Sodium hexametaphosphate ( sodium cyclo-hexaphosphate ) SHMP [ CAS: 68915-31-1 ]

Sodium Hexametaphosphate

Other Names: SHMP, Sodium polymetaphosphate, Hexasodium metaphosphate, Graham's salt
Chemical Formula: (NaPO₃)₆ (idealized cyclic hexamer: Na₆P₆O₁₈)
Molar Mass: 611.77 g/mol (for Na₆P₆O₁₈)
CAS Number: 10124-56-8
Appearance: White powder, granules or glassy solid
Odor: Odorless
SMILES: [Na+].[Na+].[Na+].[Na+].[Na+].[Na+].[O-]P1(=O)OP([O-])(=O)OP([O-])(=O)OP([O-])(=O)OP([O-])(=O)OP([O-])(=O)O1

Sodium hexametaphosphate is an inorganic condensed phosphate commonly abbreviated as SHMP. Although the name is traditionally associated with the cyclic hexamer (NaPO₃)₆, commercial sodium hexametaphosphate is generally a glassy mixture of polymeric sodium metaphosphates rather than a single, structurally uniform hexamer. The phosphate units are connected through P–O–P linkages, producing condensed phosphate structures whose chain length and composition can vary. In aqueous systems these condensed phosphates can undergo hydrolysis, progressively forming shorter-chain phosphate species.

SHMP is particularly important in colloid and surface chemistry because of its ability to adsorb onto mineral surfaces, modify surface charge and interact with multivalent metal ions. Experimental and computational studies demonstrate that adsorption of polyphosphate species can increase the negative surface charge of mineral particles and consequently enhance electrostatic repulsion between them. These properties explain its effectiveness as a dispersing and deflocculating agent in many inorganic suspensions.

Chemical and physical properties

Sodium hexametaphosphate is normally encountered as a white powder, granular material or transparent glassy solid. It is readily soluble in water, where it produces polyphosphate-containing solutions. The exact physicochemical behavior of commercial material depends on phosphate-chain distribution, concentration, pH, temperature and the presence of dissolved metal ions.

In aqueous solution, condensed phosphate structures are not indefinitely stable. Hydrolytic cleavage of P–O–P bonds produces progressively shorter phosphate chains and ultimately simpler phosphate species. Consequently, the chemical composition of an SHMP solution may evolve with time and depends strongly on solution conditions.

A major physicochemical characteristic of SHMP is its interaction with positively charged sites on mineral surfaces and with dissolved multivalent cations. Studies of kaolinite have shown that polyphosphate species adsorb on surface sites and increase the magnitude of the negative surface charge. The resulting increase in electrostatic repulsion reduces particle aggregation and promotes dispersion.

Studies involving calcium-containing systems have additionally demonstrated complex formation between SHMP-derived phosphate species and Ca²⁺. Such interactions can alter particle zeta potential, adsorption behavior and suspension rheology. Similar adsorption and ion-interaction phenomena have been investigated for illite, montmorillonite and other mineral phases.

Applications

Sodium hexametaphosphate is widely investigated and applied as an inorganic dispersant and deflocculant. In concentrated clay suspensions, adsorption of polyphosphate species modifies particle surface charge and reduces attractive interactions between particles, thereby influencing viscosity, flow behavior and suspension stability.

In ceramic and mineral-processing systems, SHMP is used to control the dispersion of fine mineral particles. Research on kaolinite, illite and montmorillonite demonstrates that its effectiveness is associated with phosphate adsorption, modification of electrostatic interactions and complexation of dissolved multivalent ions.

SHMP is also used in mineral separation processes, where its adsorption on mineral surfaces can modify particle–particle interactions. Studies of kaolinite-containing systems show that increasing negative surface charge can enhance electrostatic repulsion and limit undesirable aggregation or surface coating by fine mineral particles.

In cementitious and other inorganic suspensions, SHMP can function as a dispersing and rheology-modifying additive. Experimental work with calcium aluminate cement has demonstrated that interactions between phosphate species and Ca²⁺, together with adsorption on particle surfaces, can change zeta potential, reduce slurry viscosity and influence hydration kinetics.

Because of its strong interactions with mineral surfaces and metal ions, sodium hexametaphosphate is also a useful research reagent in colloid chemistry, surface science and materials science. It is employed in studies of adsorption, zeta potential, particle dispersion, mineral–solution interfaces, rheological behavior and polyphosphate hydrolysis.

Scientific references

Han, Y.; Liu, W.; Zhou, J.; Chen, J. “Interactions between kaolinite Al–OH surface and sodium hexametaphosphate.” Applied Surface Science, 2016, 387, 759–765. DOI: 10.1016/j.apsusc.2016.07.002.

Andreola, F.; Castellini, E.; Ferreira, J.M.F.; Olhero, S.; Romagnoli, M. “Effect of sodium hexametaphosphate and ageing on the rheological behaviour of kaolin dispersions.” Applied Clay Science, 2006, 31, 56–64. DOI: 10.1016/j.clay.2005.08.004.

“Sodium hexametaphosphate interaction with 2:1 clay minerals illite and montmorillonite.” Applied Clay Science, 2013, 83–84, 162–170. DOI: 10.1016/j.clay.2013.08.031.

“Thermodynamic aspects of the adsorption of hexametaphosphate on kaolinite.” Journal of Colloid and Interface Science, 2005/2006. DOI associated with the published study: 10.1016/j.jcis.2005.07.019.

“Effects of Sodium Hexametaphosphate Addition on the Dispersion and Hydration of Pure Calcium Aluminate Cement.” Materials, 2020, 13, 5229.

Safety

Signal Word: None required under GHS
GHS Classification: Not classified as hazardous under standard conditions

General Hazards:

• Dust may cause mechanical respiratory and eye irritation

• Prolonged skin contact may cause dryness or mild irritation

Precautionary Measures:

• Storage: Store in a cool, dry, and well-ventilated area. Avoid exposure to moisture

• Handling: Use gloves, safety goggles, and a dust mask when handling bulk quantities

• First Aid:

  • Eye Contact: Rinse with water for several minutes

  • Skin Contact: Wash with soap and water

  • Inhalation: Move to fresh air if irritation occurs

  • Ingestion: Rinse mouth and seek medical advice if large amounts are consumed

Environmental Considerations

Sodium hexametaphosphate contains phosphorus, which may contribute to eutrophication in aquatic environments if released in excessive quantities. Disposal should follow local environmental regulations to minimize impact on surface waters.