Figure 2: Second-derivative absorbance spectra (N″, AC″, and H″) for (a) sulfides (Na2S solution at 235 µmol/L), (b) sulfites (Na2SO3 solution at 490 µmol/L), and (c) thiosulfates (Na2S2O3 solution at 500 µmol/L) between 220 and 300 nm at 25°C in three different buffered media (H, pH = 1.0; AC, pH = 4.7; and N, pH = 9.2). Note that H″ and AC″ spectra merge in (a) while N″ and AC″ are superposed in (c). Sulfides display a maximum at 250 nm for a pH of 9.2 (N″), while a minimum is reached for sulfites at 278 nm for a pH of 1 (H″). For thiosulfates, maxima are found at 244 nm for the three media (N″, H″, and AC″). Optical path length and Δλ were of 10 mm and 8 nm, respectively.