Potassium heptafluorotantalate

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Potassium heptafluorotantalate
Names
IUPAC name
Dipotassium heptafluorotantalate
Systematic IUPAC name
Dipotassium heptafluorotantalum(2-)
Other names
Potassium heptafluorotantalate(V)
Potassium fluorotantalate
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.037.245 Edit this at Wikidata
EC Number
  • 240-986-1
  • InChI=1S/7FH.2K.Ta/h7*1H;;;/q;;;;;;;2*+1;+5/p-7
  • F[Ta-2](F)(F)(F)(F)(F)F.[K+].[K+]
Properties
K2[TaF7]
Molar mass 392.13 g/mol
Appearance white solid
Density 4.56 g/mL at 25 °C
Melting point 630 to 820 °C (1,166 to 1,508 °F; 903 to 1,093 K)
0.5 g/100 mL (15 °C)[1]
Hazards
GHS labelling:
GHS06: ToxicGHS07: Exclamation mark
Danger
H301, H315, H319, H331, H335
P261, P264, P270, P271, P280, P301+P310, P302+P352, P304+P340, P305+P351+P338, P311, P312, P321, P330, P332+P313, P337+P313, P362, P403+P233, P405, P501
Lethal dose or concentration (LD, LC):
110 mg/kg (Oral: rat)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Potassium heptafluorotantalate is an inorganic compound with the formula K2[TaF7]. It is the potassium salt of the heptafluorotantalate anion [TaF7]2−. This white, water-soluble solid is an intermediate in the purification of tantalum from its ores and is the precursor to the metal.[2]

Preparation

Industrial

Further information: Tantalum § Production and fabrication

Potassium heptafluorotantalate is an intermediate in the industrial production of metallic tantalum. Its production involves leaching tantalum ores, such as columbite and tantalite, with hydrofluoric acid and sulfuric acid to produce the water-soluble hydrogen heptafluorotantalate.[2]

Ta2O5 + 14 HF → 2 H2[TaF7] + 5 H2O

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

H2[TaF7] + 2 KF → K2[TaF7] + 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 and wet methods. The anhydrous method involves the reaction of tantalum oxide with potassium bifluoride or ammonium bifluoride according to the following equation:[1][3]

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

The method was originally reported by

Berzelius.[4]

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

tantalum pentachloride
:

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

Structure

Potassium heptafluorotantalate exists in at least two

square prisms
.

At temperatures above 230 °C this converts to β-K2[TaF7], which is

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.[6][7]

Reactions

K2[TaF7] is primarily used to produce metallic tantalum by

reduction with sodium. This takes place at approximately 800 °C in molten salt and proceeds via a number of potential pathways.[8]

K2[TaF7] + 5 Na → Ta + 5
NaF + 2 KF

K2[TaF7] is susceptible to hydrolysis. For example, a boiling aqueous solution of K2[TaF7] yields 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

  1. ^ a b Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 256.
  2. ^
    ISBN 9780080529028
    .
  3. doi:10.1016/S0022-1139(03)00190-8
  4. ^ J. J. Berzelius Pogg. Ann. 4, 6 (1825#.
  5. doi:10.1016/0022-4596#87)90333-1
    .
  6. doi:10.1107/S1600536806009147
  7. doi:10.1107/S0021889808005876
    .
  8. doi:10.1557/JMR.1998.0459
    .
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