Resonance Rayleigh scattering method for the determination of trace amounts of cadmium with iodide–rhodamine dye systems Original Research Article

Resonance Rayleigh scattering (RRS) of cadmium(II)–iodide–rhodamine dye systems and their analytical application have been studied. In dilute phosphoric acid, rhodamine dyes such as rhodamine B (BRhB), ethylrhodamine B (ERhB), butylrhodamine B (BRhB) and rhodamine 6G (Rh6G) have characteristic RRS spectra and their maximum RRS wavelengths (λmax) are at 575 nm for RhB, ERhB and BRhB, and 545 nm for Rh6G. In the presence of a large excess of I−, cadmium(II) forms [CdI4]2−, which further reacts with a rhodamine dye to form an ion-association compound. This results in significant enhancement of RRS intensity and the appearance of new spectra with λmax at 610 nm for RhB, ERhB and BRhB systems and 585 nm for the Rh6G system. The characteristics of the RRS spectra of the ion-association compounds, the affecting factors and the optimum conditions of these reactions have been investigated. The intensity of RRS is directly proportional to the concentration of cadmium in the range 0–1.0 μg/25 ml for all the systems. The methods have high sensitivities and the detection limits for cadmium are 0.60–1.26 ng ml−1 depending on the reaction system; the Rh6G system is the most sensitive. A new method for the determination of trace cadmium on the basis of RRS is described.