Polyphosphate-fertilizer solution stability with time, temperature, and pH

Ammonium polyphosphate fertilizers are gaining popularity in agricultural industry due to ease of application and yield benefits in calcareous soils. In this study, the effects of temperature, pH, and time on the stability of ammonium polyphosphate-fertilizer solution simulating a range of storage conditions were evaluated. Ion chromatography was used for the speciation of orthophosphate, pyrophosphate, and tripolyphosphate in the polyphosphate-fertilizer solution over time. Polyphosphate solutions were very stable when the pH was maintained close to neutral (pH 6.4) and the temperature was less than 25 degrees C. In contrast, at the lowest pH (2.3) and highest temperature (50 degrees C) almost all tripolyphosphate and 96% of pyrophosphate was hydrolyzed after 28 d. The hydrolysis rate constant for tripolyphosphate at 50 degrees C was calculated to be 9.2 x 10(-7) s(-1) and the half-life 20 d. At 25 degrees C, the half-life of tripolyphosphate was 34 d at pH 2.3 and 174 d at pH 5.4. The activation energy at pH 2.3 was 12.7 kJ mol(-1). The results demonstrate that increasing temperature and decreasing pH have a deleterious effect on the stability of condensed P species in polyphosphate fertilizer. The effect of acidification on polyphosphate-fertilizer composition requires consideration when formulating mixed ammonium polyphosphate blends with acids and trace elements for application in the field.