Sodium polyphosphate

Ignition of the residue left on evaporation of a solution of sodium hydroxide or carbonate with sufficient phosphoric acid gives a vitreous polyphosphate in an impure condition. It dissolves slowly in cold water, but more rapidly on warming. The solution is almost neutral to methyl-orange and to phenolphthalein. It has been found impossible to purify the substance or determine its composition, on account of its energetic action on vessels of glass, porcelain, silica, nickel, and platinum, but it probably has the formula Na₄P₆O₁₇.