A modified equation for the calculation of the stability constants of W(VI)–NTA, Mo(VI)–NTA and Mo(VI)–glutamic acid complexes at different ionic strengths

Abstract

The equilibria between molybdenum(VI) and tungsten(VI) with nitrilotriacetic acid (NTA) and glutamic acid are studied in aqueous solution at 25 °C and different ionic strengths ranging from (0.1 to 1.0) mol dm − 3 (NaClO 4). Stoichiometry and stability of the complexes formed are determined from a combination of potentiometric, polarimetric and spectrophotometric measurements. Tungsten(VI) forms a mononuclear complex with NTA of the type (WO 3L 3−) at pH = 7.5. The complexation of molybdenum(VI) with nitrilotriacetic acid and glutamic acid occurs at pH = 6.0. By introducing two empirical parameters C and D in these three complex formation reactions the dependence of the dissociation and stability constants on ionic strength is described by a Debye–Huckel type equation. Finally a comparison has been made between the calculated and experimental values of stability constants.