry that involves the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives. These compounds may contain any number of other elements, including hydrogen, nitrogen, oxygen, the halogens as well as. Due to the affinity of silicon Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. It is the eighth most common element in the universe by mass, but silicon very rarely occurs as the pure free element in nature. Silicon is more widely for fluoride, and the ability of silicon to expand its coordination number, silyl ether protecting groups A protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis can be easily removed by the fluoride sources such as sodium fluoride Sodium fluoride is the inorganic compound with the formula NaF. This colorless solid is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride and tetra-n-butylammonium fluoride (TBAF).

Enzyme inhibitors

In biochemistry Biochemistry is the study of the chemical processes in living organisms. It deals with the structures and functions of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules. Over the last 40 years biochemistry has become so successful at explaining living processes that now almost all areas of the life, fluoride salts are commonly used to inhibit Enzyme inhibitors are molecules that bind to enzymes and decrease their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides. Not all molecules that bind to enzymes are inhibitors; enzyme activators bind to enzymes and the activity of phosphatases A phosphatase is an enzyme that removes a phosphate group from its substrate by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group . This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their substrates by using energetic molecules like ATP. A, such as serine Serine is an organic compound with the formula H /threonine Threonine is an α-amino acid with the chemical formula HO2CCH(NH2)CH(OH)CH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar. Together with serine and tyrosine, threonine is one of three proteinogenic amino acids bearing an alcohol group phosphatases.^[7] It may do this by replacing the nucleophilic hydroxyl ion in these enzymes' active sites.^[8] Beryllium fluoride and aluminium fluoride are also used as phosphatase inhibitors, since these compounds are structural mimics of the phosphate A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry. At elevated temperatures group and can act as analogues of the transition state The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always go on to form products of the reaction.^[9][10]

Inorganic fluorides

Sulfur hexafluoride Sulfur hexafluoride is an inorganic, colorless, odorless, non-toxic and non-flammable gas (under standard conditions). SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in nonpolar organic solvents is an inert, nontoxic insulator that is used in electrical transformers. Uranium hexafluoride Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure (STP), is highly toxic, reacts violently with water and is corrosive to most metals. It is used in the separation of isotopes of uranium between the fissile In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons (in the form of gamma rays), as well. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation isotope U-235 Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238 it is fissile, i.e. it can sustain fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature and the non-fissile isotope U-238 in preparation of nuclear reactor A nuclear reactor is a device to initiate, control, and sustain a nuclear chain reaction. The most common use of nuclear reactors is for the generation of electrical power and for the power in some ships (see Nuclear marine propulsion). This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam fuel Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. Nuclear fuels are the most dense sources of energy available to humans. Nuclear fuel in a nuclear fuel cycle can refer to the material or to physical objects composed of the fuel material, perhaps mixed with and atomic bombs A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter; a modern thermonuclear weapon weighing little more than a thousand kilograms can produce an explosion. The volatility of fluorides of uranium and other elements may also be used for nuclear fuel reprocessing.

Fluoropolymers

Fluoropolymers such as polytetrafluoroethylene, Teflon In chemistry, polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon, are used as chemically inert and biocompatible Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used, or to more empirical clinical success of a whole device in which the material or materials feature. The ambiguity of the term reflects the ongoing development materials for a variety of applications, including as surgical implants In medicine, a prosthesis is an artificial extension that replaces a missing body part. It is part of the field of biomechatronics, the science of using mechanical devices with human muscle, skeleton, and nervous systems to assist or enhance motor control lost by trauma, disease, or defect. Prostheses are typically used to replace parts lost by such as coronary bypass grafts,^[11] and a replacement for soft tissue In medicine, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, not being bone in cosmetic Plastic surgery is a medical specialty concerned with the correction or restoration of form and function. While famous for aesthetic surgery, plastic surgery also includes many types of reconstructive surgery, hand surgery, microsurgery, and the treatment of burns. The word "plastic" derives from the Greek plastikos meaning to mould or and reconstructive surgery Although plastic surgery and plastic surgeons are involved in many aspects of reconstructive surgery, there are other branches of surgery that also perform reconstructive procedures. For example, orthopedic surgeons reconstruct the hip and other joints, repair tendons such as those in the rotator cuff and perform osteotomies to straighten.^[12] These compounds are also commonly used as non-stick surfaces in cookware and bakeware, and the fluoropolymer fabric Gore-Tex used in breathable garments for outdoor use.

Cavity prevention

Fluoride-containing compounds are used in topical and systemic fluoride therapy for preventing tooth decay. They are used for water fluoridation and in many products associated with oral hygiene.^[13] Originally, sodium fluoride was used to fluoridate water; however, hexafluorosilicic acid (H₂SiF₆) and its salt sodium hexafluorosilicate (Na₂SiF₆) are more commonly used additives, especially in the United States. The fluoridation of water is known to prevent tooth decay^[14][15] and is considered by the U.S. Centers for Disease Control and Prevention as "one of 10 great public health achievements of the 20th century".^[16][17] In some countries where large, centralized water systems are uncommon, fluoride is delivered to the populace by fluoridating table salt. Fluoridation of water is not without critics, however (see Opposition to water fluoridation).^[18]

Biomedical applications

Positron emission tomography is commonly carried out using fluoride-containing pharmaceuticals such as fluorodeoxyglucose, which is labelled with the radioactive isotope fluorine-18 that emits positrons when it decays into ¹⁸O.

Numerous drugs contain fluorine including antipsychotics such as fluphenazine, HIV protease inhibitors such as tipranavir, antibiotics such as ofloxacin and trovafloxacin, and anesthetics such as halothane.^[19] Fluorine is incorporated in the drug structures to reduce drug metabolism, as the strong C-F bond resists deactivation in the liver by cytochrome P450 oxidases.^[20]

Toxicology

Fluoride-containing compounds are so diverse that it is not possible to generalize on their toxicity, which depends on their reactivity and structure, and in the case of salts, their solubility and ability to release fluoride ions.

Soluble fluoride salts, of which NaF is the most common, are mildly toxic but have resulted in both accidental and suicidal deaths from acute poisoning.^[6] While the minimum fatal dose in humans is not known, a case of a fatal poisoning of an adult with 4 grams of NaF is documented.^[21] For Sodium fluorosilicate (Na₂SiF₆), the 50% lethal dose (LD50) orally in rats is 0.125 g/kg, corresponding to 12.5 g for a 100 kg adult.^[22] The fatal period ranges from 5 min to 12 hours.^[21] The mechanism of toxicity involves the combination of the fluoride anion with the calcium ions in the blood to form insoluble calcium fluoride, resulting in hypocalcemia; calcium is indispensable for the function of the nervous system, and the condition can be fatal. Treatment may involve oral administration of dilute calcium hydroxide or calcium chloride to prevent further absorption, and injection of calcium gluconate to increase the calcium levels in the blood.^[21] Hydrogen fluoride is more dangerous than salts such as NaF because it is corrosive and volatile, and can result in fatal exposure through inhalation or upon contact with the skin; calcium gluconate gel is the usual antidote.^[23]

In the higher doses used to treat osteoporosis, sodium fluoride can cause pain in the legs and incomplete stress fractures when the doses are too high; it also irritates the stomach, sometimes so severely as to cause ulcers. Slow-release and enteric-coated versions of sodium fluoride do not have gastric side effects in any significant way, and have milder and less frequent complications in the bones.^[24] In the lower doses used for water fluoridation, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development; this is mostly mild and is unlikely to represent any real effect on aesthetic appearance or on public health.^[25]

See also

• Dental fluorosis
• Fluoride therapy
• Fluoride deficiency
• Fluoride poisoning
• Water fluoridation
• Halide
• PTFE
• Total Ionic Strength Adjustment Buffer

References

1. ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0080379419 p. 804
2. ^ Khriachtchev, Leonid; Mika Pettersson, Nino Runeberg, Jan Lundell & Markku Räsänen (24 August 2000). "A stable argon compound". Nature 406 (6798): 874–876. doi:10.1038/35022551. PMID 10972285. http://www.nature.com/nature/journal/v406/n6798/abs/406874a0.html.
3. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
4. ^ Fluoride in Drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality. World Health Organization, 2004, page 2. Page accessed on February 22, 2007.
5. ^ Environmental Health Criteria 227: Fluorides. World Health Organization, 2002, page 38. Page accessed on February 22, 2007.
6. ^ ^{a b} Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, Renée; Cuer, Jean Pierre (2005). "Fluorine Compounds, Inorganic title = Ullmann’s Encyclopedia of Industrial Chemistry". Wiley-VCH, Weinheim. p. 307. doi:10.1002/14356007.a11.
7. ^ Nakai C, Thomas JA (1974). "Properties of a phosphoprotein phosphatase from bovine heart with activity on glycogen synthase, phosphorylase, and histone". J. Biol. Chem. 249 (20): 6459–67. PMID 4370977. http://www.jbc.org/cgi/pmidlookup?view=long&pmid=4370977.
8. ^ Schenk G, Elliott TW, Leung E, et al. (2008). "Crystal structures of a purple acid phosphatase, representing different steps of this enzyme's catalytic cycle". BMC Struct. Biol. 8 (1): 6. doi:10.1186/1472-6807-8-6. PMID 18234116. PMC 2267794. http://www.biomedcentral.com/1472-6807/8/6.
9. ^ Wang W, Cho HS, Kim R, et al. (2002). "Structural characterization of the reaction pathway in phosphoserine phosphatase: crystallographic "snapshots" of intermediate states". J. Mol. Biol. 319 (2): 421–31. doi:10.1016/S0022-2836(02)00324-8. PMID 12051918. http://linkinghub.elsevier.com/retrieve/pii/S0022-2836(02)00324-8.
10. ^ Cho H, Wang W, Kim R, et al. (2001). "BeF(3)(-) acts as a phosphate analog in proteins phosphorylated on aspartate: structure of a BeF(3)(-) complex with phosphoserine phosphatase". Proc. Natl. Acad. Sci. U.S.A. 98 (15): 8525–30. doi:10.1073/pnas.131213698. PMID 11438683. PMC 37469. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=11438683.
11. ^ Kannan RY, Salacinski HJ, Butler PE, Hamilton G, Seifalian AM (2005). "Current status of prosthetic bypass grafts: a review". J. Biomed. Mater. Res. Part B Appl. Biomater. 74 (1): 570–81. doi:10.1002/jbm.b.30247. PMID 15889440.
12. ^ Singh S., Baker J. L. (2000). "Use of expanded polytetrafluoroethylene in aesthetic surgery of the face". Clin Plast Surg 27 (4): 579–93. PMID 11039891.
13. ^ McDonagh M. S., Whiting P. F., Wilson P. M., Sutton A. J., Chestnutt I., Cooper J., Misso K., Bradley M., Treasure E., & Kleijnen J. (2000). "Systematic review of water fluoridation". British Medical Journal 321 (7265): 855–859. doi:10.1136/bmj.321.7265.855. PMID 11021861.
14. ^ Griffin SO, Regnier E, Griffin PM, Huntley V (2007). "Effectiveness of fluoride in preventing caries in adults". J. Dent. Res. 86 (5): 410–5. doi:10.1177/154405910708600504. PMID 17452559.
15. ^ Winston A. E., Bhaskar S. N. (1 November 1998). "Caries prevention in the 21st century". J. Am. Dent. Assoc. 129 (11): 1579–87. PMID 9818575. http://jada.ada.org/cgi/pmidlookup?view=long&pmid=9818575.
16. ^ Community Water Fluoridation - Oral Health
17. ^ http://www.cdc.gov/about/history/tengpha.htm
18. ^ Newbrun E (1996). "The fluoridation war: a scientific dispute or a religious argument?". J. Public Health Dent. 56 (5 Spec No): 246–52. doi:10.1111/j.1752-7325.1996.tb02447.x. PMID 9034969.
19. ^ Park BK, Kitteringham NR, O'Neill PM (2001). "Metabolism of fluorine-containing drugs". Annu. Rev. Pharmacol. Toxicol. 41: 443–70. doi:10.1146/annurev.pharmtox.41.1.443. PMID 11264465.
20. ^ Fisher MB, Henne KR, Boer J (2006). "The complexities inherent in attempts to decrease drug clearance by blocking sites of CYP-mediated metabolism". Curr. Opin. Drug Discov. Devel. 9 (1): 101–9. PMID 16445122.
21. ^ ^{a b c} I. M. Rabinowitch. Acute Fluoride Poisoning. Can Med Assoc J. 1945, 52, 345–349. [1]
22. ^ The Merck Index, 12th edition, Merck & Co., Inc., 1996
23. ^ Muriale L, Lee E, Genovese J, Trend S. Fatality due to acute fluoride poisoning following dermal contact with hydrofluoric acid in a palynology laboratory. Ann Occup Hyg. 1996 40, 705–710. PMID 8958774.
24. ^ Murray TM, Ste-Marie LG. Prevention and management of osteoporosis: consensus statements from the Scientific Advisory Board of the Osteoporosis Society of Canada. 7. Fluoride therapy for osteoporosis. CMAJ. 1996;155(7):949–54. PMID 8837545.
25. ^ National Health and Medical Research Council (Australia). A systematic review of the efficacy and safety of fluoridation [PDF]. 2007. ISBN 1864964154. Summary: Yeung CA. A systematic review of the efficacy and safety of fluoridation. Evid Based Dent. 2008;9(2):39–43. doi:10.1038/sj.ebd.6400578. PMID 18584000. Lay summary: NHMRC, 2007.

External links

• Fluoride in groundwater worldwide - IGRAC International Groundwater Resources Assessment Centre

Categories: Fluorides | Anions

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