Tripotassium arsenate, is an inorganic compound with the formula . It is the normal potassium salt of arsenic acid and consists of potassium ions and isolated arsenate, , tetrahedra. It is a colorless, water-soluble crystalline solid.
Preparation Tripotassium arsenate can be prepared by complete neutralization of arsenic acid with potassium hydroxide:
It can also be obtained by reacting arsenic acid with excess potassium carbonate.
Anhydrous has additionally been synthesized by oxidation of elemental arsenic with potassium superoxide under high-temperature solid-state conditions.
Tripotassium arsenate can also form during oxidative alkaline treatment of arsenic-containing minerals such as arsenopyrite. In the presence of potassium carbonate and oxygen, arsenic may be converted into while iron and sulfur are oxidized to iron oxide and sulfur dioxide.
Properties Crystal structure Tripotassium arsenate crystallizes in the orthorhombic crystal system, space group Cccm (No. 66).
Single-crystal diffraction gives lattice parameters a = 10.6011 Å, b = 11.3521 Å, c = 16.9401 Å with sixteen formula units per unit cell.
The structure contains isolated tetrahedra, with As–O distances of approximately 1.67–1.71 Å. These tetrahedra and the potassium ions form a nearly cubic close-packed arrangement.
The calculated crystallographic density is approximately 3.34 g/cm3.
Reactions Addition of silver nitrate to an aqueous solution of potassium arsenate precipitates reddish-brown silver arsenate:
This precipitation reaction was historically used as a qualitative test for arsenate ions.
Under acidic conditions arsenate can be reduced to arsenite by iodide, with liberation of iodine. The underlying redox process can be represented ionically as:
Acid salts Partial neutralization of arsenic acid gives acidic potassium arsenates such as dipotassium arsenate, , and monopotassium arsenate, . These are distinct compounds from the fully neutralized .
Safety Tripotassium arsenate is toxic because it contains pentavalent arsenic. Exposure to soluble inorganic arsenic compounds can cause serious acute and chronic health effects.
See also Arsenic acid Monopotassium arsenate Dipotassium arsenate Silver arsenate Arsenate Arsenite
References