Potassium heptafluorotantalate is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...

with the formula K₂ aF₇ It is the potassium salt of the heptafluorotantalate

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

aF₇sup>2−. This white, water-soluble solid is an intermediate in the purification of

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

from its ores and is the precursor to the metal.

Preparation

Industrial

Potassium heptafluorotantalate is an intermediate in the industrial production of metallic tantalum. Its production involves leaching tantalum ores, such as

columbite Columbite, also called niobite, niobite-tantalite and columbate [], is a black mineral group that is an ore of niobium. It has a submetallic Lustre (mineralogy), luster and a high density and is a niobate of iron and manganese. This mineral group w ...

and

tantalite The mineral group tantalite Fe,_manganese.html"_;"title="iron.html"_;"title="iron">Fe,_manganese">Mn)Tantalum">Ta2oxygen.html" ;"title="manganese">Mn)Tantalum.html" ;"title="iron">Fe,_manganese.html" ;"title="iron.html" ;"title="iron">Fe, mang ...

, with

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...

and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

to produce the water-soluble hydrogen pentafluorotantalate. : Ta₂O₅ + 14 HF → 2 H₂ aF₇ + 5 H₂O This solution is subjected to a number of liquid-liquid extraction steps to remove metallic impurities (most importantly

niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...

) before being treated with

potassium fluoride Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali halide and occurs naturally as the rare ...

to produce K₂ aF₇ : H₂ aF₇+ 2 KF → K₂ aF₇+ 2 HF

Lab-scale

Hydrofluoric acid is both corrosive and toxic, making it unappealing to work with; as such a number of alternative processes have been developed for small-scale syntheses. Potassium heptafluorotantalate can be produced by both

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achi ...

and wet methods. The anhydrous method involves the reaction of tantalum oxide with

potassium bifluoride Potassium bifluoride is the inorganic compound with the formula . This colourless salt consists of the potassium cation () and the bifluoride anion (). The salt is used as an etchant for glass. Sodium bifluoride is related and is also of commercial ...

ammonium bifluoride Ammonium hydrogen fluoride is the inorganic compound with the formula or . It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass- etchant and an intermediate in a once-contemplated route to hydrofluoric acid. Structu ...

according to the following equation: :Ta₂O₅ + 4 KHF₂ + 6 HF → 2 K₂ aF₇ + 5 H₂O The method was originally reported by Berzelius. K₂ aF₇can also be precipitated from solutions in hydrofluoric acid provided that the concentration of HF is below about 42%. Solutions having higher concentrations of HF yield potassium hexafluorotantalate TaF₆ The K-salt can be also precipitated from a solution in

of tantalum pentachloride: : 5 HF + 2 KF + TaCl₅ → K₂ aF₇ + 5 HCl

Structure

Potassium heptafluorotantalate exists in at least two polymorphs. α-K₂ aF₇is the most common form and crystallises in the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

''P''2₁/c

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...

. The structure is composed of aF₇sup>2− units interconnected by potassium ions. aF₇sup>2− polyhedra may be described as monocapped

trigonal prism In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...

s with the capping atom located on one of the rectangular faces. Potassium atoms are 9-coordinated and may be viewed as distorted monocapped

square prism In geometry, a cuboid is a hexahedron, a six-faced solid. Its faces are quadrilaterals. Cuboid means "like a cube", in the sense that by adjusting the length of the edges or the angles between edges and faces a cuboid can be transformed into a cub ...

s. At temperatures above 230 °C this converts to β-K₂ aF₇ which is

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...

(space group: ''P''nma). This structure also consists of potassium ions and the complex anion aF₇sup>2−. The structure of the 7-coordinate aF₇sup>2− units is essentially unchanged. However the potassium atoms now exist in 2 environments where they coordinate to either 11 or 8 fluorine atoms.

Reactions

K₂ aF₇is primarily used to produce metallic tantalum by reduction with

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

. This takes place at approximately 800 °C in

molten salt Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.Journal of Chemical T ...

and proceeds via a number of potential pathways. : K₂ aF₇+ 5 Na → Ta + 5 NaF + 2 KF K₂ aF₇is susceptible to hydrolysis. For example, a boiling aqueous solution of K₂ aF₇yields potassium oxyfluorotantalate (K₂Ta₂O₃F₆), known as “ Marignac’s salt”. In order to prevent hydrolysis and co-precipitation of potassium oxyfluorotantalate, a small excess of HF is added to the solution.

References

{{Tantalum compounds Fluoro complexes Tantalates Potassium compounds Fluorometallates