Potentiometric and UV-Vis spectroscopy studies of citrate with the hexaquo Fe3+ and Cr3+ metal ions

We are reporting the interaction and behavior of citrate with the hexaqo Fe(NO3)(3), and Cr(NO3)(3) (Fe3+ and Cr3+ 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 Fe3+/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 Cr3+/citrate systems showed no isosbestic points. To our surprise, it appeared that the Cr3+/citric acid system and the Cr3+/sodium citrate system had different UV-Vis patterns. The perturbation in the (4)A(2g) --> T-4(1g) electronic transition is dramatically different when comparing the two systems. It also appeared that there is a minor difference in the Ag-4(2g) --> T-4(2g) electronic transition between the two systems.