Abstract

Diammonium phosphate, systematically named diammonium hydrogen phosphate with chemical formula (NH₄)₂HPO₄, represents an important inorganic ammonium phosphate salt with molar mass 132.06 g·mol⁻¹. This compound crystallizes as colorless monoclinic crystals with density 1.619 g·cm⁻³ and demonstrates high water solubility, reaching 106.7 g per 100 mL at 70°C. The material decomposes at 155°C rather than melting, exhibiting dissociation pressures following the equilibrium (NH₄)₂HPO₄(s) ⇌ NH₃(g) + NH₄H₂PO₄(s). Diammonium phosphate serves as a principal phosphorus and nitrogen source in agricultural fertilizers, typically formulated as 18-46-0 (N-P₂O₅-K₂O), and finds additional applications as a fire retardant, fluxing agent, and yeast nutrient. The compound occurs naturally as the rare mineral phosphammite in guano deposits.

Introduction

Diammonium phosphate occupies a significant position in industrial and agricultural chemistry as one of the most extensively produced ammonium phosphate salts. Classified as an inorganic compound, this material represents the partially neutralized form of phosphoric acid with ammonia, occupying the intermediate position between monoammonium phosphate (NH₄H₂PO₄) and triammonium phosphate ((NH₄)₃PO₄). The compound's commercial importance stems from its dual nutrient content, providing both nitrogen and phosphorus in readily plant-available forms. Industrial production typically involves the direct reaction of ammonia with phosphoric acid, with global production exceeding 20 million metric tons annually. The compound's thermal dissociation behavior and acid-base properties have been extensively studied since the early 20th century, with particular attention to its decomposition pathways and stability under various environmental conditions.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The diammonium phosphate molecule consists of two ammonium cations (NH₄⁺) and one hydrogen phosphate anion (HPO₄²⁻). The hydrogen phosphate anion exhibits tetrahedral geometry around the central phosphorus atom, consistent with VSEPR theory predictions for AX₄-type species. The P-O bond lengths measure approximately 1.54 Å for P-OH and 1.51 Å for P=O bonds, with O-P-O bond angles near the ideal tetrahedral angle of 109.5°. The ammonium ions adopt regular tetrahedral symmetry with N-H bond lengths of 1.03 Å and H-N-H angles of 109.5°. Crystallographic analysis reveals that the compound forms monoclinic crystals with space group P2₁/c, with unit cell parameters a = 7.441 Å, b = 7.693 Å, c = 7.012 Å, and β = 112.17°. The hydrogen phosphate anion carries formal charges of +5 on phosphorus, -2 on each oxygen atom, and +1 on the hydrogen atom, resulting in a net charge of -2. The ammonium cations each carry formal charges of -3 on nitrogen and +1 on each hydrogen atom.

Chemical Bonding and Intermolecular Forces

The hydrogen phosphate anion contains both covalent and ionic bonding characteristics. The phosphorus-oxygen bonds display significant double bond character due to pπ-dπ backbonding, particularly for the terminal P=O bond. The P-O bond dissociation energies range from 335-464 kJ·mol⁻¹ depending on the specific bonding environment. The ammonium ions engage in extensive hydrogen bonding with phosphate oxygen atoms, with N-H···O hydrogen bond distances measuring approximately 2.87-2.92 Å. These hydrogen bonds create a three-dimensional network in the crystal structure, contributing significantly to the compound's stability and physical properties. The hydrogen bonding energy averages 17-25 kJ·mol⁻¹ per hydrogen bond. The compound exhibits polar character with a calculated molecular dipole moment of approximately 4.2 D for the isolated ion pair. Van der Waals interactions between methyl groups contribute approximately 4 kJ·mol⁻¹ to the lattice energy.

Physical Properties

Phase Behavior and Thermodynamic Properties

Diammonium phosphate presents as colorless to white monoclinic crystals with a characteristic saline taste. The compound does not melt but undergoes decomposition beginning at approximately 70°C, with complete decomposition occurring at 155°C. The standard enthalpy of formation (ΔH_f°) measures -1566.91 kJ·mol⁻¹ at 298.15 K. The heat capacity (C_p) follows the equation C_p = 98.5 + 0.247T J·mol⁻¹·K⁻¹ in the temperature range 273-373 K. The density measures 1.619 g·cm⁻³ at 20°C, with a refractive index of 1.52 for the crystalline material. The compound exhibits high water solubility, increasing from 57.5 g per 100 mL at 10°C to 106.7 g per 100 mL at 70°C. Solubility in organic solvents remains negligible, with complete insolubility in ethanol, acetone, and liquid ammonia. The dissociation pressure of ammonia follows the relationship log(P/mmHg) = 8.74 - 3420/T, reaching approximately 5 mmHg at 100°C.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrational modes at 3100-2800 cm⁻¹ (N-H stretching), 1450-1400 cm⁻¹ (N-H bending), 1100-1000 cm⁻¹ (P-O stretching), and 900-800 cm⁻¹ (P-O-H deformation). The P=O stretching vibration appears as a strong band at 1075 cm⁻¹, while O-H stretching appears as a broad band centered at 2850 cm⁻¹. Solid-state ³¹P NMR spectroscopy shows a single resonance at approximately 5.2 ppm relative to 85% H₃PO₄, consistent with the hydrogen phosphate environment. ¹H NMR in D₂O solution displays a singlet at 7.1 ppm for ammonium protons and a broad signal at 5.8 ppm for the exchangeable phosphate proton. UV-Vis spectroscopy shows no significant absorption above 200 nm, consistent with the absence of chromophores. Mass spectrometric analysis of thermally decomposed samples shows characteristic fragments at m/z 98 (PO₄⁺), 80 (PO₃⁺), and 17 (NH₃⁺).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Diammonium phosphate demonstrates thermal instability through reversible dissociation according to the equilibrium (NH₄)₂HPO₄(s) ⇌ NH₃(g) + NH₄H₂PO₄(s). The forward reaction follows first-order kinetics with an activation energy of 89.4 kJ·mol⁻¹ and pre-exponential factor of 2.7×10¹¹ s⁻¹. Complete decomposition occurs above 155°C through multiple pathways, primarily yielding polyphosphates, ammonia, and water vapor. The compound undergoes acid-base reactions with strong acids to form monoammonium phosphate and with strong bases to release ammonia gas. Reaction with metal ions typically results in precipitation of corresponding metal phosphates, with solubility products following the trend of metal phosphate solubility. Hydrolysis in aqueous solution proceeds slowly, maintaining a pH of approximately 8.0 in fresh solutions, though gradual acidification occurs through ammonium oxidation.

Acid-Base and Redox Properties

The hydrogen phosphate anion acts as a weak acid with pK_a values of 2.15, 7.20, and 12.35 for the three dissociation steps of phosphoric acid. In diammonium phosphate, the second dissociation predominates, giving the compound buffering capacity in the pH range 6.2-8.2. The ammonium ions serve as weak acids with pK_a of 9.25, creating a complex buffering system. The compound exhibits no significant redox activity under standard conditions, with standard reduction potentials indicating stability against common oxidants and reductants. The electrochemical behavior shows irreversible oxidation waves at +1.2 V versus SCE and reduction waves at -1.8 V versus SCE in aqueous electrolyte. Stability under acidic conditions persists down to pH 4.0, while alkaline conditions above pH 8.5 promote ammonia evolution.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory preparation typically involves the neutralization of phosphoric acid with ammonia in aqueous solution. The stoichiometric reaction requires two equivalents of ammonia per equivalent of phosphoric acid: H₃PO₄ + 2NH₃ → (NH₄)₂HPO₄. The reaction proceeds exothermically with ΔH = -87 kJ·mol⁻¹. Crystallization occurs through slow evaporation of water at temperatures below 70°C to prevent decomposition. Alternative synthetic routes include the reaction of ammonium carbonate with phosphoric acid: 2(NH₄)₂CO₃ + 2H₃PO₄ → 2(NH₄)₂HPO₄ + 2CO₂ + 2H₂O. Purification typically involves recrystallization from water, with yields exceeding 95%. The product purity can be verified through titration methods, with typical impurities including monoammonium phosphate and various metal phosphates.

Industrial Production Methods

Industrial production employs continuous processes using wet-process phosphoric acid and anhydrous ammonia. The reaction occurs in stainless steel reactors at temperatures maintained between 50-80°C to optimize reaction kinetics while minimizing ammonia loss. The process typically operates at pH 7.5-8.0 to ensure complete conversion to the diammonium species. Crystallization occurs through cooling crystallization or evaporative crystallization, with particle size control achieved through careful manipulation of supersaturation levels. Modern plants utilize fluidized bed dryers to reduce moisture content below 1%. Production costs primarily depend on phosphoric acid and ammonia prices, with typical energy consumption of 1.2-1.8 GJ per metric ton of product. Environmental considerations include ammonia scrubbing systems and wastewater treatment for phosphate recovery.

Analytical Methods and Characterization

Identification and Quantification

Standard identification methods include X-ray diffraction using JCPDS pattern 00-012-0254 for crystalline characterization. Quantitative analysis typically employs gravimetric methods through precipitation as ammonium phosphomolybdate or titration with standard sodium hydroxide using phenolphthalein indicator. Spectrophotometric methods based on the molybdenum blue method achieve detection limits of 0.1 mg·L⁻¹ for phosphate determination. Ammonium content determination utilizes distillation-titration methods or ion-selective electrodes with detection limits of 0.5 mg·L⁻¹. Chromatographic methods include ion chromatography with conductivity detection, achieving separation of phosphate and ammonium ions within 15 minutes. Nuclear magnetic resonance spectroscopy provides non-destructive quantification with precision of ±2% for both phosphorus and nitrogen content.

Purity Assessment and Quality Control

Commercial specifications typically require minimum concentrations of 18% nitrogen and 46% P₂O₅ equivalent, with maximum limits for heavy metals including arsenic (10 mg·kg⁻¹), cadmium (10 mg·kg⁻¹), and lead (25 mg·kg⁻¹). Moisture content specifications generally require less than 1.5% water. Common impurities include monoammonium phosphate, iron phosphates, aluminum phosphates, and various silicate compounds. Stability testing indicates satisfactory storage characteristics for up to 12 months in sealed containers under dry conditions. The product exhibits slight hygroscopicity, requiring packaging with moisture barriers. Quality control protocols include periodic sampling for particle size distribution analysis, typically requiring 90% of particles between 1-4 mm for fertilizer applications.

Applications and Uses

Industrial and Commercial Applications

Diammonium phosphate serves primarily as a high-analysis fertilizer, providing both nitrogen (18%) and phosphorus (20%) in plant-available forms. The global market exceeds 25 million metric tons annually, with principal applications in cereal crops, vegetables, and fruit production. As a fire retardant, the compound finds use in forest fire control through aerial application, where it functions by promoting char formation and reducing flammable volatile production. The material serves as a yeast nutrient in fermentation processes, particularly in winemaking and mead production, where it provides assimilable nitrogen at concentrations of 100-300 mg·L⁻¹. Metallurgical applications include use as a flux for soldering tin, copper, zinc, and brass alloys at temperatures between 200-400°C. Textile industry applications include use as a mordant for acid dyes on wool fibers, improving color fastness and uniformity.

Research Applications and Emerging Uses

Recent research explores diammonium phosphate as a precursor for nitrogen-doped carbon materials through controlled thermal decomposition. Catalysis research investigates supported diammonium phosphate systems for dehydration reactions and acid catalysis. Materials science applications include development of ammonium phosphate-based flame-retardant coatings for polymers and textiles with loading levels of 15-30%. Emerging environmental applications involve use in phosphate recovery systems from wastewater through precipitation processes. Electrochemical research examines ammonium phosphate electrolytes for intermediate-temperature fuel cells operating at 150-250°C. Ceramics research utilizes diammonium phosphate as a binding agent for refractory materials through formation of aluminum phosphate phases. Patent activity remains active in controlled-release fertilizer formulations incorporating polymer coatings or matrix encapsulation.

Historical Development and Discovery

The development of diammonium phosphate coincides with the emergence of the modern fertilizer industry in the early 20th century. Initial patent literature from the 1920s describes methods for producing ammonium phosphates through reaction of ammonia with phosphoric acid. Commercial production began in the 1930s following developments in phosphoric acid production technology. Wartime demands during World War II accelerated process development for both military and agricultural applications. The 1950s saw significant process improvements through the development of continuous crystallization and granulation technologies. Environmental concerns in the 1970s led to improved emission control technologies and waste reduction practices. Recent decades have witnessed optimization of production processes for energy efficiency and product quality consistency. The compound's fire retardant applications developed progressively throughout the latter half of the 20th century, with standardized application protocols emerging in the 1990s.

Conclusion

Diammonium phosphate represents a chemically versatile compound with significant industrial importance, particularly in agricultural applications. Its unique combination of nitrogen and phosphorus in a single compound, coupled with favorable physical properties including high water solubility and handling characteristics, ensures its continued prominence in fertilizer markets. The compound's thermal dissociation behavior and acid-base properties provide interesting chemical characteristics that enable diverse applications beyond agriculture. Current research directions focus on developing enhanced efficiency formulations through coating technologies, exploring new applications in materials science, and improving production sustainability. Future challenges include reducing the carbon footprint of production processes, developing value-added specialty products, and addressing environmental concerns related to phosphorus runoff. The fundamental chemistry of diammonium phosphate continues to offer opportunities for scientific investigation and technological innovation.