Potentiometric and UV–Vis Spectroscopy Studies of Citrate with the Hexaquo Fe^3 + and Cr^3 + Metal Ions

Abstract

We are reporting the interaction and behavior of citrate with the hexaqo Fe(NO₃)₃, and Cr(NO₃)₃ (Fe^3 + and Cr^3 + will be used as shorthand notation for the two metal ions) using ultraviolet and visible spectroscopy (UV–Vis). The identity of the proposed metal complexes was confirmed by using the potentiometric titration technique. With the Fe^3 +/citric acid system, it appeared that the simple 1:1 complex and the bis‐complex predominate. From the UV–Vis we observed the presence of two isosbestic points, one at 320 nm above the physiological pH, and another at 350 nm in the acidic pH range of 2.10 to 3.73. The appearance of an isosbestic point at 350 nm in the acidic region is novel. These isosbestic points indicate the presence of major species at a certain pH range. In contrast to the Fe^3 +/citrate system, the Cr^3 +/citrate systems showed no isosbestic points. To our surprise, it appeared that the Cr^3 +/citric acid system and the Cr^3 +/sodium citrate system had different UV–Vis patterns. The perturbation in the ⁴A_2g→⁴T_1g electronic transition is dramatically different when comparing the two systems. It also appeared that there is a minor difference in the ⁴A_2g→⁴T_2g electronic transition between the two systems.

Acknowledgments

The author would like to thank the Nebraska Academy of Science for the two‐year science grant that was used to purchase chemicals and the stability constants database software used. The author would like to give special thanks Dr. David Hales of Hendrix College, Conway Arkansas, and Dr. Wesley Harris of the University of Missouri–St. Louis for their useful discussion on the Fe^3 +/citrate system.