Polyphosphoric acids, ammonium salts

Water solubility

Description of key information

The water solubility of ammonium polyphosphate was determined according to EU Method A.6 (Water solubility) and under the conditions of GLP. 

Additional information

The water solubility of the test item has been determined to be greater than 50% w/w of solution at 20.0 ± 0.5 °C. On evaluation of the influence of pH on solubility at pH 4 and pH 9, this highly soluble, inorganic salt was demonstrated, on dissolution, to exceed the buffering capacity of buffer solutions and as such, solubility and final solution pH values were essentially identical to those achieved using unbuffered water.

On preparation of test item and solvent mixtures for purified water, pH 4 buffer solution and pH 9 buffer solution, over a nominal concentration range of 10% w/w to 60% w/w, all samples remained apparently saturated with a visible, undissolved excess remaining. However, the quantity of undissolved material in these initial preliminary visual assessment samples did not increase significantly on increasing the initial saturation concentration from 10% w/w to 60% w/w. Therefore it was suspected that the visual assessment of solubility was influenced by a minor insoluble impurity and/or residue. On repeating the procedure and filtering the resulting solutions for analysis, the analysed test item concentrations for the sample solution filtrates remained very similar to the theoretical prepared concentrations at which each sample solution was initially saturated. This allowed the undissolved residues to be confidently attributed to a minor component or components and/or impurity or impurities within the complex mixture of which the test item is composed. The majority of the test item could therefore be confirmed as being highly soluble.