Composition and stability constant of ferrous-bathophenanthroline complex

Abstract

Although the red colored complex of ferrous ion and bathophenanthroline is used extensively for the determination of iron, the composition and the stability constant thereof have hitherto been left undetermined. In the present paper, the spectrophotometric study on the ferrous-bathophenanthroline complex in 10% ethanol solution has been carried out. The composition of the complex is determined by both the continuous variation method and Molland's method. Since the combining ratio of ferrous ion and bathophenanthroline is one to three in buffered solutions of pH 2 to 6, the red complex is considered to be tris- (4, 7-diphenyl-1, 10-phenanthroline) -iron (II). Following the discussion on the effect of pH on absorbancy, the stability constant of the complex is evaluated to be 1021.8at 18°C. Bathophenanthroline behaves as a mono-acid base in the solution, and the acidity constant of bathophenanthrolium ion is spectrophotometrically estimated to be 10-4.80 at 18°C. © 1961, The Japan Society for Analytical Chemistry. All rights reserved.