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; Fe3+:citric acid in 1:1 ratio with concentration of 2.5×10−4 mol · L−1, 0.4922 mol · L−1 NaOH solution, and pKw=13.781±0.006 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∼4.0 to pH∼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.