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Strontium hydroxide

Strontium hydroxide is an inorganic compound with the chemical formula Sr(OH)₂ and a molecular weight of 121.63 g/mol. It exists as a white, deliquescent, tetragonal crystalline solid that is highly soluble in water, forming a strong alkaline solution. The anhydrous form has a density of 3.625 g/cm³, a melting point of 375 °C, and decomposes at around 710 °C.^[1]^[2]^[3] Strontium hydroxide is typically produced by reacting strontium oxide (SrO) with water or by heating strontium carbonate (SrCO₃) or strontium sulfide (SrS) with steam. It often occurs as the octahydrate form, Sr(OH)₂·8H₂O, which has a molecular weight of 265.76 g/mol and a density of 1.9 g/mL. This compound is chemically reactive, acting as a base in neutralization reactions and exhibiting caustic properties due to its hydroxide ions.^[1]^[4] Key applications of strontium hydroxide include refining sugar from beet molasses by forming soluble saccharides, serving as a stabilizer in plastics, glass, and adhesives, and acting as a raw material for ceramics and other strontium salts. It is also used in the production of soaps and greases, as a chemical reagent, and in advanced materials like sol-gel precursors for thin films and nanomaterials in optoelectronics and catalysis. Due to its corrosiveness, it causes severe skin and eye burns and requires careful handling with protective equipment.^[5]^[4]^[1]

Properties

Physical properties

Strontium hydroxide occurs as prismatic, colorless, deliquescent crystals.^[1] The compound exists in anhydrous, monohydrate, and octahydrate forms, with molar masses of 121.63 g/mol, 139.65 g/mol, and 265.76 g/mol, respectively.^[5]^[2]^[6] The density of the anhydrous form is 3.625 g/cm³, while the octahydrate has a density of 1.90 g/cm³.^[1]^[7] The anhydrous form melts at 535 °C and decomposes at 710 °C, whereas the octahydrate melts at approximately 100 °C (375 K).^[1]^[7]

Property	Anhydrous	Octahydrate
Density (g/cm³)	3.625	1.90
Melting point (°C)	535 (decomposes at 710)	100

Strontium hydroxide exhibits increasing solubility in water with rising temperature, with values of 0.41 g/100 mL at 0 °C and 21.83 g/100 mL at 100 °C; it is insoluble in acetone.^[8]

Chemical properties

Strontium hydroxide, Sr(OH)₂, functions as a strong base in aqueous solutions, fully dissociating into Sr²⁺ and 2 OH⁻ ions, which results in highly alkaline conditions with a pH typically ranging from 13 to 14 depending on concentration.^[1]^[9] This dissociation enables its use in neutralization reactions and contributes to its corrosive nature toward acids and certain salts.^[1] The compound exhibits strong reactivity with acids, undergoing neutralization to form the corresponding strontium salt and water; for instance, it reacts with hydrochloric acid according to the equation:

\text{Sr(OH)}_2 + 2 \text{HCl} \rightarrow \text{SrCl}_2 + 2 \text{H}_2\text{O}

^[1] It also reacts vigorously with ammonium salts, such as ammonium chloride, liberating ammonia gas in a double displacement reaction:

\text{Sr(OH)}_2 + 2 \text{NH}_4\text{Cl} \rightarrow \text{SrCl}_2 + 2 \text{NH}_3 + 2 \text{H}_2\text{O}

^[10] This exothermic process is commonly employed to generate ammonia in laboratory settings. Upon heating, strontium hydroxide undergoes thermal decomposition, breaking down into strontium oxide and water vapor, as represented by:

\text{Sr(OH)}_2 \rightarrow \text{SrO} + \text{H}_2\text{O}

This occurs at approximately 710°C for the anhydrous form, with the reaction proceeding more readily in the absence of moisture.^[1] Strontium hydroxide is deliquescent, readily absorbing atmospheric moisture to form hydrated crystals, which underscores its hygroscopic behavior and necessitates storage in dry conditions to prevent degradation.^[1]

Structure and hydrates

Crystal structure

Strontium hydroxide is an ionic compound composed of Sr²⁺ cations and OH⁻ anions organized in a three-dimensional lattice. The anhydrous form adopts an orthorhombic crystal system, characterized by the space group Pnma (No. 62).^[11]^[1] This structure features four formula units per unit cell, with the Sr²⁺ ions positioned in a distorted environment that reflects the compound's ionic bonding nature.^[11] In the crystal lattice, each Sr²⁺ cation is coordinated by seven OH⁻ anions, forming a capped octahedral polyhedron around the metal center. The Sr-O bond distances in this coordination sphere average approximately 2.60 Å, contributing to the overall stability of the anhydrous phase. This sevenfold coordination is influenced by the relatively large size of the Sr²⁺ ion, which allows for an expanded coordination sphere compared to smaller alkaline earth analogs. The lattice parameters of anhydrous Sr(OH)₂ are notably larger than those of calcium hydroxide, Ca(OH)₂, primarily due to the greater ionic radius of Sr²⁺ (1.18 Å in sixfold coordination) relative to Ca²⁺ (1.00 Å in sixfold coordination). While Ca(OH)₂ exhibits a trigonal structure with sixfold coordination, the orthorhombic arrangement in Sr(OH)₂ accommodates the increased interionic distances, highlighting the role of cation size in dictating structural motifs among group 2 hydroxides.^[12]

Hydrated forms

Strontium hydroxide forms several hydrated phases, with the monohydrate and octahydrate being the most prominent. The octahydrate, Sr(OH)₂·8H₂O, is stable at room temperature under normal conditions and crystallizes from aqueous solutions. It exhibits a density of 1.90 g/cm³ and dehydrates at approximately 375 K.^[2] The monohydrate, Sr(OH)₂·H₂O, is stable above approximately 85 °C (358 K), though it can partially decompose to the anhydrous form upon prolonged evacuation or heating. It is typically obtained through hydration processes conducted at relatively higher temperatures compared to the octahydrate.^[13] The octahydrate, Sr(OH)₂·8H₂O, represents the form stable in hydrated environments up to 85 °C. It crystallizes in the tetragonal space group P4/ncc.^[14] In its crystal structure, the Sr²⁺ ions are coordinated by eight water molecules in a square antiprism arrangement, forming double layers separated by hydroxide ions along the c-axis, with extensive hydrogen bonding stabilizing the lattice.^[14] Phase transitions between these hydrates occur with temperature changes, reflecting their thermal stability ranges. The octahydrate is the stable phase up to approximately 85 °C (358 K); above this temperature, the monohydrate is favored, losing its water content around 110 °C to yield the anhydrous form; the anhydrous form predominates above 110 °C.^[15]^[13] These transitions highlight the compound's sensitivity to hydration levels and thermal conditions, influencing its practical handling.

Preparation

Laboratory methods

Strontium hydroxide can be prepared in the laboratory by precipitation from soluble strontium salts, such as strontium nitrate, using a strong base like sodium hydroxide. The reaction proceeds as follows:

\mathrm{Sr(NO_3)_2 + 2 NaOH \rightarrow Sr(OH)_2 \downarrow + 2 NaNO_3}

This method involves dissolving strontium nitrate in water and slowly adding an aqueous solution of sodium hydroxide until precipitation is complete, resulting in fine white crystals of strontium hydroxide.^[2] The precipitate is then filtered, washed with cold water to remove sodium nitrate impurities, and dried.^[8] An alternative laboratory synthesis involves the hydration of strontium oxide with water. The reaction is:

\mathrm{SrO + H_2O \rightarrow Sr(OH)_2}

Strontium oxide is typically exposed to water vapor under controlled humidity conditions or added cautiously to liquid water, as the reaction is exothermic. This approach is suitable for small-scale preparations and yields strontium hydroxide directly, though care must be taken to avoid excessive heat buildup.^[1] Purification of the crude strontium hydroxide, often obtained as the octahydrate, can be achieved by recrystallization from hot water to isolate the monohydrate form, Sr(OH)₂·H₂O. The solid is dissolved in approximately 2.2 mL of hot water per gram and the solution cooled to 0°C, promoting crystallization of the purified monohydrate.^[1]^[13] Yields for the precipitation method are near quantitative due to the low solubility of strontium hydroxide in cold water (0.41 g/100 mL at 0°C), though filtration is essential to separate the product from soluble byproducts and ensure high purity.^[2] The hydration method also provides high yields but may require additional drying steps to achieve the desired hydrate form.^[8]

Industrial production

Strontium hydroxide is produced industrially primarily by the hydration of strontium oxide, which is obtained by reductive roasting of celestite ore (SrSO₄) with a reducing agent such as coke at 1000–1350 °C, converting the sulfate to oxide while releasing reduced sulfur compounds such as SO₂. The resulting strontium oxide is then hydrated with water under boiling conditions (around 90 °C) or under pressure at elevated temperatures up to 400 °C: SrO + H₂O → Sr(OH)₂. This method allows for efficient extraction from the primary mineral source, celestite, which is abundant but requires beneficiation to >90% purity prior to processing.^[16] An alternative industrial route involves heating strontium carbonate (SrCO₃) or strontium sulfide (SrS) with steam at temperatures around 500–600 °C:

\mathrm{SrCO_3 + H_2O \rightarrow Sr(OH)_2 + CO_2}

This process directly yields strontium hydroxide and is used in some facilities processing strontium compounds.^[1]^[8] Strontium hydroxide is generally manufactured as a byproduct during broader strontium compound processing, with global annual output on the order of thousands of tons, reflecting its niche role compared to dominant products like strontium carbonate. U.S. imports of strontium oxide, hydroxide, and peroxide totaled about 67 metric tons in 2019, with overall imports of strontium compounds (in strontium content) estimated at 3,700 metric tons in 2023, indicating limited large-scale domestic production.^[17]^[18] Production costs are influenced primarily by the supply of celestite ore, which accounts for much of the raw material expense, and the energy-intensive steps involved in high-temperature roasting and pressurized hydration. Energy costs for calcination and reduction can represent a significant portion of overall expenses, particularly in regions with volatile fuel prices.^[17]

Applications

Sugar refining

Strontium hydroxide plays a key role in the purification of beet sugar molasses by precipitating impurities such as proteins and pectins, facilitating the recovery of higher-purity sucrose. In the Strontian process, also known as the Scheibler process, strontium hydroxide is added to the molasses to form insoluble strontium salts with these organic impurities, which are then removed through filtration, leaving a clearer solution for further sugar extraction.^[1]^[19] The process involves heating the molasses with an excess of strontium hydroxide solution (typically 20-25%) to above 100°C, promoting the formation of granular precipitates. After filtration and washing, the strontium is recycled by treating the precipitates with carbon dioxide to form strontium carbonate, which is subsequently calcined and redissolved in water to regenerate the hydroxide for reuse. This recycling step minimizes material losses and makes the process economically viable despite the higher cost of strontium compared to alternatives.^[19]^[20] Adopted widely in European refineries since the late 19th and early 20th centuries, particularly in Germany, the method improved sucrose yields by enabling direct production of marketable pure sugar from molasses, outperforming earlier lime-based techniques that required additional cooling and osmosis steps. The process maintains alkaline conditions, typically controlling pH to 10-11, to optimize precipitation without excessive sugar degradation. Overall, it enhances sugar recovery efficiency relative to lime alone by avoiding prolonged processing times and reducing non-sugar residues in the final product. The Strontian process, though innovative for its time, has largely been supplanted by more economical calcium hydroxide-based methods in contemporary sugar production.^[19]^[21]^[20]

Other industrial uses

Strontium hydroxide serves as a stabilizer in the production of plastics, particularly polyvinyl chloride (PVC) and polyethylene, where it neutralizes acidic degradation products formed during processing, thereby extending the material's shelf life and thermal stability.^[22]^[23] As a source of strontium ions, it is employed in the manufacture of ceramics and glass, where it contributes to enhancing optical properties, hardness, and light refraction in specialized formulations.^[24]^[25] In pyrotechnics, strontium hydroxide acts as a precursor for colorants that produce vivid red hues in fireworks and flares.^[26] In the production of soaps and adhesives, strontium hydroxide functions as a saponifying agent, reacting with fatty acids to form metallic soaps that improve texture and adhesion properties.^[1]^[27] Strontium hydroxide is utilized in the paint and oil industries to accelerate the oxidation of drying oils, promoting faster film formation and enhancing the durability of coatings.^[28]^[25] It is used in the formulation of high-performance lubricants and greases, where it aids in producing strontium-based thickeners for improved mechanical stability under extreme conditions.^[27] Additionally, as a precursor for strontium salts, it supports the development of electronic components, including ceramics for solid oxide fuel cells and other oxide materials in advanced devices.^[29]^[30]

Safety

Health hazards

Strontium hydroxide is a strong base with caustic properties that can cause severe chemical burns upon direct contact with skin, eyes, and mucous membranes due to its alkaline nature and ability to generate heat through exothermic reactions with moisture.^[5] Skin exposure typically results in redness, pain, and blistering, potentially leading to necrosis if not promptly treated, while eye contact may cause immediate and irreversible damage, including corneal opacity and vision loss.^[31] These effects stem from the compound's pH exceeding 12 in solution, promoting tissue saponification and protein denaturation.^[32] Inhalation of strontium hydroxide dust or mist irritates the respiratory tract, causing coughing, shortness of breath, and inflammation of the upper airways, with potential progression to pulmonary edema in severe cases due to caustic damage to lung tissues.^[33] Ingestion is highly hazardous, leading to corrosive injury throughout the gastrointestinal tract, manifesting as severe abdominal pain, vomiting, diarrhea, and possible perforation or hemorrhage; the absorbed strontium ions mimic calcium in physiological processes, potentially disrupting bone metabolism by incorporating into hydroxyapatite and interfering with mineralization at elevated exposure levels.^[5]^[34] Toxicity assessments indicate moderate acute oral toxicity, with an LD50 greater than 2,000 mg/kg in rats, reflecting primarily local corrosive effects rather than systemic poisoning.^[32] Under the Globally Harmonized System (GHS), it is classified as a skin corrosion/irritation Category 1 substance and causes serious eye damage (Category 1), with additional warnings for acute toxicity if swallowed (Category 4).^[5] Long-term exposure may result in strontium accumulation in bones, potentially altering skeletal development in vulnerable populations such as children, though stable strontium exhibits low overall systemic toxicity compared to more hazardous analogs like barium hydroxide, with no evidence of carcinogenicity or reproductive harm at typical occupational levels.^[34]^[32]

Handling and storage

Strontium hydroxide should be handled in well-ventilated areas or under a chemical fume hood to minimize dust formation and inhalation risks, with operators wearing appropriate personal protective equipment including chemical-resistant gloves, safety goggles, protective clothing, and a NIOSH-approved respirator if dust levels may exceed safe thresholds.^[35]^[36]^[31] For storage, keep the compound in tightly sealed, airtight containers made of compatible materials in a cool, dry, well-ventilated area away from incompatible substances such as acids, strong oxidizers, and sources of moisture, as it is deliquescent and can react exothermically.^[35]^[31]^[36] In the event of a spill, evacuate non-essential personnel, ventilate the area, and use personal protective equipment to sweep up the material without generating dust, placing it into suitable sealed containers for disposal while avoiding contact with water to prevent heat generation from potential reactions.^[35]^[36]^[31] As a corrosive hazardous material, strontium hydroxide must be managed in accordance with OSHA Hazard Communication Standard (29 CFR 1910.1200) and equivalent ECHA regulations, including proper labeling of containers with GHS pictograms for skin corrosion and eye damage.^[35]^[36] Disposal involves neutralizing the waste with a dilute acid under controlled conditions if feasible, followed by treatment as non-hazardous after pH verification, and ultimate disposition at an approved facility per local, state, and federal regulations to prevent environmental release.^[35]^[36]^[31]

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