Determination of nanomolar dissolved sulfides in water by coupling the classical methylene blue method with...

In this study, we proposed a novel method for the determination of nanomolar dissolved sulfides, including H₂S, HS^-, and S^2- (defined as S(-II)) in water by coupling the classical methylene blue (MB) method with surface-enhanced Raman spectroscopy (SERS) detection. Overall, the following analytical procedures were employed: i) precipitation of S(-II) as zinc sulfide, ii) centrifugation to collect zinc sulfide, iii) derivatization of S(-II) to MB by the reaction with N, N-dimethyl-p-phenylenediamine in the presence of FeCl₃ under acidic conditions, and iv) SERS detection. Parameters affecting the derivatization and SERS detection were optimized. Under the optimized conditions, a linear range of 12.3 nmol/L-200 nmol/L for S(-II) was obtained with a correlation coefficient (R²) of 0.99. Limits of detection and quantification of the developed method were estimated to be 3.7 nmol/L and 12.3 nmol/L, respectively. In addition, the proposed method demonstrated excellent tolerance to coexisting substances, such as NO₂^-, NO₃^-, SO₃^2-, and other common ions. The proposed method demonstrates immense promise for the determination of nanomolar S(-II) in surface waters and wastewater.