Pyro- and Tripolyphosphates and Citrates as the Complexing Agents for Micronutrients in Liquid Fertilizers

Abstract

The conditional stability constants of tripolyphosphate, pyrophosphate, and citrate complexes of Zn2+, Mn2+, Co2+, and Fe2+ as function of pH have been determined. The polarographic method was employed in these studies, and the ionic strength of the solution used was 0.5 mol/dm3. P3O105- complexes with the respective micronutrients were found to show higher conditional stability constants more often than P2O74- and citrate complexes. In comparison with complexes containing other cations, conditional stability constants of complexes with zinc studied in this work showed higher values. Distinct from other micronutrients, Cu2+ ions were observed as significantly accelerating the hydrolysis process of P3O105- in a range of pH from 4.5 to 10.0. Hence, the degree of hydrolysis increased together with the pH value of their solutions. In the case of other micronutrients, the degree of hydrolysis of P3O105- was lower for higher pH values of their solutions. Additionally, it was stated that the structure of Fe2+ tripolyphosphate complexes differs from complexes with other central ions. The preparation conditions of liquid fertilizers stable over long periods of time, containing macroelements and micronutrients in the complexing form by polyphosphates, have been elaborated. Stability of the solutions depended on the content of individual components, pH, and kind of complexing agent. It remained closely related to stability constants of complexes.