Ammonium ferric sulfate dodecahydrate, commonly known as iron ammonium sulfate, is a representative compound crystal with a chemical formula of NH₄Fe(SO₄)₂·12H₂O. Its appearance is elegant, ranging from a light purple to colorless transparent octahedral crystals. It is relatively stable in the air. As an important trivalent iron salt, it can ionize Fe³⁺, NH₄⁺ and SO₄²⁻ ions in aqueous solution, thus having significant acidity and oxidizing properties. This compound plays a crucial role in analytical chemistry, and is often used as a reference substance in redox titration methods, such as for determining the content of reducing substances in samples. Additionally, it is also applied in industrial water treatment, mordanting agents, and as a raw material for the laboratory preparation of other iron compounds. The twelve water molecules in its hydrate play a decisive role in the stability and solubility of its crystal structure.
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Chemical Properties and Physical Characteristics
Dodecahydrate of Ammonium Iron(Sulfate)₂ (chemical formula: NH₄Fe(SO₄)₂·12H₂O) is an inorganic compound belonging to a complex salt category. Its crystal structure is colorless to pale purple transparent octahedron, with a glassy luster, regular shape and stability. This substance exists in a solid state at room temperature, with a melting point range of 39-41°C and a boiling point of 230°C (at which all the crystalline water is lost). Its density is 1.71 g/cm³, and its refractive index is 1.4854, indicating a tightly packed crystal structure and unique optical properties.
In terms of solubility, ammonium ferric sulfate dodecahydrate is highly soluble in water. At 20℃, its solubility reaches 1240 g/L. The solution is yellow to reddish-brown, with a pH value of approximately 1.8 (100 g/L aqueous solution), indicating a strong acidic nature. Due to the hydrolysis of iron ions (Fe³⁺) in the aqueous solution, it may form iron hydroxide precipitates, and it needs to be stored in an acidic condition to avoid decomposition. Additionally, this substance is almost insoluble in organic solvents such as ethanol, and this property makes it an auxiliary reagent for separation or purification in organic synthesis.
Synthesis Methods and Industrial Production
The industrial synthesis of ammonium ferric sulfate dodecahydrate is mainly achieved through a double decomposition reaction. The key steps are as follows:
Raw material preparation: Dissolve ferrous sulfate (FeSO₄·7H₂O) in hot pure water (70-80°C) to form a saturated solution.
Oxidation reaction: Add industrial sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂, concentration 28-30%) to the solution to oxidize ferrous ions (Fe²⁺) to iron ions (Fe³⁺). The reaction endpoint is confirmed by potassium ferrocyanide test (no blue precipitate formation indicates complete oxidation of ferrous).
Crystallization purification: Heat the solution to 70-80°C to remove the remaining hydrogen peroxide, then evaporate and concentrate it to a saturated state. Slowly add ammonium sulfate ((NH₄)₂SO₄) hot solution, stir and cool to 0-10°C (avoiding temperatures above 10°C to prevent large crystals) to precipitate the crude product. The crude product is dissolved in 50°C hot water, and then controlled density (1.28-1.30) and temperature (45-55°C) are used to add a small amount of hydrogen peroxide to further oxidize impurities. Filter and let it stand to crystallize, finally obtaining small particle crystals. The drying process should be carried out at room temperature to avoid loss of crystallization water.
The key parameters of this process include reaction temperature, oxidant dosage and crystallization rate, which need to be precisely controlled to obtain high-purity products (purity can reach over 99%). Industrial-grade products are usually packaged in 25 kg per barrel for easy storage and transportation.
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Core Application Domains
Analytical Chemistry Domain
Halogen determination indicators: In volumetric methods (such as Mohr's method and Fehling's method), ferric ammonium sulfate is used as the endpoint indicator. The iron ions react with thiocyanate ions (SCN⁻) to form a red complex (Fe(SCN)₃), indicating the end point of the titration. For example, in the determination of chloride ions (Cl⁻), an excess of silver nitrate (AgNO₃) titrant reacts with ammonium thiocyanate (NH₄SCN), and the appearance of red indicates the end point.
Standard solution of ferric ions preparation: Its high purity and stability make it an ideal raw material for preparing iron ion standard solutions, which are used for calibrating analytical instruments or as titration reagents.
Industrial Production Domain
Quality control of nitrogen fertilizers: In the production of soda ash (Na₂CO₃), this substance is used to detect the iron ion content in the product to ensure that the purity of nitrogen fertilizers meets the standards.
Analysis of metal salt solutions: By forming precipitates or complexes, impurities such as cyanide (CN⁻) in metal salt solutions can be separated and determined, improving product quality.
Materials Science Domain
Mordanting agents: In the textile printing and dyeing industry, the iron ions can combine with dye molecules to enhance the adhesion of dyes on fibers, improving dyeing fastness and uniformity.
Catalyst carriers: As a precursor of iron-based catalysts, it is used in chemical reactions such as ammonia synthesis and Fischer-Tropsch synthesis to improve catalytic efficiency and selectivity.
Safety Operations and Storage Specifications
Personal Protection
Operators must wear protective gloves, goggles, and a respirator to avoid skin contact or inhalation of dust. If contact with eyes, immediately rinse with plenty of water and seek medical attention; if skin contact occurs, wash with soapy water and change contaminated clothing.
Storage Conditions
The product should be stored in a cool, ventilated warehouse, with a temperature not exceeding 37°C, and kept away from fire sources and heat sources. The warehouse should be equipped with lightning protection equipment, and the exhaust system should have an anti-static grounding device. It is prohibited to store with oxidants or edible chemicals to prevent dangerous reactions.
Emergency Handling
In case of leakage, isolate the leakage source and establish a warning zone. Use sand or inert materials to absorb the leaked substance to prevent it from flowing into sewers or water bodies. When extinguishing the fire, use dry powder, foam, or carbon dioxide extinguishing agents. Water spraying directly is prohibited to prevent the spread of fire.
Market Trends and Price Trends
The market price of dicalcium sulfate ferric ammonium sulfate is significantly affected by purity, packaging specifications, and suppliers. Taking the Chinese market as an example, the price range of industrial-grade products (purity ≥ 99%) is 4-72 yuan per kilogram. The quotations from suppliers in Hubei region are relatively concentrated. For instance, Hubei Jushi Technology Co., Ltd. offers products packaged in 25 kg drums, with a price of 4 yuan per kilogram; while Hubei Qifei Pharmaceutical Chemical Co., Ltd. offers similar products at 72 yuan per kilogram, with the difference mainly due to brand premium and purity level. Additionally, some suppliers offer negotiated prices, which can be negotiated based on the purchase volume, suitable for large-scale procurement needs.
Future Development Direction
With the further refinement of analytical chemistry and industrial production, the application scope of ammonium ferric sulfate dodecahydrate will expand further. For instance, in environmental monitoring, the demand for it as a test reagent for heavy metal ions may increase; in the field of new energy, the research on iron-based catalysts may promote its application as a catalyst carrier. At the same time, the development of green synthesis processes (such as reducing the amount of hydrogen peroxide and optimizing crystallization conditions) will lower production costs and enhance market competitiveness.