Removing Micronutrient Metal Cation Interferences Prior to Titrimetric Determination of Polyphosphate Chain Length

Evaluation of chain lengths of polymeric phosphates is essential for the characterization of polyphosphate-based fertilizers. The titrimetric method, which is the most reliable and simple means of determining number average chain lengths ( n̄ ), cannot be applied to solutions containing micronutrient ions. In this investigation, methods have been proposed for the elimination of metal ion interference prior to the titrimetric determination of the average chain length of polyphosphate solutions. When Cu 2+, Fe 3+, Mn 2+, Mg 2+, or Mo 6+ are present in the solutions, solvent extraction of their complexes with 8-hydroxyquinoline at appropriate pH, prior to titration, was very effective. The interference due to Zn 2+ was conveniently masked by the addition of potassium ferrocyanide and the titration carried out in the presence of the insoluble complexes. These methods do not introduce any perceptible additional error in the determination of n̄ . They can also be conveniently combined when more than one interfering ion is present in the polyphosphate solution. These methods, therefore, offer a rapid, simple, and reliable means for analyzing micronutrient polyphosphate fertilizers.