Stability Constants for the Equilibrium Models of Iron(III) with Several Biological Buffers in Aqueous Solutions

TLDR: In this paper, the stability of Fe(III with two buffer families, tris(hydroxymethyl)aminomethane (TRIS), N-[tris[tris]-methyl]-2-aminoethanesulfonic acid (TES), TAPS, and N-tris-methyl]-3amino-2-hydroxypropanesulfonic acids (TAPSO), were studied by potentiometric measurements.

Abstract: The complexation equilibria of Fe(III) with two buffer families, which are ubiquitous in biological system studies, were studied by potentiometric measurements at a constant ionic strength of I = 0.1 mol·dm−3 NaNO3 in aqueous solutions at 298.15 K. The members of TRIS family are tris(hydroxymethyl)aminomethane (TRIS), N-[tris(hydroxymethyl)methyl]-2-aminoethanesulfonic acid (TES), N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS), N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic acid (TAPSO), and N-tris(hydroxymethyl)methyl-4-aminobutanesulfonic acid (TABS) buffers. The members of morpholine family are 4-morpholineethanesulfonic acid (MES), 4-morpholinepropanesulfonic acid (MOPS), 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO), and 4-(N-morpholino) butanesulfonic acid (MOBS) buffers. The overall stability constants were determined from pH-metric data using the least-squares curve-fitting program HYPERQUAD 2008. Based on the best-fit results, the species formed at equilibrium are ML, ML2, ML2H−1, and ML3 in the systems with TRIS family buffers. The complex species ML, ML2, ML2H−1, and MLH−1 are formed in the MOPSO-containing system, while ML, ML2, and ML2H−1 are formed in the systems with MES, MOPS, and MOBS. The stabilities of the complexes fall in the order TABS > TRIS > TAPS > TAPSO > TES and MOBS > MOPS > MOPSO > MES for the TRIS family and morpholine families, respectively.