Diazotisation: ArNH2 + NaNO2 + 2HCl (0-5°C) -> ArN2+Cl- + NaCl + 2H2O. Temperature MUST be 0-5°C — above this, the diazonium salt decomposes to phenol + N2. Why aromatic diazonium salts are stable but aliphatic are not: In ArN2+, the positive charge is delocalised into the benzene ring (resonance stabilisation). Aliphatic RN2+ has no such stabilisation and decomposes instantly to R+ + N2 (N2 is an excellent leaving group). Storage: Diazonium salts are usually used immediately after preparation. Some stable forms exist: ArN2+BF4- (tetrafluoroborate) can be isolated as a solid and stored. This form is used in the Balz-Schiemann reaction.