Hydroxylammonium nitrate
Hydroxylammonium nitrate (HAN), with the chemical formula [NH₃OH]⁺[NO₃]⁻ or H₂NOH·HNO₃ and a molar mass of 96.04 g/mol, is a white, odorless solid salt formed from hydroxylamine and nitric acid.[1] It has a density of 1.84 g/cm³, a melting point of 48 °C, and is highly soluble in water, with solubility of 587 g/L at 20 °C.[2][3] HAN is primarily utilized as a key oxidizer in aqueous monopropellant formulations for spacecraft and rocket propulsion systems, offering a less toxic alternative to hydrazine-based propellants with higher density impulse (up to 60% greater) and lower environmental impact.[4] These HAN-based propellants, such as those containing 60-85 wt% HAN with fuels like triethanolammonium nitrate or methanol, decompose exothermically upon catalysis to produce hot gases for thrust, enabling applications in satellite attitude control and upper-stage engines; as of 2025, research continues on electrically controlled variants.[5][6] Additionally, HAN serves as a reducing agent in the PUREX process for nuclear fuel reprocessing, converting plutonium(IV) to plutonium(III) at sites like Savannah River and Los Alamos, though its use has been limited due to safety concerns.[5] Despite its advantages, HAN poses significant hazards as an explosive material prone to autocatalytic decomposition, particularly in concentrated solutions (>2-3 M) or at elevated temperatures (>40 °C), potentially leading to violent explosions as seen in incidents like the 1997 Plutonium Reclamation Facility accident.[5] It is acutely toxic via oral and dermal routes, carcinogenic (category 2), and harmful to aquatic life with long-lasting effects, necessitating strict handling protocols including temperature control below 40 °C, iron impurity limits (<5 ppm), and use of dilute solutions with excess nitric acid.[7] Storage requires sealed, vented containers to prevent concentration via evaporation, and personal protective equipment such as gloves, eyewear, and respirators is mandatory.[5]Properties
Physical properties
Hydroxylammonium nitrate (HAN) is an ionic compound with the chemical formula [NH₃OH]⁺[NO₃]⁻, also represented as NH₃OH·HNO₃, and a molar mass of 96.04 g/mol. In its pure form, it appears as a colorless, hygroscopic solid that readily absorbs moisture from the air, potentially leading to deliquescence in humid environments.[5] This hygroscopic nature necessitates careful handling to prevent unintended concentration changes.[5] The solid has a density of 1.84 g/cm³.[8] Its melting point is 48 °C, above which it decomposes rather than boiling, rendering a boiling point inapplicable.[9] HAN exhibits high solubility in water, exceeding 587 g/L at 20 °C, forming clear, colorless aqueous solutions that are commercially available and typically range from 20% to 60% concentration by weight for various applications.[3] It is miscible with water in all proportions at room temperature.[8] The compound shows slight solubility in alcohols, such as butanol.[10]Chemical properties
Hydroxylammonium nitrate (HAN) is an ionic salt composed of the hydroxylammonium cation (NH₃OH⁺), derived from hydroxylamine as a reducing agent, and the nitrate anion (NO₃⁻), an oxidizing agent from nitric acid. This dual nature results in inherent internal redox instability, where the reducing and oxidizing components can react with each other, predisposing the compound to spontaneous decomposition under certain conditions.[11][12] The thermal decomposition of HAN is exothermic, producing nitrogen gas, water, and acidity, though actual decomposition pathways may involve intermediates like nitrous acid or nitrous oxide depending on conditions.[12][13] Aqueous solutions of HAN exhibit acidic pH values, typically in the range of 1–2, arising from the hydrolysis of the hydroxylammonium ion, which partially dissociates to release protons.[11] The redox potential underscores the compound's reactivity, with the hydroxylammonium ion functioning as a reductant (e.g., capable of reducing Pu(IV) to Pu(III) or Fe(III) to Fe(II)) and the nitrate ion serving as an oxidant, facilitating electron transfer processes intrinsic to its instability.[12] HAN demonstrates high sensitivity to catalysts, decomposing rapidly in the presence of metal oxides such as platinum or iridium, which lower the activation energy for decomposition, as well as under acidic or basic conditions that accelerate proton transfer or alter ionic equilibria.[14] Regarding thermal stability, HAN melts at 48 °C and remains thermally stable up to higher temperatures, with exothermic decomposition typically initiating above 80 °C depending on conditions such as purity and form, potentially leading to explosive gas evolution if the material is confined.[15]Preparation
Laboratory methods
Laboratory methods for synthesizing hydroxylammonium nitrate (HAN) focus on small-scale procedures suitable for research environments, emphasizing precise control to mitigate thermal decomposition and oxidation risks associated with the compound's instability. A standard neutralization route involves the reaction of hydroxylamine hydrochloride with silver nitrate to form HAN and precipitate silver chloride, followed by filtration to remove the insoluble AgCl and evaporation under reduced pressure to isolate the product:\ce{NH2OH \cdot HCl + AgNO3 -> NH3OH+ NO3- + AgCl}
This double displacement method is conducted at low temperatures (0–10 °C) with stirring to ensure complete reaction and minimize side products. Yields typically reach 70–80% after purification.[16] Catalytic reduction represents another key laboratory approach, where nitric oxide or nitrogen dioxide is reduced with hydrogen over a platinum catalyst in an aqueous acidic medium. The reaction occurs in a stirred reactor at 30–50 °C and near-atmospheric pressure, with a H₂:NO molar ratio of approximately 1.7–2:1 and continuous addition of nitric acid to maintain 1–2 N free acidity. The supported platinum catalyst, often sulfur-poisoned for selectivity, enables hydroxylamine concentrations of 110–130 g/L, with NO-based yields up to 85% and space-time yields of 66 g NH₂OH/g Pt/hour. An inert atmosphere, such as nitrogen purging, is employed to prevent unwanted oxidation.[17] Electrolytic reduction of nitric acid to HAN is achieved in a divided electrochemical cell, where the catholyte consists of 0.2–0.5 mol/L excess nitric acid, and the cathode potential is controlled at -1 to -1.5 V versus saturated calomel electrode (SCE) using a suitable electrode material like platinum or glassy carbon. Operating at 15–30 °C and current densities of 0.2–1 kA/m², this method produces HAN solutions up to 39 wt.% with current efficiencies of 67–78%. The process requires an inert gas cover to avoid aerial oxidation during electrolysis.[18] Ion exchange provides a versatile lab technique, typically by passing a solution of hydroxylammonium sulfate through an anion exchange resin loaded with nitrate ions, effecting the exchange:
\ce{(NH3OH)2SO4 + 2 NO3^- (resin) -> 2 NH3OHNO3 + SO4^{2-} (resin)}
The procedure is performed at 0–10 °C under an inert atmosphere, with effluent monitored for complete ion swap via conductivity or titration. Yields exceed 90% for small batches. Alternatively, metathesis with barium or sodium nitrate in aqueous or organic media (e.g., butanol) precipitates the sulfate salt for facile separation.[19][10] A recent advancement (as of 2024) involves selective electrosynthesis of hydroxylamine from nitrate ions using specialized electrocatalysts (e.g., copper-based materials modified for selectivity) in aqueous media at ambient conditions, achieving faradaic efficiencies over 80% for NH₂OH by suppressing hydrogen evolution and over-reduction to ammonia. The resulting hydroxylamine can be directly neutralized with nitric acid to form HAN solutions.[20] Across these methods, overall yields range from 70–90%, influenced by reaction scale and purity of starting materials. Post-synthesis purification commonly involves recrystallization from water-ethanol mixtures (e.g., 1:1 ratio), exploiting HAN's solubility to exclude impurities like ammonium nitrate, followed by drying under vacuum at low temperature.[21]