First of all, you may be wondering two things...what is sodium hexametaphosphate? And just what is a dispersing agent?
To start, sodium hexametaphosphate (SHMP) is a hexamer of composition (NaPO3)6.Sodium hexametaphosphate of commerce is typically a mixture of polymeric metaphosphates, of which the hexamer is one, and is usually the compound referred to by this name. It is more correctly termed sodium polymetaphosphate. It is prepared by melting monosodium orthophosphate, followed by rapid cooling.
Sodium hexametaphosphate (SHMP) is used as a sequestrant and has applications in a wide variety of industries, including as a food additive in which it is used under the E number E452i. Sodium carbonate is sometimes added to SHMP to raise the pH to 8.0-8.6, which produces a number of SHMP products used for water softening and detergents. Also used as a dispersing agent to break down clay and other soil types.
What is a dispersing agent, or deffloculating agent? A dispersing agent prevents flocculation, or the combining of suspended matter into aggregates large enough for gravity to accelerate their settling out.
Divalent and trivalent cations such as sodium hexametaphosphate are used in water solutions to facilitate flocculation of insoluble particles in suspension, especially colloidal size particles like clay and organic matter that have negative surface charges. Addition of divalent or trivalent cations is frequently used in water treatment to remove a combination of inorganic and organic matter as well as living microscopic organisms through flocculation followed by filtration.
Dispersing agents such as sodium hexametaphosphate are more commonly used in laboratory procedures to sustain suspensions and estimate particle size distribution. They are commonly used as a component of detergents, for keeping pigments dispersed in paints and in photography where they find special applications to prevent spotting of films and photographs. While you can't see it SHMP is hiding just beyond the shadows of your drinking water and likely in the paint you have used in your house.