The various phosphate forms are all made from the same basic compound, phosphoric acid. Phosphoric acid is produced from phosphate rock which is mined and processed. There are two typical processes used to produce phosphoric acid, wet processing or (dry) furnace processing.   The wet process uses sulfuric acid to digest phosphate rock and then precipitates and filters the impurities. Most of the material produced by this method goes into the fertilizer industry.   The furnace process heats a high purity phosphate rock inside an electric furnace to produce a phosphorous pentoxide which is then collected and dissolved in water or a dilute solution of phosphoric acid to produce a stronger phosphoric acid. This high purity method is the basic building block for the production of orthophosphate used for potable applications.   Under a controlled sequence of temperature and time, salts are carefully added to make condensed phosphates or what are termed polyphosphates. During condensation, water is driven off.  This is sometimes referred to as dehydration.   Condensed phosphates are all comprised of multiple phosphates. Sodium and Potassium are the most common salts added. At 300-400 degrees F, orthophosphate is condensed to form pyrophosphates. Pyrophosphates are the most basic polyphosphate with only two phosphates in the chain.   At 400-500 degrees F, if used with the correct time, pyrophosphate will condense to tripolyphosphate.   Above 1200 degrees F. tripolyphosphate will condense to hexametaphosphate and even higher metaphosphates.   Polyphosphates in general provide many functions. They control iron and manganese, soften water and remove scale.  They are known as sequestering agents or chelating agents which form soluble complexes without precipitation.   Usually, iron, manganese, and calcium require a 1:1 mole ratio with phosphate in order to be sequestered. However, polyphosphates also function as crystal modifiers which use only a fraction of that ratio to effectively modify the crystalline structure of divalent scale forming compounds.   By modifying the structure, these compounds will not precipitate into scale and actually will stay in solution (they repel each other keeping them in suspension). In theory, SHMP (sodium hexametaphosphate) requires 500mg/L to sequester 200 mg/L of calcium as calcium carbonate. However, only 2 to 4 mg/L of SHMP is used to modify crystalline growth.   Polyphosphates serve three functions in municipal systems. Sequestering agents Crystal modifiers Corrosion inhibitors