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Sodium Metaphosphate NaPO3 is an inorganic compound with the molecular formula (NaPO3)n. It is a high-molecular-weight sodium polyphosphate composed of two long-chain metaphosphates with opposite helical directions and a common axis. The Na2O/P2O5 ratio is about 1.0. It is colorless transparent crystal or white powder. It is easy to absorb moisture in the air. Soluble in water and inorganic acids, the aqueous solution is alkaline, insoluble in ethanol and organic solvents. It can be obtained by high-temperature melt polymerization of sodium dihydrogen phosphate. It is used for the softening of boiler water and the separation and purification of certain heavy metal ions. Food grade Sodium Metaphosphate NaPO3 can be used as food emulsifier, chelating agent and tissue modifier.

Accurately weigh about 800mg of the sample, transfer it into a 500ml volumetric flask, add 100ml of water and 25ml of nitric acid, and boil on a heating plate for 10min. After cooling, dilute to volume with water and mix well. Pipette 20.0ml of this solution into a 500ml Erlenmeyer flask, add 100ml of water, and heat to just boiling. While stirring, add 50ml of quinolimonone test solution (TS-202), cover with a watch glass, and boil for 1min in a well-ventilated cabinet. Cool to room temperature under constant stirring, and filter through a Gooch crucible (or a medium-porosity porous glass crucible) with constant weight, and wash 5 times with 25 ml of water. Dry at about 225°C for 30 minutes, weigh after cooling. Each mg of precipitate is equivalent to 32.074 μg of phosphorus pentoxide (P205).