A quantitative method for determination of Co(II) based on the inner filter effect of reagents on the Raman scattering signals of water

It is known that Raman scattering signals are one of main interference sources leading up to determination errors in spectrofluorometry, and thus the signals can be easily detected with a common spectrofluorometer. In this contribution, we propose a quantitative method based on the inner filter effect (IFE) of reagents on the Raman scattering signals of solvent by taking the complexation of divalent cobalt ion with 4-(5-chloro-2-pyridyl)azo]-1,3-diaminobenzene (5-Cl-PADAB) as a model system. By adjusting the excitation wavelength of the spectrofluorometer, we could easily detect the Raman scattering signals of water at 424 nm where the maximum absorption of 5-Cl-PADAB reagent is located. In a solution of 5-Cl-PADAB, the Raman scattering signals of water are decreased owing to the IFE of 5-Cl-PADAB. If Co(II), which could form the binary complex of Co(II)/5-Cl-PADAB and consumes the 5-Cl-PADAB reagent, is present in such a case for a given amount of 5-Cl-PADAB solution, recovered Raman scattering signals could be observed and measured with a spectrofluorometer. It was found that the intensity of the enhanced Raman scattering signals is proportional to the Co(II) concentration over the range from 2.0 × 10⁻⁷ mol L⁻¹ to 1.0 × 10⁻⁵ mol L⁻¹, and the detection limit could reach 1.2 × 10⁻⁷ mol L⁻¹. With that, Co(II) in samples could be detected with R.S.D. values lower than 2.6% and recoveries over the range of 97.2-104.7%.