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Solid-State Decomposition Kinetics of Phenyl Phosphorodiamidate¹

ABSTRACT

Kinetic studies were conducted to determine the effect of temperature, phenyl phosphorodiamidate (PPDA) production impurities, and relative humidity on the solid-state decomposition of the urease inhibitor, PPDA, alone and in urea-PPDA mixtures (960 g kg^–1 of urea and 40 g kg^–1 of PPDA). In general, the solid-state decomposition of pure PPDA obeys first-order reaction kinetics, whereas the decomposition of the urea-PPDA mixtures obeys zero-order kinetics. Rate constants (d^–1) for pure PPDA ranged from 0.05 at 70°C to 4.78 at 100°C. Rate constants (decomposition d^–1) for urea-PPDA mixtures ranged from 7.26% at 50°C to 1200% at 90°C. The activation energy (kJ mol^–1) is 164 for pure PPDA and 127 for urea-PPDA mixtures. The presence of production impurities, ammonium chloride (NH₄Cl) and diphenyl phosphoramidate (C₆H₅O)₂·PO·NH₂, increased the decomposition rate for pure PPDA, but did not significantly affect the decomposition of the urea-PPDA mixtures. Increasing relative humidity dramatically increased the decomposition rate for PPDA. These results suggest that researchers should minimize the time between preparation and testing of urea-PPDA mixtures, because at room temperature over half of the PPDA can be lost in 1 yr.

¹ Contribution from the National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, AL 35660.

² Research Chemist, Co-op Student, Co-op Student, and Research Chemist, respectively, National Fertilizer Development Center, TVA, Muscle Shoals, AL.

Received for publication August 6, 1985.