Ammonium arsenate

Ammonium arsenate is an inorganic compound with the formula ((NH₄)₃AsO₄, typically encountered as a trihydrate, (NH₄)₃AsO₄·3H₂O. It is a colorless, water-soluble crystalline solid that decomposes upon heating, releasing ammonia and forming arsenic-containing residues.

Ammonium arsenate

Names
Other names Ammonium orthoarsenate
Identifiers
CAS Number	24719-13-9 Y
3D model (JSmol)	Interactive image
ChemSpider	11663170 Y
ECHA InfoCard	100.029.152
EC Number	246-428-3
PubChem CID	18762179
CompTox Dashboard (EPA)	DTXSID0036895
InChI InChI=1S/AsH3O4.3H3N/c2-1(3,4)5;;;/h(H3,2,3,4,5);3*1H3 Y Key: SNSGFXVMSAYXTC-UHFFFAOYSA-N Y InChI=1/AsH3O4.2H3N/c2-1(3,4)5;;/h(H3,2,3,4,5);2*1H3 Key: XPVHUBFHKQQSDA-UHFFFAOYAT
SMILES [NH4+].[NH4+].[NH4+].[O-][As](=O)([O-])[O-]
Properties
Chemical formula	(NH4)3AsO4 . 3 H2O
Molar mass	247.1 (trihydrate)
Solubility in water	Soluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Classified as an IARC Group 1 carcinogen, it is highly toxic and poses significant health and environmental risks.^[1][2]

Historically used in pesticides and analytical chemistry, its applications are now limited due to toxicity concerns. Ammonium arsenate occurs rarely in nature and is primarily synthesized for research or industrial purposes. Its chemistry, environmental behavior, and analytical detection are of interest in toxicology, environmental chemistry, and biogeochemistry.^[3]

It is prepared by treating a concentrated solution of arsenic acid with ammonia, resulting in precipitation of colorless crystals of the trihydrate.^[4] Upon heating, it releases ammonia.

Acid salts are also known, including diammonium arsenate and ammonium dihydrogen arsenate.

Chemistry and structure

Ammonium arsenate consists of ammonium cations (NH₄⁺) and the arsenate anion (AsO₄^3-), where arsenic is in the +5 oxidation state, tetrahedrally coordinated to four oxygen atoms. The trihydrate form, (NH₄)₃AsO₄·3H₂O, is the most common, featuring a crystal lattice stabilized by hydrogen bonding between water molecules, ammonium ions, and arsenate tetrahedra. X-ray diffraction (XRD) studies indicate an orthorhombic crystal system, with structural similarities to ammonium phosphate due to the analogous tetrahedral geometry of AsO₄^3- and PO₄^3-. Acid salts, such as diammonium arsenate ((NH₄)₂HAsO₄) and ammonium dihydrogen arsenate (NH₄H₂AsO₄), form under specific conditions and exhibit greater acidity. Aqueous solutions of ammonium arsenate are mildly acidic (pH ~4–6) and can neutralize bases with moderate exothermic release. The compound is highly soluble in water but decomposes in hot water, yielding arsenious oxides and ammonia gas.^[3][2][5]

Synthesis

Ammonium arsenate is synthesized by reacting concentrated arsenic acid (H₃AsO₄) with ammonia (NH₃) in aqueous solution, resulting in the precipitation of colorless trihydrate crystals.

The reaction proceeds at room temperature to prevent decomposition:

H₃AsO₄ + 3NH₃ → (NH₄)₃AsO₄ + 3H₂O

Acid salts, such as diammonium arsenate, are produced by adjusting the ammonia-to-arsenic acid ratio (e.g., 2:1). Purification involves recrystallization, with composition verified by inductively coupled plasma mass spectrometry (ICP-MS) or Fourier-transform infrared spectroscopy (FTIR), which detect As–O and N–H vibrational modes. Industrial synthesis is uncommon due to toxicity, but laboratory preparation supports studies in arsenic speciation and toxicology. Heating above 100 °C releases ammonia, forming ammonium hydrogen arsenate or arsenic oxides, necessitating precautions to avoid toxic fumes.^[2][5][6]

Analysis

Quantifying ammonium arsenate in environmental and biological samples demands precise techniques due to its low concentrations and matrix interferences. Ion exchange chromatography coupled with inductively coupled plasma mass spectrometry (LC-ICP-MS) is the gold standard for speciating arsenate and arsenite, achieving detection limits in the parts-per-billion range. X-ray absorption spectroscopy (XAS) determines arsenic oxidation states (As(V) vs. As(III)) in solids or solutions. Fourier-transform infrared spectroscopy (FTIR) detects As–O and N–H bonds, confirming the compound's presence. Environmental samples are extracted using hydroxylammonium hydrochloride or ammonium oxalate to preserve speciation, followed by high-performance liquid chromatography (HPLC) with hydride generation-atomic fluorescence spectroscopy (HG-AFS). Radioisotope assays with [⁷³As]arsenate track microbial transformations in soils, aiding biogeochemical studies. These methods are essential for monitoring contamination and assessing exposure risks.^[5]

Recent studies have developed portable whole-cell biosensors (WCBs) with ArsR-regulated luciferase genes, detecting arsenate in soils at 10–50 μg/L.^[7][8]

Environmental features

Geologic rarity

Ammonium arsenate is extremely rare in nature, as ammonium and arsenate ions are typically found in distinct geochemical settings. Trace occurrences have been documented in arsenic-rich environments, such as acid mine drainage (AMD) sites or geothermal springs, where microbial activity or industrial runoff introduces ammonia and arsenate. For instance, in AMD from gold mines in Dushan, Guizhou, China, transient ammonium arsenate-like phases form in sulfate-rich waters with elevated ammonium from fertilizer runoff. These are minor compared to arsenate minerals like scorodite (FeAsO₄·2H₂O) or tooeleite (Fe₆(AsO₃)₄SO₄(OH)₄·4H₂O). The compound's instability under typical environmental pH and redox conditions favors arsenite or sulfide phases, limiting its natural prevalence.^[7][8]

Nevertheless, ammonium arsenate may form biogenically. A 2023 study on rice paddies in Guizhou, China, identified anaerobic bacterial metabolism that coupled ammonium oxidation to arsenate reduction (As-ammox).^[7][8]

Industrial applications

In the early 20th century, ammonium arsenate was used as a pesticide and herbicide in agriculture, exploiting arsenic's toxicity to control insects and weeds. Its use was discontinued in most countries by the 1980s due to health risks and the availability of safer alternatives. In analytical chemistry, it serves as a standard for arsenate ion speciation, particularly in ion chromatography coupled with ICP-MS to differentiate arsenate (As(V)) from arsenite (As(III)). It is also employed in crystallographic studies as a growth agent for arsenate-containing crystals, leveraging its structural analogy to phosphates. Limited modern applications include its use as a precursor in synthesizing other arsenic compounds for toxicological or environmental research. Its handling is strictly regulated due to its carcinogenic classification.^[5][9][10]

Health impacts

Ammonium arsenate's high solubility and toxicity pose significant environmental and health risks. It can contaminate waterways through runoff from historical pesticide applications or industrial spills, releasing arsenate ions that persist in sediments or groundwater. A 2021 study found that ammonium-rich waters enhance arsenic mobility by stabilizing arsenate, increasing its bioavailability in agricultural soils, such as rice paddies in Guizhou, China. Arsenate mimics phosphate in biochemical pathways, inhibiting enzymes like pyruvate dehydrogenase, causing cellular damage. As an IARC Group 1 carcinogen, chronic exposure via inhalation, ingestion, or dermal contact is associated with skin, lung, and bladder cancers. Acute exposure leads to severe gastrointestinal, neurological, and cardiovascular effects, with potential lethality. Decomposition products, such as arsenic oxides, further exacerbate environmental contamination. Strict containment is required to prevent exposure.^{[3][7][8][11]}