Consider the white scale accumulating inside water pipes over time, the stubborn metal ion deposits in industrial equipment, or the various challenges in food preservation. These seemingly disparate problems may share a common solution—sodium hexametaphosphate (SHMP).
Sodium hexametaphosphate, also known as polyphosphate, is a widely used inorganic compound with the molecular formula (NaPO₃)ₓ (where x ≥ 6). Its CAS registry number is 10124-56-8. At room temperature and pressure, pure SHMP appears as a white, odorless glassy solid or crystalline powder. It is water-soluble though dissolves relatively slowly, with its aqueous solution showing a tendency to hydrolyze, particularly under high temperatures or acidic conditions. The pH of its solution typically ranges between 6.0-7.0 (1% solution).
- Molecular weight: As a polymeric phosphate, SHMP's molecular weight varies with its degree of polymerization (x value). Industrial-grade SHMP typically has an average molecular weight of approximately 611.77 g/mol.
- Melting point: 628°C (1,162°F; 901 K)
- Density: ~2.484 g/cm³
- Solubility: Readily soluble in water with increasing solubility at higher temperatures. Hydrolyzes in water, gradually releasing phosphate ions.
- Stability: Relatively stable in dry conditions but hygroscopic in humid environments. Hydrolyzes faster under high temperatures or acidic conditions, decomposing into orthophosphates.
Industrial production primarily employs two methods:
Sodium dihydrogen phosphate (NaH₂PO₄) is heated to 600-700°C for dehydration and polymerization. The molten product is then cooled and crushed. This method offers simplicity and lower costs but yields relatively lower purity.
Phosphorus pentoxide (P₂O₅) reacts with sodium carbonate (Na₂CO₃) in water, followed by evaporation, crystallization, and drying. This higher-cost method produces high-purity SHMP.
SHMP serves as a crucial water treatment agent, chelating calcium and magnesium ions to prevent scale formation and soften water. It also disperses suspended solids and inhibits pipe corrosion, making it valuable in industrial cooling systems, boiler water treatment, and reverse osmosis membrane antiscalants.
As a detergent builder, SHMP softens water by sequestering hardness ions, enhancing surfactant performance and preventing soil redeposition on fabrics.
Approved as a multifunctional food additive, SHMP serves as:
- Water retention agent: Improves moisture retention in meat products
- Emulsifier: Stabilizes oil-water mixtures
- Chelator: Prevents metal-induced discoloration and spoilage
- pH regulator: Adjusts acidity for improved stability
Used in meat, dairy, beverages, and baked goods, its application levels are strictly regulated worldwide.
Additional applications include:
- Textiles: Dyeing auxiliary for improved color fastness
- Petroleum: Drilling mud dispersant and viscosity reducer
- Ceramics/Paper: Slurry dispersant for improved processing
- Metal treatment: Cleaning and corrosion inhibition
Extensive studies confirm SHMP's safety within prescribed limits, though excessive intake may cause digestive discomfort. Global regulations, including China's GB 2760-2014 and equivalent EU/US standards, strictly govern its usage across industries.
- Store in dry, cool, ventilated areas
- Avoid contact with strong acids, bases, or oxidizers
- Use protective equipment during handling