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The presence of Al3+, as an impurity, on the temperature of the polymorphic transformation of pentasodium tripolyphosphate Na5P3O10-II (low-temperature Form-II) into Na5P3O10-I (high-temperature Form-I) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The effect of Al3+ on the phase transformation was recorded between ignition temperatures of 350 and 450 °C. Al-free powders, ignited up to 450 °C, did not contain Form-I, while powders containing 0.5 or 1.0 wt % of Al were mixtures of Form-I/Form-II indicating that the presence of Al acted in favor of Form-I formation. Phase composition at temperatures of ignition higher than 500 °C was not dependent on Al presence. Na5P3O10 in all of the products ignited at 500−550 °C, whether Al-free or containing Al, was in Form-I. The presence of Al caused lower crystallinity and inhibited the grain growth of pentasodium tripolyphosphate while powder was ignited. Both phenomena are believed to act in favor of phase transformation at lower ignition temperatures.

The effect of Fe 3+ and Al 3+ , usually present as impurity ions in wet-process phosphoric acid, on formation and polymorphic transformation of sodium tripolyphosphate (STPP ) Na 5 P 3 O 10 ) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The simultaneous appearance of both high-(Form-I) and low-(Form-II) temperature modifications at the beginning of STPP formation at the temperature range of 250-300 °C in the impurity-free as well as in Al-and Fe-containing samples was recorded. Form-I in the impurity-free STPP, containing both crystallographic forms, underwent low-temperature transformation into Form-II around 350 °C, while Form-II was stable up to 450 °C and transformed into high-temperature Form-I above 450 °C. Ignition of the impurity-containing STPP revealed that Al 3+ and Fe 3+ stabilized Form-I so that the low-temperature transformation (Form-I f Form-II) did not occur, while Form-II transformed into high-temperature Form-I at a lower temperature (below 450 °C) than in the impurity-free STPP temperature.

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