Diammonium phosphate
Diammonium phosphate (DAP), chemically known as diammonium hydrogen phosphate with the formula (NH₄)₂HPO₄, is an inorganic salt that serves primarily as a highly soluble fertilizer delivering essential nitrogen (18%) and phosphorus (46% P₂O₅) nutrients to crops.[1][2] It appears as a white or colorless crystalline solid with a molar mass of 132.06 g/mol, a density of 1.619 g/cm³, and high water solubility (approximately 58.8–69.5 g/100 mL at 20–25°C), making it readily available for plant uptake upon soil application.[1][2] The compound has a slightly alkaline pH of 7.5–8 in solution and, upon heating, loses ammonia above approximately 70 °C to form monoammonium phosphate, with further decomposition around 155 °C releasing phosphorus oxides, nitrogen oxides, and additional ammonia.[1][2][3] Produced through the neutralization reaction of phosphoric acid with ammonia in a 1:2 molar ratio, often via the wet process using phosphate rock, sulfur, and ammonia, DAP is one of the most common phosphorus fertilizers globally due to its cost-effectiveness and nutrient density.[1][2] In agriculture, it supports root development, flowering, and fruiting in a wide range of crops, including grains, vegetables, and turfgrass, though careful placement is needed to avoid seedling damage from high concentrations.[2][4] Beyond farming, DAP functions as a fire retardant in mixtures applied to forests or textiles, a yeast nutrient in brewing and winemaking to enhance fermentation, and a food-grade additive for nutrient supplementation in processed foods and pharmaceuticals.[1][4] It also finds applications in cosmetics, pesticides, and water treatment, underscoring its versatility as a multi-purpose chemical compound.[1]Chemical identity
Formula and nomenclature
Diammonium phosphate has the chemical formula (NH_4)_2HPO_4.[1] Its IUPAC name is diammonium hydrogen phosphate.[5] It is also known by other names, including DAP, ammonium phosphate dibasic, and ammonium monohydrogen phosphate.[1] Diammonium phosphate is classified as a water-soluble inorganic salt, specifically the diammonium salt of phosphoric acid, formed from the reaction of ammonia and phosphoric acid.[1][2] The molar mass of diammonium phosphate is 132.06 g/mol.[5]Molecular structure
Diammonium phosphate is an ionic compound consisting of two ammonium cations (\ce{NH4+}) and one hydrogen phosphate anion (\ce{HPO4^2-}).[6] The compound forms colorless monoclinic crystals in the holohedral class $2/m, with the detailed crystal structure belonging to the space group P2_1/c and unit cell dimensions a = 11.043 Å, b = 6.700 Å, c = 8.031 Å, and \beta = 113.42^\circ.[7] In this arrangement, the structure comprises discrete tetrahedral \ce{PO4} and \ce{NH4} units, where the phosphate tetrahedron features one elongated P–O bond (1.587 Å) indicative of the protonated oxygen in \ce{HPO4^2-}. These tetrahedra are interconnected through an extensive network of hydrogen bonds, including a short O–H⋯O interaction (2.615 Å) and multiple N–H⋯O bonds with N⋯O distances ranging from 2.789 Å to 2.965 Å, stabilizing the ionic lattice. Ball-and-stick models of the structure illustrate the tetrahedral geometry of the ammonium ions, with nitrogen centrally bonded to four hydrogens, and the distorted tetrahedral hydrogen phosphate anion, where phosphorus is coordinated to three oxygen atoms with double-bond character and one protonated hydroxyl group.[6]Physical and chemical properties
Physical characteristics
Diammonium phosphate is typically observed as a white to off-white crystalline powder or granules at standard temperature and pressure.[6] This appearance arises from its formation into compact, free-flowing particles during industrial processing, facilitating handling and storage.[8] The compound possesses a density of 1.619 g/cm³, contributing to its relatively compact bulk form.[6] It emits a mild ammonia-like odor, attributable to the presence of ammonium ions.[6] Diammonium phosphate exhibits a hygroscopic nature, which can lead to caking and agglomeration when exposed to humid conditions, potentially affecting its flowability.[9] The material adopts a monoclinic crystal structure.[10]Solubility and stability
Diammonium phosphate demonstrates significant solubility in water, dissolving at a rate of 57.5 g/100 mL at 10 °C and increasing to 106.7 g/100 mL at 70 °C, which facilitates its use in aqueous applications.[1][11] It is, however, insoluble in common organic solvents such as alcohol, acetone, and liquid ammonia, limiting its solubility to polar media.[1][11] Aqueous solutions of diammonium phosphate exhibit a pH around 8, rendering them mildly basic due to the hydrolysis of the phosphate ions, with a minor contribution from the ammonium ions.[2] Regarding stability, diammonium phosphate remains stable under dry storage conditions but is hygroscopic and prone to decomposition in moist environments, where it absorbs water and releases ammonia gas.[12][13]Thermal decomposition
Diammonium phosphate exhibits thermal instability and begins to decompose at approximately 70 °C, primarily through the release of ammonia gas, resulting in the formation of monoammonium phosphate.[14] This initial stage involves the dissociation of the ammonium ions and follows the reversible equilibrium reaction: (\ce{NH4})_2\ce{HPO4(s)} \rightleftharpoons \ce{NH3(g)} + \ce{(NH4)H2PO4(s)} At 100 °C, the dissociation pressure over the solid is approximately 5 mmHg, indicating limited volatility under these conditions.[15] Upon further heating, the compound undergoes more complete decomposition around 155 °C, where it releases phosphorus oxides such as P₄O₁₀, nitrogen oxides (NOₓ), and additional ammonia.[14] Diammonium phosphate does not possess a true melting point, as decomposition occurs prior to any phase transition to a liquid state.[8] This behavior underscores its unsuitability for applications requiring high thermal endurance without gaseous emissions.Production
Laboratory synthesis
Diammonium phosphate, with the formula (NH₄)₂HPO₄, is synthesized in the laboratory through the neutralization reaction of phosphoric acid with ammonia:\ce{H3PO4 + 2 NH3 -> (NH4)2HPO4}
This reaction proceeds in aqueous solution, where ammonia acts as a base to partially deprotonate the phosphoric acid, forming the dibasic ammonium salt.[16] The standard laboratory procedure involves titrating a dilute phosphoric acid solution with ammonia solution to determine the equivalence point, ensuring a 1:2 molar ratio of acid to base. Typically, 10 cm³ of 1 mol dm⁻³ ammonia solution is placed in a conical flask with a pH indicator such as methyl orange, and titrated with 1 mol dm⁻³ phosphoric acid until the endpoint is reached, recording the volume required. For preparation, double the ammonia volume (e.g., 20 cm³) is then mixed with the corresponding acid volume in an evaporating basin. The mixture is gently heated on a water bath or tripod to evaporate to about one-fifth its volume, avoiding boiling to prevent decomposition, and allowed to cool slowly for crystal formation. The resulting crystals are filtered using vacuum or gravity filtration, washed with cold water, and dried at room temperature. This method yields colorless, crystalline (NH₄)₂HPO₄ suitable for analytical purposes.[16] To favor the formation of the dibasic form over monoammonium phosphate (NH₄H₂PO₄) or triammonium phosphate ((NH₄)₃PO₄), the reaction pH is controlled between 7.5 and 8.0 during neutralization. This range ensures the second proton of phosphoric acid is predominantly removed while minimizing excess ammoniation, as monitored by a pH meter or indicator; deviations can lead to mixed salt products. Ammonia gas can alternatively be bubbled into phosphoric acid solution under controlled cooling to manage the exothermic reaction and maintain this pH. For purification, the crude product is recrystallized from hot distilled water. The crystals are dissolved in minimal boiling water, filtered hot to remove insoluble impurities, and the filtrate is cooled to induce recrystallization. The purified crystals are then filtered, washed, and dried, achieving high purity by removing residual acids or other ammonium phosphates. This step is essential for obtaining analytically pure diammonium phosphate, with yields typically around 70-80% based on the limiting reactant.[17]