Potassium heptafluorotantalate: Difference between revisions

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 444634516

| ImageFile = K2TaF7.png

| ImageSize =240

| IUPACName = Dipotassium heptafluorotantalate

| SystematicName = Dipotassium heptafluorotantalum(2-)

| OtherNames = Potassium heptafluorotantalate(V)<br/>Potassium fluorotantalate

|Section1={{Chembox Identifiers

| CASNo_Ref = {{cascite|correct|??}}

| CASNo = 16924-00-8

| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}

| ChemSpiderID = 28541740

| EC_number = 240-986-1

| PubChem = 28146

| SMILES = F[Ta-2](F)(F)(F)(F)(F)F.[K+].[K+]

| InChI = 1S/7FH.2K.Ta/h7*1H;;;/q;;;;;;;2*+1;+5/p-7}}

|Section2={{Chembox Properties

| Formula = K₂[TaF₇]

| MolarMass = 392.13 g/mol

| Appearance = white solid

| Density = 4.56 g/mL at 25 °C

| MeltingPtC = 630 to 820

| MeltingPt_notes =

| BoilingPt =

| pKa =

| Solubility = 0.5 g/100 mL (15 °C)<ref name=Brauer>{{cite book|author=W. Kwasnik|chapter=Potassium Heptafluorotantalate (V)|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY, NY|volume=2pages=256}}</ref>

:{{chem2|Nb2O5 + 4 KHF2 + 6 HF -> 2K2NbF7 + 5 H2O}}.</ref>}}

|Section7={{Chembox Hazards

| ExternalSDS =

| MainHazards =

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| NFPA-F =

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| LD50 = 110 mg/kg (Oral: rat)

| PEL =

| GHSPictograms = {{GHS06}}{{GHS07}}

| GHSSignalWord = Danger

| HPhrases = {{H-phrases|301|315|319|331|335}}

| PPhrases = {{P-phrases|261|264|270|271|280|301+310|302+352|304+340|305+351+338|311|312|321|330|332+313|337+313|362|403+233|405|501}}

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'''Potassium heptafluorotantalate''' is an [[inorganic compound]] with the formula K₂[TaF₇]. It is the potassium salt of the heptafluorotantalate [[fluoroanion|anion]] [TaF₇]²⁻. This white, water-soluble solid is an intermediate in the purification of [[tantalum]] from its [[ore]]s and is the precursor to the metal.<ref name= Agulyanski/>

==Preparation==

===Industrial===

{{further|Tantalum#Production and fabrication}}

Potassium heptafluorotantalate is an intermediate in the industrial production of metallic tantalum. Its production involves [[Leaching (metallurgy)|leaching]] tantalum ores, such as [[columbite]] and [[tantalite]], with [[hydrofluoric acid]] and [[sulfuric acid]] to produce the water-soluble hydrogen heptafluorotantalate.<ref name= Agulyanski>{{cite book|author=Anthony Agulyanski|editor=Anatoly Agulyanski|chapter=Fluorine chemistry in the processing of tantalum and niobium|title=Chemistry of Tantalum and Niobium Fluoride Compounds|year=2004|publisher=Elsevier|location=Burlington|isbn=9780080529028|edition=1st}}</ref>

: Ta₂O₅ + 14 HF → 2 H₂[TaF₇] + 5 H₂O

This solution is subjected to a number of [[Liquid–liquid extraction|liquid-liquid extraction]] steps to remove metallic impurities (most importantly [[niobium]]) before being treated with [[potassium fluoride]] to produce K₂[TaF₇]

: H₂[TaF₇] + 2 [[Potassium fluoride|KF]] → K₂[TaF₇] + 2 [[Hydrogen fluoride|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]] and wet methods. The anhydrous method involves the reaction of tantalum oxide with [[potassium bifluoride]] or [[ammonium bifluoride]] according to the following equation:<ref name=Brauer/><ref>Agulyansky, A. "Potassium fluorotantalate in solid, dissolved and molten conditions" J. Fluorine Chemistry 2003, 155-161. {{doi|10.1016/S0022-1139(03)00190-8}}</ref>

:Ta₂O₅ + 4 KHF₂ + 6 HF → 2 K₂[TaF₇] + 5 H₂O

The method was originally reported by [[Berzelius]].<ref>J. J. Berzelius Pogg. Ann. 4, 6 (1825#.</ref>

K₂[TaF₇] can 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 [KTaF₆]. The K-salt can be also precipitated from a solution in [[hydrofluoric acid]] of [[tantalum pentachloride]]:

: 5 HF + 2 KF + TaCl₅ → K₂[TaF₇] + 5 HCl

Potassium heptafluorotantalate exists in at least two [[Polymorphism (materials science)|polymorphs]]. α-K₂[TaF₇] is the most common form and crystallises in the [[monoclinic]] ''P''2₁/c [[space group]]. The structure is composed of [TaF₇]²⁻ units interconnected by potassium ions. [TaF₇]²⁻ polyhedra may be described as monocapped [[trigonal prism]]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]]s. In terms of the coordination sphere of the heavy metal, '''potassium heptafluoroniobate''' is similar to the tantalum salt.<ref>{{cite journal|last=Torardi|first=C.C.|author2=Brixner, L.H. |author3=Blasse, G. |author-link3=George Blasse |title=Structure and luminescence of K₂TaF₇ and K₂NbF₇|journal=Journal of Solid State Chemistry|year=1987|volume=67|issue=1|pages=21–25|doi=

10.1016/0022-459687)90333-1}}</ref>

At temperatures above 230&nbsp;°C this converts to β-K₂[TaF₇], which is [[orthorhombic]] (space group: ''P''nma). This structure also consists of potassium ions and the [[coordination compound|complex anion]] [TaF₇]²⁻. The structure of the 7-coordinate [TaF₇]²⁻ units is essentially unchanged. However the potassium atoms now exist in 2 environments where they coordinate to either 11 or 8 fluorine atoms.<ref>Langer, V. Smrčok, L. Boča, M. "Dipotassium heptafluorotantalate#V#, β-K₂TaF₇, at 509K" Acta Crystallographica Section E 2006, E62, i91-i93. {{doi|10.1107/S1600536806009147}}</ref><ref>{{cite journal|last=Smrčok|first=Ľubomír|author2=Brunelli, Michela |author3=Boča, Miroslav |author4= Kucharík, Marian |title=Structure of K₂TaF₇ at 993 K: the combined use of synchrotron powder data and solid-state DFT calculations|journal=Journal of Applied Crystallography|date=8 April 2008|volume=41|issue=3|pages=634–636|doi=10.1107/S0021889808005876}}</ref>

==Reactions==

K₂[TaF₇] is primarily used to produce metallic tantalum by [[reduction-oxidation|reduction]] with [[sodium]]. This takes place at approximately 800&nbsp;°C in [[molten salt]] and proceeds via a number of potential pathways.<ref>{{cite journal|last=Okabe|first=Toru H.|author2=Sadoway, Donald R.|title=Metallothermic reduction as an electronically mediated reaction|journal=Journal of Materials Research|year=1998|volume=13|issue=12|pages=3372–3377|doi=10.1557/JMR.1998.0459|bibcode = 1998JMatR..13.3372O }}</ref>

: K₂[TaF₇] + 5 Na → Ta + 5 [[NaF]] + 2 [[Potassium fluoride|KF]]

K₂[TaF₇] is susceptible to hydrolysis. For example, a boiling aqueous solution of K₂[TaF₇] yields potassium oxyfluorotantalate (K₂Ta₂O₃F₆), known as “[[Jean Charles Galissard de Marignac|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==

{{Reflist}}

{{potassium compounds}}

{{fluorine compounds}}

{{Tantalum compounds}}

[[Category:Fluoro complexes]]

[[Category:Tantalates]]

[[Category:Potassium compounds]]

[[Category:Fluorometallates]]