Fe(CN)₆^3–(aq) + N₂H₅⁺(aq) → Fe(CN)₆^4–(aq) + N₂(g)

acidic environment

[Fe(CN)₆^3–(aq) + e^– → Fe(CN)₆^4–(aq)] × 4

N₂H₅⁺(aq) → N₂(g) + 5 H⁺(aq) + 4 e^–

sum and eliminate electrons

4 Fe(CN)₆^3–(aq) + N₂H₅⁺(aq) → 4 Fe(CN)₆^4–(aq) + N₂(g) + 5 H⁺(aq)