Effect of Ligands on the Redox Reaction of Metal Ions and the Use of a Ligand Buffer for Improving the End-Point Detection in the Potentiometric Titration of Vanadium(V) with Iron(II)

The effect of complexing agents on the titration of vanadium(V) with iron(II) was studied. Complexing agents which form more stable complexes with iron(III) than iron(II) produce a sharper titration of vanadium(V) with Iron(II). Citrate, pyrophosphate and EDTA improve the potential break at the equivalence point. The presence of excess EDTA decreases the formal potential of the V(V)/V(IV) couple due to the formation of vanadium(V)-EDTA complexes. The use of a ligand buffer with zinc(II) in excess over EDTA is very effective for improving the end-point detection in the titration, preventing the formation of vanadium(V)-EDTA complexes.