Reagent: benzenesulphonyl chloride (C6H5SO2Cl) in NaOH. 1° amine: RNH2 + C6H5SO2Cl -> C6H5SO2NHR (sulphonamide with one N-H). This N-H is acidic (electron withdrawal by SO2 group) -> dissolves in NaOH (forms soluble sodium salt). 2° amine: R2NH + C6H5SO2Cl -> C6H5SO2NR2 (sulphonamide with no N-H). Cannot form sodium salt -> insoluble in NaOH. 3° amine: No reaction with C6H5SO2Cl (no N-H to react). Summary: 1° -> product dissolves in NaOH; 2° -> product doesn't dissolve; 3° -> no reaction. This is a cleaner distinction than the HNO2 test for practical purposes.