Sodium Hexametaphosphate

Brief Overview

Sodium hexametaphosphate (SHMP) is a hexamer with the chemical formula (NaPO₃)₆. It is also called Graham’s salt or hexasodium metaphosphate. It is white and granular in appearance and hydrolyses in an aqueous solution during acidic conditions. It is soluble in water while insoluble in organic solvents. It is a sequestrant that binds with calcium ions.

Manufacturing Process

1. Sodium dihydrogen phosphate method

The soda solution was the first subject to a neutralization reaction with phosphate acid at 80 ~ 100 ℃ for two hours; the resulting sodium dihydrogen phosphate solution was concentrated by evaporation, cooled and crystallized to obtain sodium dihydrogen phosphate dihydrate and heated to 110 ~ 230 ℃ to remove two crystal water; further, be subject to heating to remove the structural water; further being heated to 620 ℃ for dehydration to result in the molten sodium metaphosphate and further polymerized into sodium hexametaphosphate, and discharged; apply chilling shock for 650 ℃ to 60 ~ 80 ℃ for flaking, and crush to obtain the hexametaphosphate products.

Its reaction formula is as below: 

Na₂CO₃ + 2H₃PO₄ + H₂O → 2NaH₂O₄.2H₂O + CO₂

NaH₂PO₄.2H₂O → Na₂H₂PO₄ + 2H₂O

2NaH₂PO₄ → Na₂H₂P₂O₇ + H₂O

Na₂H₂P₂O₇ → 2NaPO₃ + H₂O

6NaPO₃  → (NaPO₃)₆

2. Phosphorus pentoxide method

Put the yellow phosphorus into a steam of dry air for combustion and oxidation. The cooling obtained phosphorus pentoxide is mixed with soda in a certain ratio (Na₂O : P₂O₅ = 1 ~ 1.1). Put the mixed powder in a graphite crucible and heat it indirectly to have it dehydrated and agglomerated. The resulting sodium hexametaphosphate is subject to flaking upon chilling shock and pulverized to obtain industrial hexametaphosphate products.

The reaction formula is as below:

P₄ + 5O₂ → 2P₂O₅

P₂O₅ + Na₂CO₃ → 2NaPO₃ + CO₂

6NaPO₃  → (NaPO₃)₆

Food Industry

Sodium hexametaphosphate is used as a sequestrant that improves food products' quality and stability. It is used under E number E425i. Artificial maple syrup, canned milk, cheese powders and dips, imitation cheese, whipped topping, packaged egg whites, roast beef, fish fillets, fruit jelly, frozen desserts, salad dressing, herring, breakfast cereal, ice cream, beer, and bottled beverages, among other foods, can contain sodium hexametaphosphate.

Other Application

Sodium hexametaphosphate is used for water softening and detergents and as a dispersing agent to break down clay and other soil types. One of the lesser-known uses for sodium hexametaphosphate is as a deflocculant in making terra sigillata, a ceramic technique using a fine particled slip.