Synthesis and pH-metric determination of the stability constant of iron (iii) plant-based oxalato complex

Abstract

Calvin-Bjerrum’s pH-metric titration technique, as modified by Irving and Rossoti, was used to determine the stability constant of synthesized iron (iii) plant-based oxalato complex in 50 % (v/v) ethanol-water solvent, at 300K and ionic strength of 0.1 M. Soluble oxalate was extracted from Spinacia Oleracea (Spinach) and the ligand, oxalic acid (as confirmed), was crystallized using sulphuric acid liberation method. The overall stability constant of the complex, logK was found to be 11.90 and the free energy, ΔG, of the formation as -6.8359 x 10⁴JK^-1mol^-1. The proton-ligand stability constants, pK^H calculated using half integral method at proton-ligand formation number, nA = 0.5 and 1.5 are 1.50 and 4.40 for pk1^H and pK2^H respectively and 1.50 and 5.80 for point wise method. The stability constant of the complex shows that it is relatively stable. The negative free energy shows that the reaction for the formation of ferric oxalate is spontaneous, favouring the formation of the complex. With this result, it can be said that the consumption of half-boiled vegetables like Spinach may enhanced the formation of oxalate-related kidney stones which are serious health challenges nowadays since a stable complex can easily be formed.

Keywords: pH-metry, oxalato, stability constants, iron (iii) complex, kidney stones