The characteristics of ultraviolet absorption and electronic excitation of sulfate at high concentrations.

The variation of spectra and the characteristics of electronic excitation are critical for establishing a model for quantifying sulfate at high concentrations. The absorption characteristics of sulfate are affected by the optical pathlength and sulfate concentration. The absorption coefficient declines by approximately 86.09-96.20% with an increasing concentration (0-130 g/L) at different optical pathlengths (1-100 mm). Moreover, a high sensitivity and accuracy can be achieved at weak absorption wavelengths or at lower optical pathlengths when high concentrations of sulfate are detected. In addition, the maximum absorption wavelength of sulfate redshifts by approximately 0-10 nm with an increasing concentration and optical pathlength, which is significantly affected by the optical pathlength. The (H2SO4)n‧(H2O)4-n models were established at the PBEPBE/6-311++G(d, p) level of theory. There absorption spectra were calculated by the time-dependent density functional theory (TD-DFT) method. As a result, the maximum absorption wavelength redshifted from 180.16 nm to 192.71 nm with an increasing sulfate concentration, and the corresponding absorption coefficient demonstrated a declining trend. Furthermore, the electron-hole and natural bond orbital (NBO) analysis indicate that the type of electronic excitation changes from a n(O) → σ*(S-O) localized excitation to n → σ* charge-transfer excitation as the sulfate concentration increases. This study provides a theoretical foundation for understanding the spectral behavior of sulfates and constructing the quantification models or methods that can also be applied to analyze the spectroscopy of other chemicals.