Accurate potentiometric studies of chromium-citrate and ferric-citrate complexes in aqueous solutions at physiological and alkaline pH values

Speciation diagram of Fe3+-citrate complexes in aqueous solutions is presented. The conditions set forth for the speciation diagram were; Fe 3+ :citric acid in 1:1 ratio with concentration of 2.5x10(-4) mol . L-1 , 0.4922 mol . L-1 NaOH solution, and pKw=13.7810.006 taken from ( J. Sol. Chem. 1974, 3, 191-214). The speciation diagram is showing that the dimeric species is the dominant species in solutions from pH similar to 4.0 to pH similar to 12.0. In addition the speciation diagram for the free citrate presented is in a good agreement with what had been reported in the literature ( Critical Stability Constants Database, Version 6.0 , NIST, 2001). In a fairly recent development ( Science , 2002, 295, 1715-1719), the integral outer membrane cell receptor for iron citrate was crystallized. The interpretation of the crystallographic data collected led to the conclusion that a reasonable model for the species anchored into the cleft of the cell receptor is the dimer Fe3+-citrate species. The data on the Cr3+-citrate system is also in a fairly good agreement with the dimer model presented in aqueous solutions.