[NS]:[N₂H⁺] ratio as a function of S_gas at steady-state from chemical models with with n₄ = 1 (top) and 10 (bottom) and ζ₁₇ = 1, 3, and 10 (left to right). In each panel, five values of the C/O ratio (0.4–1.2) are shown. The observed NS:N₂H⁺ column density ratios (with ± 1σ uncertainties) toward L1521E, L1498, and L1544 (central position), are shown as hashed areas, and the lower limit in L1521B is indicated with an arrow. The black, straight, line shows our analytical prediction given in Eq. (13).