THERMAL-BEHAVIOR OF ALKALINE-EARTH METAL CYCLO-TETRAPHOSPHATES

The effect of water vapor on the thermal behavior of alkaline-earth metal cyclo-tetraphosphates, M2P4O12.nH2O (n = 7.0, 3.5, 5.5, and 4.0 for M = Mg, Ca, Sr, and Ba, respectively) (P4m) was examined using DTA-TG, X-ray diffraction analysis, isothermal heating in an electric furnance, and HPLC-FIA. The calcination reaction of each P4m proceeded via ring opening of P4m with elimination of water of crystallization, followed by condensation of soluble oligophosphates (P(oligo)) to insoluble polyphosphates (P(high poly)). The rates of these reactions were faster for Mg2P4m than for other metal cyclo-phosphates. The decomposition of P4m was appreciably faster in humid air than in dry air, because of the availability of water for hydrolysis and for breaking the P-O-P linkage. However, the effect of water vapor from the atmosphere was not conspicuous, since the atmosphere contains enough water (theor. 20 mg L-1) for the hydrolysis of P4m. The products of calcination of Ba2P4m.4H2O were hardly influenced by water vapor from the atmosphere, because the water of crystallization was not easily released in the Ba system compared with the other metal salts.