Schematic chemical network for HCO⁺, DCO⁺, N₂D⁺ and N₂H⁺ in different regions of protostellar envelopes: at high temperatures (T > 30 K) CO is in the gas-phase and H is dominating over H₂D⁺ (the reaction in Eq. (1) proceeds to the left) enhancing HCO⁺ and destroying N₂H⁺. At intermediate temperatures (T = 20 − 30 K) CO depletion and H₂D⁺ production starts to kick-in (the reaction in Eq. (1) proceeds to the right), enhancing N₂H⁺ (less destruction through CO) and the deuterated species (DCO⁺ in particular, through reactions between H and CO). At low temperatures CO depletion is very significant and N₂D⁺ is enhanced relative to the other species.