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Ammonium phosphomolybdate

Ammonium phosphomolybdate is an inorganic compound and the ammonium salt of phosphomolybdic acid, with the chemical formula (NH₄)₃[PMo₁₂O₄₀] · xH₂O, typically appearing as a yellow crystalline powder that decomposes upon heating.^[1] It forms a characteristic yellow precipitate when phosphate ions react with ammonium molybdate in acidic solution, enabling its primary role in analytical chemistry for the gravimetric determination of phosphorus content in samples such as fertilizers, soils, and biological materials.^[2] Beyond traditional analysis, the compound has gained attention for its applications in nuclear waste treatment as a selective ion exchanger for removing cesium isotopes, such as ¹³⁷Cs, from acidic radioactive effluents.^[3]

Chemical Properties

Ammonium phosphomolybdate exhibits a molecular weight of approximately 1876.35 g/mol (anhydrous basis) and is slightly soluble in water but more soluble in alkaline solutions, while remaining insoluble in acids.^[4] Its structure consists of a central phosphate tetrahedron surrounded by twelve molybdenum oxide octahedra, forming a Keggin-type heteropolyoxometalate anion [PMo₁₂O₄₀]³⁻ balanced by ammonium cations, which contributes to its stability and reactivity in aqueous media.^[5] The compound is an oxidizing agent and must be stored at cool temperatures (2–8°C) to prevent decomposition.^[4]

Applications

In analytical chemistry, ammonium phosphomolybdate is precipitated from phosphate-containing solutions, filtered, dried, and weighed to quantify phosphorus, with the precipitate often ignited to molybdenum trioxide (MoO₃) for precise mass measurement; this method is valued for its accuracy in low-concentration samples.^[2] For nuclear waste management, it demonstrates high selectivity for cesium ions, achieving over 99.9% removal efficiency in acidic conditions (e.g., 0.1–1 N nitric acid), with an ion exchange capacity of about 1.6 meq/g, making it suitable for treating fuel reprocessing wastes when supported on carriers like silica gel.^[3] Emerging uses include its incorporation into composite materials, such as with metal-organic frameworks or polymers, to enhance cesium adsorption in wastewater, and as a dielectric in electronic memory devices due to its semiconducting properties.^[6]^[5] It also serves as a catalyst precursor in organic synthesis, such as for glycerol dehydration to acrolein.^[4]

Safety and Handling

The compound is classified as an irritant, causing skin, eye, and respiratory irritation upon exposure, and as a mild oxidizer; handling requires protective equipment, and it should be avoided in confined spaces to prevent dust inhalation.^[4] In laboratory settings, it is transported under non-hazardous classifications but requires proper ventilation during use in analytical procedures.^[1]

Chemical structure and properties

Molecular structure

Ammonium phosphomolybdate is classified as a heteropolyoxometalate salt, featuring the Keggin-type polyanion [ \ce{PMo_{12}O_{40}^{3-}} ] counterbalanced by three ammonium cations \ce{(NH4+)} to form the neutral compound \ce{(NH4)3PMo_{12}O_{40}}.^[5] The core of the polyanion consists of a central \ce{[PO4]^{3-}} tetrahedron surrounded by twelve \ce{MoO6} octahedra, which share edges and corners to create a robust, cage-like framework characteristic of the \alpha-Keggin structure.^[5] This arrangement exhibits tetrahedral (T_d) symmetry, with the phosphate group encapsulated at the center and the molybdenum centers adopting a distorted octahedral coordination. The overall crystal structure is cubic with space group P\bar{n}3m.^[5] Key structural features include distinct Mo-O bond lengths that reflect the bonding environments: terminal Mo=O bonds average approximately 1.68 Å, bridging Mo-O-Mo bonds within the octahedra are around 1.90 Å, and the longer Mo-O-P bonds connecting to the central tetrahedron measure about 2.43 Å.^[7] These bond lengths contribute to the stability of the polyanion, with oxygen atoms categorized into terminal, bridging, and central types based on their connectivity. The compound is commonly encountered in its hydrated form, \ce{(NH4)3PMo_{12}O_{40} \cdot 6 H2O}, where the water molecules integrate into the crystal lattice, influencing the overall packing without altering the core polyanion geometry.

Physical properties

Ammonium phosphomolybdate typically appears as yellow crystals or a fine yellow powder, which takes on a darker yellow hue in its hydrated form.^[8] The compound is hygroscopic and commonly occurs as a hexahydrate, though other hydration states are possible depending on preparation conditions.^[9] Its molar mass is 1876.35 g/mol for the anhydrous form.^[10] The density of the trihydrate is approximately 3.15 g/cm³.^[11] Ammonium phosphomolybdate decomposes upon heating without melting, with thermal decomposition onset occurring between 100 and 200 °C, involving initial loss of crystal water followed by formation of phosphomolybdenum oxides.^[12] In terms of solubility, it exhibits very low solubility in water and is practically insoluble in nitric acid (0.5–0.7 g/L in 0.1 N nitric acid), but dissolves readily in alkali hydroxides.^[3]^[9]

Chemical properties

Ammonium phosphomolybdate, as the ammonium salt of phosphomolybdic acid, exhibits an acidic nature characteristic of heteropolyacid salts, with the phosphomolybdate anion [PMo₁₂O₄₀]³⁻ contributing to its corrosive properties and ability to cause burns upon contact.^[5]^[13] This acidity arises from the proton-donating capability of the Keggin structure, making it behave as a weak heteropolyacid salt suitable for catalytic applications involving proton transfer.^[14] The compound demonstrates high stability in acidic media, such as nitric, hydrochloric, sulfuric, and hydrofluoric acids at concentrations up to 1.5 N, where it maintains its structure and low solubility (0.5–0.7 g/L in 0.1 N acids).^[3] However, it decomposes in strong basic environments, becoming soluble in alkaline solutions due to disruption of the polyanion framework.^[4]^[15] Thermal decomposition occurs in distinct stages, beginning with the loss of crystal water, followed by release of ammonia, and culminating in oxide formation; in air, it is stable up to approximately 425°C, then decomposes between 425–750°C to yield molybdenum oxide, phosphoric acid derivatives (such as phosphorus oxides), and oxidized ammonia products (nitrogen and water), with molybdenum trioxide (MoO₃) subliming above 750°C to form greenish-white crystals.^[3]^[16] In inert atmospheres, the process similarly starts with water loss, but ammonia partially reduces the intermediate phosphomolybdenum oxide to a nonstoichiometric suboxide, with remaining ammonia evolving as N₂ and H₂.^[16] Ammonium phosphomolybdate possesses ion-exchange capabilities as a selective cation exchanger, particularly for metals like cesium, with an exchange capacity of 1.6 milliequivalents (211 mg Cs) per gram and distribution coefficients exceeding 1000 in acidic solutions, enabling >99.9% removal from simulated wastes.^[3]^[17] The molybdate framework imparts redox properties, allowing the compound to act as an oxidizing agent and electron reservoir through reversible one- or two-electron transfers between Mo(VI) and Mo(V) states, as evidenced by photochemical reduction under UV irradiation from yellow to green forms.^[13]^[5] This behavior facilitates electron transfer in catalytic processes, with the Keggin anion's HOMO-LUMO gap of 4.79 eV supporting its role in redox-active materials.^[5]

Synthesis and preparation

Laboratory synthesis

Ammonium phosphomolybdate is typically synthesized in the laboratory by reacting ammonium molybdate with phosphoric acid in the presence of nitric acid under aqueous conditions. The primary reagents are ammonium orthomolybdate, ((NH₄)₂MoO₄), phosphoric acid (H₃PO₄), and nitric acid (HNO₃), with the latter serving to acidify the medium and facilitate the formation of the Keggin-type heteropolyacid structure. In practice, the commercially available ammonium heptamolybdate tetrahydrate, ((NH₄)₆Mo₇O₂₄ · 4H₂O), is often used as the molybdenum source, as it dissolves and equilibrates to the reactive molybdate species in acidic solution.^[18]^[5] The balanced chemical equation for the reaction using ammonium orthomolybdate is:

\mathrm{H_3PO_4 + 12 (NH_4)_2MoO_4 + 21 HNO_3 \rightarrow (NH_4)_3PMo_{12}O_{40} + 21 NH_4NO_3 + 12 H_2O}

This stoichiometry accounts for the assembly of the [PMo₁₂O₄₀]³⁻ anion, with ammonium ions balancing the charge and excess nitrate forming soluble ammonium nitrate byproduct.^[18] The procedure begins by dissolving ammonium molybdate (e.g., 2.3 g of the heptamolybdate tetrahydrate) in dilute nitric acid (approximately 1 M, 40 mL) to form a clear solution. A solution of phosphoric acid (or an equivalent phosphate source like 0.15 g NaH₂PO₄ · 2H₂O in 20 mL water, adjusted for acidity) is then added slowly with stirring. The mixture is heated to 60–80 °C for 1 hour to promote condensation and heteropolyanion formation, followed by cooling to room temperature, which induces precipitation of the bright yellow crystalline product. The precipitate is collected by filtration, washed with cold distilled water to remove soluble byproducts, and dried at 60 °C.^[5] Yields are generally high (often >90% based on phosphate), making this method suitable for both preparative and analytical scales; the rapid precipitation even at low phosphate concentrations (as low as 10⁻⁴ M) underpins its use as a qualitative test for phosphate ions in solution, where the yellow color confirms PO₄³⁻ presence. Purity of the initial precipitate is good due to the selective formation and low solubility, though traces of nitrate or unreacted molybdate may require monitoring via elemental analysis for critical applications.^[5]

Purification methods

Purification of ammonium phosphomolybdate, (NH₄)₃PMo₁₂O₄₀, typically begins with the isolation of the yellow precipitate formed during synthesis, followed by refinement steps to achieve high purity by removing soluble byproducts such as ammonium nitrate. The crude product is collected via filtration under reduced pressure to separate the solid from the reaction mixture. Subsequent washing with cold water or dilute nitric acid effectively eliminates residual ammonium nitrate and other ionic impurities, as the byproduct is highly soluble in these media while the target compound exhibits low solubility.^[18] This step is crucial to prevent contamination that could affect subsequent applications, with multiple wash cycles often employed until the filtrate shows no nitrate presence via qualitative testing. For further refinement, recrystallization is performed using solvents that exploit the compound's solubility profile: very slightly soluble in water but more soluble in dilute alkali solutions, and insoluble in alcohols.^[9] The precipitate is dissolved in hot dilute ammonium hydroxide or sodium hydroxide solution, followed by cooling to induce crystallization, which helps remove nitrate and other soluble impurities co-precipitated during synthesis. Alternatively, water-ethanol mixtures are used, where the compound is dissolved in hot water and ethanol is added to reduce solubility and promote crystal growth, yielding purer crystals upon filtration.^[9] These methods ensure the removal of trace contaminants, with yields typically exceeding 80% for well-controlled conditions. The purified crystals are then dried under vacuum at low temperatures (below 100°C) to control the hydration state, as the compound exists as a hexahydrate and overheating can lead to decomposition or loss of ammonium ions. Vacuum drying minimizes exposure to moisture, preserving the stoichiometric composition. Post-purification characterization confirms purity and structure: X-ray diffraction (XRD) verifies the Keggin-type cubic structure; infrared (IR) spectroscopy identifies Mo-O bonds; and elemental analysis determines the P:Mo:N ratio, typically matching the theoretical 1:12:3 composition within 0.5% error.^[19] Challenges in purification include the compound's hygroscopic nature, which causes it to absorb atmospheric moisture and form variable hydrates, complicating handling and storage—thus, operations are conducted in a dry atmosphere or desiccator.^[20] Additionally, if alternative reagents like sodium molybdate are used in synthesis, complete removal of sodium ions requires ion-exchange resin treatment or exhaustive washing with acidified ethanol to avoid lattice incorporation, as residual sodium can alter catalytic properties.^[9] These issues demand careful control to maintain batch-to-batch reproducibility.

Applications

Analytical uses

Ammonium phosphomolybdate serves as a key reagent in the qualitative detection of phosphate ions through the formation of a bright yellow precipitate in acidic medium. When a solution containing phosphate is treated with ammonium molybdate in the presence of nitric acid, the orthophosphate ions react to produce ammonium phosphomolybdate, (NH₄)₃[PMo₁₂O₄₀], which appears as a canary-yellow crystalline precipitate, confirming the presence of phosphorus.^[21] This test is highly sensitive and is commonly performed by heating the mixture to around 60°C to accelerate precipitation without interference from excess acidity.^[22] In quantitative gravimetric analysis, ammonium phosphomolybdate is precipitated from phosphate-containing samples for phosphorus determination by weighing the dried precipitate. The process involves adding ammonium molybdate to an acidic solution of the sample at controlled temperatures (typically 50–70°C) to form the yellow complex, which is then filtered, washed with dilute potassium nitrate, dried at 200–250°C, and weighed; the phosphorus content is calculated from the mass using the factor 1.65% P in (NH₄)₃PMo₁₂O₄₀.^[22]^[23]^[24] This method is accurate for phosphorus levels above 0.1% in samples like fertilizers, steels, and organic materials, offering high specificity when performed under standardized conditions.^[25]^[26] Spectrophotometric methods utilize the ammonium phosphomolybdate complex for colorimetric phosphorus quantification, often by reducing it to a blue phosphomolybdate species (molybdenum blue) for enhanced sensitivity. In acidic medium, phosphate reacts with ammonium molybdate and a reducing agent like ascorbic acid or hydrazine to form the reduced complex, which absorbs at 700–880 nm; absorbance is measured to determine phosphorus concentrations as low as 0.01 mg/L.^[27]^[28] This approach is widely adopted for trace analysis due to its simplicity and compatibility with automated flow injection systems.^[29] The method's selectivity can be affected by interferences from arsenate and silicate ions, which form analogous yellow precipitates under similar conditions. Arsenate produces a similar phosphomolybdate-like complex, while silicate forms silicomolybdate, both competing with phosphate; borate may also interfere at high concentrations.^[30]^[31] To mitigate these, masking agents such as tartaric acid or malic acid are added to complex silicate and arsenate, preventing their reaction with molybdate; for instance, tartaric acid effectively suppresses silica interference in freshwater samples.^[32]^[33] These analytical techniques find extensive use in environmental monitoring for assessing phosphate levels in wastewater and natural waters to evaluate eutrophication risks, where concentrations are typically regulated below 0.1 mg/L.^[27]^[29] In food analysis, they quantify phosphorus in products like meat, dairy, and fertilizers to ensure nutritional compliance and quality control, with detection limits suitable for regulatory standards.^[34]^[35]

Materials science and catalysis

Ammonium phosphomolybdate (APM), with the formula (NH₄)₃PMo₁₂O₄₀, serves as a promising dielectric material in thin-film configurations due to its ability to exhibit dielectric crossover and resistive switching behaviors suitable for memory devices. In Au/APM/Au sandwich structures, the yellow form of APM (YAPM) displays a high dielectric constant of 390 at 100 Hz and 40°C, which decreases to 200 upon UV irradiation, transitioning to the green form (GAPM) and enabling a shift in AC conductivity from quantum mechanical tunneling to hopping conduction mechanisms. This UV-induced change is attributed to Maxwell–Wagner polarization and enhanced dipolar relaxation, with the dielectric loss tangent increasing from 1.5 to 6.7. For resistive switching, YAPM-based devices achieve an ON/OFF current ratio of 2 × 10² at 3.0 V, with the OFF state governed by Schottky emission (barrier height ≈ 0.52 eV) and the ON state by the Poole–Frenkel mechanism; these devices demonstrate stability over 10³ cycles under 3 V pulses and can be locked into a stable ON state after 300 s of UV exposure.^[36] The photocatalytic properties of APM enable the reduction of metal ions such as Cr(VI) to Cr(III) under UV light, alongside the degradation of organic pollutants in aqueous solutions. In polymer composites with APM, the material facilitates efficient Cr(VI) reduction, leveraging its tunable band gap energy (reduced from 3.2 eV in pure APM to lower values in hybrids) to enhance charge separation and electron transfer for environmental remediation. APM-mediated photocatalysis also achieves near-complete degradation of dyes like Janus green B, with the process driven by the compound's redox-active Mo centers that generate reactive species under irradiation. When incorporated into metal-organic frameworks such as MIL-88B(Fe) with polyethylene glycol, APM boosts the photocatalytic removal of persistent antibiotics like tetracycline, attaining degradation efficiencies exceeding 90% under visible or UV light due to improved surface area and charge carrier dynamics. Sn-doped variants of APM have emerged as selective adsorbents for cesium (Cs⁺) in nuclear waste remediation, operating via ion-exchange mechanisms that exploit the compound's negatively charged framework. Synthesized through co-precipitation, these Sn-doped materials exhibit high Cs⁺ uptake capacities, with distribution coefficients indicating strong selectivity over competing ions like Na⁺ and K⁺ in acidic solutions typical of radioactive effluents. The doping enhances structural stability and pore accessibility, enabling efficient Cs⁺ removal through replacement of NH₄⁺ ions, with adsorption kinetics following pseudo-second-order models and capacities reported up to several hundred mg/g under optimized conditions. In composite materials, APM is integrated into UiO-66 metal-organic frameworks to form nano-composites for the selective capture of radionuclides, particularly ¹³⁷Cs, from wastewater. The UiO-66/AMP hybrid achieves a maximum Cs⁺ adsorption capacity of 94.9 mg/g and 96.7% removal efficiency at pH 7, with ultrafast kinetics removing 90% of Cs⁺ within 5 minutes across a broad pH range (4–11). This performance stems from ion-exchange between Cs⁺ and NH₄⁺ in the AMP phase, combined with the framework's high surface area (over 1000 m²/g) and distribution coefficients of 5.8 × 10³ to 1.26 × 10⁴ mL/g in multi-ion environments, ensuring selectivity amid high concentrations of Na⁺, K⁺, Mg²⁺, and Ca²⁺. Defect-engineered versions further improve co-removal of Cs⁺ and Sr²⁺, maintaining capacities above 80 mg/g while adhering to Langmuir isotherm models for monolayer adsorption.^[37] As a heteropolyacid salt, APM contributes to catalysis through its proton conductivity and redox activity, particularly in organic synthesis and potential fuel cell applications. In Keggin-type heteropolyacid/Ni-MOF catalysts derived from phosphomolybdic acid (the parent acid of APM), the structure provides strong Brønsted acidity for esterification reactions, such as converting oleic acid to biodiesel with 86.1% yield at 160°C, following first-order kinetics with an activation energy of 64.6 kJ/mol and recyclability over 10 cycles. Analogous ammonium salts like (NH₄)₃PW₁₂O₄₀ exhibit proton conductivities up to 10⁻² S/cm at intermediate temperatures when composited with phosphoric acid, supporting their use in proton-exchange membranes for fuel cells by facilitating Grotthuss-type hopping of H⁺ ions. The redox versatility of the Mo₆O₂₂ core in APM enables multi-electron transfers in oxidation processes, enhancing efficiency in heterogeneous catalysis for fine chemical production.^[38]

History and safety

Discovery and development

Ammonium phosphomolybdate, with the formula (NH₄)₃[PMo₁₂O₄₀], was first synthesized in 1826 by Swedish chemist Jöns Jacob Berzelius during his investigations into molybdic acid and phosphates. This compound, formed by reacting ammonium molybdate with phosphoric acid, is recognized as the earliest discovered polyoxometalate, initiating a vast field of metal-oxygen cluster chemistry. Berzelius's work laid the groundwork for understanding these complex anions, though the full structural details remained elusive for over a century.^[39]^[40] In the early 19th century, ammonium phosphomolybdate quickly became a key reagent in qualitative analysis for phosphorus detection. Its characteristic yellow precipitate, formed upon addition to phosphate-containing solutions, enabled reliable identification of phosphorus in minerals, soils, and biological materials, facilitating advancements in geochemistry and biochemistry. This application persisted into the 20th century, where it was refined for both qualitative and gravimetric quantitative methods.^[41] A pivotal advancement came in the 1930s with the structural elucidation of the Keggin ion, the core anion of ammonium phosphomolybdate. Using powder X-ray crystallography, James F. Keggin determined the structure of the isostructural phosphotungstate [PW₁₂O₄₀]³⁻ in 1933–1934, revealing a framework of twelve MoO₆ (or WO₆) octahedra surrounding a central PO₄ tetrahedron. This discovery provided essential insights into the phosphomolybdate's architecture, spurring further research into polyoxometalate properties and reactivity.^[42]^[43] Following the turn of the millennium, ammonium phosphomolybdate's study has shifted toward nanomaterials and advanced applications. A 2020 study highlighted its dielectric properties, demonstrating photoinduced crossover from high- to low-dielectric states under UV irradiation, with potential for resistive switching in memory devices. In 2025, research on Sn-doped variants revealed high selectivity for cesium adsorption, achieving efficient uptake from aqueous solutions, relevant for nuclear waste remediation. These developments reflect the compound's evolution from an analytical tool to a versatile polyoxometalate in green chemistry, including as a catalyst for aerobic oxidations and solvent-free processes.^[44]^[45]^[46]

Safety considerations

Ammonium phosphomolybdate is classified under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) with a warning signal word, indicating hazards including skin irritation (H315), serious eye irritation (H319), and respiratory tract irritation (H335) from dust exposure, attributable to its irritant properties and molybdenum content. Occupational exposure limits for soluble molybdenum compounds, applicable to this substance, are set by the Occupational Safety and Health Administration (OSHA) at a permissible exposure limit (PEL) of 5 mg/m³ as an 8-hour time-weighted average (TWA), measured as molybdenum (Mo). The American Conference of Governmental Industrial Hygienists (ACGIH) establishes a threshold limit value (TLV) of 0.5 mg/m³ for the respirable fraction.^[47]^[48] Safe handling requires use in a fume hood or well-ventilated area, along with personal protective equipment such as chemical-resistant gloves, safety goggles, and respirators if airborne concentrations exceed limits; direct inhalation, skin contact, or ingestion must be avoided, with immediate flushing of affected areas using water and thorough post-handling washing recommended. The compound demonstrates low acute toxicity but serves as an irritant to skin, eyes, and the respiratory system, with potential for molybdenum accumulation from chronic exposure leading to elevated serum uric acid levels and symptoms resembling gout, though such effects are uncommon under controlled occupational conditions. Environmental hazards necessitate careful waste disposal to mitigate ecotoxicological risks, as molybdenum can adversely affect aquatic organisms. Storage should occur in a cool, dry, well-ventilated location to maintain hydration stability, with incompatibility to strong bases noted due to potential decomposition.

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James Fargher Keggin some 85 years ago published a highly unique discovery—the structure of phosphotungstic acid (Nature 1933, 131, 908–909). This structure ...
[44]
Remember the Keggin Ion? - ChemistryViews
Feb 6, 2018 · Keggin could only characterize them structurally in 1933 [1]. He solved the structure of the free acid H3PW12O40 using powder X-ray diffraction, ...Missing: phosphomolybdate 1930s<|control11|><|separator|>
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a material for dielectric crossover and resistive switching performance
The yellow ammonium phosphomolybdate [(NH4)3PMo12O40] (YAPM) is a robust and elegant compound that has found innumerable field applications.
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Novel Sn-doped ammonium phosphomolybdate
The incorporation of stannum ions replaced one-third of the inert ammonium ions in ammonium phosphomolybdate, which resulted in an increase in the volumetric ...
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[PDF] Polyoxomolybdates as Green Catalysts for Aerobic Oxidation
The present book emphasizes the different types of green heterogeneous catalysts based on phosphomolybdates as well as their use as catalysts for solvent-free.
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https://www.acgih.org/molybdenum-and-compounds/
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[PDF] Hazardous Substance Fact Sheet - NJ.gov
ACGIH: The threshold limit value (TLV) is 3 mg/m3 (as the respirable fraction) and 10 mg/m3 (as the inhalable fraction) for Molybdenum metal and insoluble.