STABILITY-CONSTANTS OF VANADIUM(V), NICKEL(II) AND IRON(II) CHELATES WITH PYRIDYLAZOPHENOL DERIVATIVES

Acid dissociation constants of 2-(2-pyridylazo)phenol (PAP) and 2-(2-pyridylazo)-5-methylphenol (PAP-5Me) and the stability constants of their chelates with vanadium(V), nickel(II), and iron(II) were determined by spectrophotometry at 293 K and mu = 0.1. The acid dissociation constants were; pK(a1)(PAP) = 2.53, pK(a2)(PAP) = 8.76, pK(a1)(PAP-5Me) = 2.67 and pK(a)2 (PAP-5 Me) = 8.95. Since iron(II)-PAP chelate was insoluble in water, its stability constant in water was determined by an extrapolation of stability constants in aqueous dioxane. Other stability constants were detemined directly in water. They were; log beta1 {V(V)-PAP} = 12.43, log beta-1 {Ni(II)-PAP} = 9.79, log beta-2 {Ni(II)-PAP} = 21.78, log beta-1 {Fe(II)-PAP} = 11.95, log beta-2 {Fe(II)-PAP} = 24.45, log beta-1 {V(V)-PAP-5Me} = 12.87, log beta-1 {Ni(II)-PAP-5Me} = 10.67, log beta-2 {Ni(II)-PAP-5Me} = 22.17, log beta-2 {Fe(II)-PAP-5Me} = 25.51.