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Lithium arsenate
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| Names | |
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| Other names
Trilithium arsenate
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| Identifiers | |
3D model (JSmol)
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| ECHA InfoCard | 100.033.416 |
| EC Number |
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PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |
| Li3AsO4 | |
| Molar mass | 159.75 g/mol |
| Appearance | Colorless crystalline solid |
| Sparingly soluble in water | |
| Structure | |
| Orthorhombic (low-temperature form) | |
| Pmn21 | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Lithium arsenate is an inorganic compound of lithium and the arsenate ion, with the formula Li3AsO4. It is a colorless crystalline solid that is only sparingly soluble in water. Several crystalline polymorphs are known, including low-temperature and high-temperature orthorhombic forms.
Preparation
[edit]Lithium arsenate can be prepared by precipitation from aqueous solutions containing lithium and arsenate ions:
- 3 Li+ + AsO3−4 → Li3AsO4↓
It can also be prepared by complete neutralization of arsenic acid with lithium hydroxide:
- 3 LiOH + H3AsO4 → Li3AsO4 + 3 H2O
Properties
[edit]Crystal structure and polymorphism
[edit]Lithium arsenate is polymorphic. The low-temperature form crystallizes in the orthorhombic crystal system, with lattice parameters approximately a = 6.27 Å, b = 5.38 Å, c = 4.95 Å.
This phase is closely related to the low-temperature forms of Li3PO4 and Li3VO4.[1]
A second orthorhombic modification occurs at elevated temperature, with approximate lattice parameters
- a = 6.28 Å
- b = 10.76 Å
- c = 5.05 Å.[1]
The low- and high-temperature structures are closely related. In both, arsenic occurs in isolated AsO4 tetrahedra and lithium is approximately tetrahedrally coordinated by oxygen.[1]
A later structural refinement assigned the low-temperature phase to space group Pmn21, with lattice parameters approximately a = 6.286 Å, b = 5.390 Å and c = 4.962 Å.[2]
High-temperature phase
[edit]At still higher temperature, Li3AsO4 forms a γ-phase related to γ-Li3PO4.[3]
Neutron-diffraction measurements at 770 and 850 °C showed that the γ-phase consists of a distorted hexagonal close-packed array of oxygen atoms in which half of the tetrahedral sites are occupied by lithium and arsenic.[3]
The lithium sublattice becomes substantially disordered at high temperature. One lithium site splits into central and off-centre positions within its tetrahedron, while another lithium species becomes distributed over pairs of face-sharing tetrahedral sites at 850 °C.[3]
The increased lithium disorder is characteristic of the high-temperature phase and is responsible for significant structural differences from the lower-temperature modifications.
Solubility
[edit]Lithium arsenate is only sparingly soluble in water, in contrast to the corresponding sodium and potassium arsenates.
It is soluble in acidic media such as aqueous acetic acid.
Safety
[edit]Lithium arsenate is toxic because it contains pentavalent arsenic. Soluble or bioavailable inorganic arsenic compounds can cause serious acute and chronic health effects.
References
[edit]- ^ a b c "Isomorphism and polymorphism of the compounds Li3PO4, Li3AsO4 and Li3VO4". Journal of Inorganic and Nuclear Chemistry. 29 (4): 915–923. 1967. doi:10.1016/0022-1902(67)80074-5.
- ^ "Polymorphism of Li3AsO4 and structure refinement of its low-temperature form". Phase Transitions. 1993.
- ^ a b c "Structure of γ-Li3AsO4 by High Temperature Powder Neutron Diffraction". Journal of Solid State Chemistry. 110 (2): 243–249. 1994. doi:10.1006/jssc.1994.1165.