Aluminum Phosphide (20859-73-8) Physical and Chemical Properties
Aluminum Phosphide
Inorganic aluminium phosphide used as a moisture‑reactive fumigant and rodenticide and as a semiconductor intermediate; requires moisture‑controlled storage and specification‑driven procurement.
| CAS Number | 20859-73-8 |
| Family | Metal phosphides |
| Typical Form | Powder or crystalline solid (moisture-sensitive) |
| Common Grades | BP, EP |
Aluminum phosphide is an inorganic binary metal phosphide belonging to the class of metal–pnictogen salts. In the solid state it adopts a cubic zinc blende (sphalerite-type) lattice and is commonly encountered as dark gray to yellow crystalline material. Bonding in the bulk is best described as largely ionic with formal charges approximating \(\mathrm{Al^{3+}}\) and \(\mathrm{P^{3-}}\) within a covalently distorted lattice; electronically it is a wide band-gap semiconductor in which electron transport and optical properties are determined by the Al–P framework and intrinsic defects.
Chemically the substance is highly moisture-sensitive: on contact with water or atmospheric humidity it hydrolyses to yield phosphine (\(\mathrm{PH_3}\)), a flammable and highly toxic gas which may ignite spontaneously in air. This hydrolytic pathway dominates its environmental and toxicological behaviour; dry material is relatively stable and of low vapour pressure, but exposure to acids, steam or humid air produces rapid and hazardous gas evolution. As an inorganic, ionic solid it is essentially non‑lipophilic and insoluble as a molecular species — risks are driven by gas formation rather than solute partitioning.
The principal industrial uses are as a fumigant, rodenticide and insecticide in stored-product protection and as a source of phosphine in certain semiconductor processes; the material’s efficacy and hazards are both a consequence of the same hydrolysis/PH3 chemistry. Common commercial grades reported for this substance include: BP, EP.
Basic Physical Properties
Density
- Reported values from experimental sources include: \(2.85\) (specific gravity) at \(59\,^\circ\mathrm{F}\) — denser than water; will sink.
- Reported: \(2.40\,\mathrm{g}\,\text{cm}^{-3}\) at \(25\,^\circ\mathrm{C}\).
- Relative density (water = 1): \(2.9\).
These values are consistent with a dense crystalline inorganic solid (zinc blende type). Bulk density and apparent density may vary with particle size, porosity and formulation (pellet/tablet vs. crystalline powder).
Melting or Decomposition Point
- “Does not melt or decompose at temperatures up to \(1832\,^\circ\mathrm{F}\).”
- Reported high temperature values: \(2,550\,^\circ\mathrm{C}\) and another high‑temperature value near \(2{,}500\,^\circ\mathrm{C}\) (reported sources vary).
Aluminum phosphide is thermally robust as a condensed phase up to very high temperatures; decomposition at elevated temperature produces phosphorus oxides and other toxic fumes. In practical handling contexts, thermal behaviour is dominated by the lower‑temperature hydrolysis pathway rather than bulk melting.
Solubility in Water
- Solubility description: reacts with water (hydrolyses).
- “Though stable when dry, it reacts with moist air, and violently with acids.”
- Solubility in water: reaction (i.e., no simple dissolution; hydrolytic decomposition).
Hydrolysis proceeds to give aluminium hydroxide (or related Al(III) species) and phosphine gas. A convenient stoichiometric representation is: \[ \mathrm{AlP} + 3\,\mathrm{H_2O} \rightarrow \mathrm{Al(OH)_3} + \mathrm{PH_3} \] The hazardous gas evolution makes direct contact with water or moisture the primary incompatibility.
Solution pH (Qualitative Behavior)
No experimentally established bulk solution pH value is available because aluminum phosphide does not dissolve as a neutral molecular solute; hydrolysis yields phosphine gas and aluminium hydroxide. Hydrolysis in neutral or basic aqueous media tends to produce colloidal/basic aluminium hydroxides, while reaction with acids is vigorous and accelerates phosphine release. Therefore pH is not a meaningful single numeric property for an aqueous “solution” of AlP.
Chemical Properties
Acid–Base Behavior
Aluminum phosphide does not display classic acid–base dissociation as a molecular solute; its chemical reactivity with protic media controls behaviour. In contact with acids (including gastric acid) AlP hydrolyses vigorously, liberating phosphine (\(\mathrm{PH_3}\)) and producing aluminium(III) species (e.g., \(\mathrm{Al^{3+}}\) as hydrated ions or \(\mathrm{Al(OH)_3}\) precipitate). The material is therefore effectively converted by protic attack into a gaseous reducing agent (phosphine) and an Lewis‑acidic aluminium residue. Reaction with strong acids can be violent and may be accompanied by explosive release of gas.
Reactivity and Stability
- Moisture-sensitive: decomposes on contact with moist air, water or steam to yield phosphine gas; hydrolysis rate increases with acidity and temperature.
- Air‑ and water‑reactive: phosphine produced is often spontaneously flammable in air.
- Reactive with oxidizing agents and mineral acids; heating to decomposition emits phosphorus oxides and aluminium fumes.
- Stable when stored dry in suitable, moisture‑free containers; vapour pressure is very low even at high temperature.
Because the hazard mode is gas evolution, reactivity control focuses on exclusion of water, humidity control, segregation from acids and oxidizers, and keeping the material dry.
Molecular and Ionic Parameters
Formula and Molecular Weight
- Molecular formula: AlP
- Molecular weight: \(57.955300\,\mathrm{g}\,\mathrm{mol}^{-1}\)
Constituent Ions
- Formal constituent ions/descriptors present in computed representations: Al as \(\mathrm{Al^{3+}}\) and P as \(\mathrm{P^{3-}}\).
- Structural descriptors in machine representations: SMILES = [Al+3].[P-3]; InChI = InChI=1S/Al.P/q+3;-3.
The solid is a covalently influenced ionic lattice rather than a discrete molecular ion pair.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS Number: 20859-73-8
- EC number: 244-088-0
- UN/NA shipping number(s): UN 1397 (also UN 3048 for pesticide formulation variants as reported)
- UNII: E23DR6L59S
- InChI: InChI=1S/Al.P/q+3;-3
- InChIKey: PGCGUUGEUBFBPP-UHFFFAOYSA-N
- SMILES: [Al+3].[P-3]
Synonyms and Common Names
Documented synonyms and trade/common names include (selected from deposited names): aluminum phosphide; aluminium phosphide; aluminum monophosphide; Aluminium monophosphide; AL‑Phos; Celphos; Phostoxin; Delicia gastoxin; Fumitoxin; ALUMINUM PHOSPHIDE (AlP). The full list of historical and depositor-supplied synonyms includes additional proprietary or regional names.
Industrial and Commercial Applications
Functional Roles and Use Sectors
Aluminum phosphide functions primarily as a fumigant and pesticide active substance (rodenticide/insecticide) for stored‑product protection and for burrow fumigation of certain pests. It is used in agricultural storage, commodity processing and pest control where a gaseous fumigant is required under controlled, sealed conditions. Secondary applications exploit its role as a source of phosphine for semiconductor research and selected industrial chemistries.
Typical Application Examples
- Fumigation of raw agricultural commodities, processed food commodities and non‑food commodities in sealed structures or containers to control storage pests.
- Outdoor fumigation of burrows to control certain rodents and moles in non‑domestic areas.
- Formulations reported in practice include pellets, tablets and bags where aluminum phosphide is the active ingredient (examples reported with active ingredient values such as 60% or ~57% in specific formulations).
- Use as a laboratory/industrial source of phosphine in semiconductor research and small‑scale chemical synthesis.
Selection for a particular application is driven by the need for a gaseous fumigant that can penetrate grain masses or enclosed spaces; its potency and hazard require trained, licensed applicators and strict procedural controls.
Safety and Handling Overview
Health and Environmental Hazards
Aluminum phosphide is an acutely toxic, water‑reactive chemical whose primary hazard arises from liberation of phosphine (\(\mathrm{PH_3}\)). Key hazard descriptors reported include: substances which in contact with water emit flammable gases (H260), and acute toxicity by ingestion/inhalation/dermal exposure (e.g., H300, H311, H330 where reported). The material has caused severe human poisonings and fatalities after ingestion or when tablets were wetted in confined spaces producing phosphine.
Representative toxicological values reported in experimental and clinical material include: LD50 (rat, oral) ≈ 11.5 mg·kg^-1; LC50 (rat, inhalation, 4 h) ≈ 15.5 mg·m^-3. Human lethal doses have been reported in low tens of mg·kg^-1 range in cases of ingestion. Phosphine exposure produces mitochondrial inhibition (cytochrome c oxidase), oxidative stress, pulmonary oedema, cardiovascular collapse and multi‑organ dysfunction. The substance is also very toxic to aquatic organisms; environmental releases pose acute ecotoxicity risks.
Acute exposure first aid and emergency response must treat inhalation of phosphine and systemic effects; there is no specific, universally accepted chemical antidote for phosphine — treatment is primarily supportive and symptomatic.
Storage and Handling Considerations
- Store dry, cool and segregated from water, moisture, acids and incompatible oxidizers; keep containers tightly closed and in a non‑sprinklered, fireproof location.
- Avoid all contact with water, steam or humid air; formulations (tablets/pellets) should be stored in airtight packaging and handled only by trained personnel using appropriate procedural controls.
- Do not use water on spills or fires involving aluminium phosphide; recommended extinguishing media for small fires and spills include dry sand, dry clay, dry ground limestone, dry chemical or carbon dioxide; Class D extinguishers may be appropriate for metal fires. For large fires, evacuate and allow controlled burn only under specialist supervision.
- Personal protective equipment for emergency response and clean‑up: positive‑pressure self‑contained breathing apparatus (SCBA) and fully encapsulating chemical protective clothing in situations where airborne phosphine may be present; dust control and local exhaust ventilation for normal handling to prevent airborne particulate and subsequent hydrolysis.
- Accidental release measures emphasize isolation, evacuation downwind, covering powder spills to keep material dry (plastic/tarp), and collection into dry, sealed containers for disposal under appropriate hazardous‑waste regulations. Do not wash into drains.
- For detailed hazard, transport and regulatory compliance, refer to the product‑specific Safety Data Sheet (SDS) and applicable local legislation.
For transport and emergency planning, aluminum phosphide is handled as a “dangerous when wet” material and is associated with regulated shipping numbers; consignments must follow applicable dangerous goods rules and specialist packaging to prevent exposure to moisture.
