Accurate Potentiometric Studies of Chromium‐Citrate and Ferric‐Citrate Complexes in Aqueous Solutions at Physiological and Alkaline pH Values

Abstract

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.

A. Peipho and B. Carlson conducted the initial work under the guidance of Y. Z. Hamada when he held an assistant professor position at Wayne State College.

Keywords:

• aqueous solutions
• citrate
• dimer
• potentiometric titrations
• speciation diagram

Acknowledgments

This work was supported in part from NSF under Grant # HRD‐0411493. We also would like to thank the Faculty at the Division of Natural and Mathematical Sciences of LeMoyne‐Owen College for reading the manuscript.

Notes

A. Peipho and B. Carlson conducted the initial work under the guidance of Y. Z. Hamada when he held an assistant professor position at Wayne State College.