is 23.79 GHz, corresponding to microwave radiation of a wavelength of 1.260 cm. The absorption at this frequency was the first microwave spectrum to be observed and was used in the first maser. Amphotericity One of the most characteristic properties of ammonia is its basicity. Ammonia is considered to be a weak base. It combines with acids to form ammonium salts; thus with hydrochloric acid it forms ammonium chloride (sal ammoniac); with nitric acid, ammonium nitrate, etc. Perfectly dry ammonia gas will not combine with perfectly dry hydrogen chloride gas; moisture is necessary to bring about the reaction. As a demonstration experiment under air with ambient moisture, opened bottles of concentrated ammonia and hydrochloric acid solutions produce a cloud of ammonium chloride, which seems to appear "out of nothing" as the salt aerosol forms where the two diffusing clouds of reagents meet between the two bottles. The salts produced by the action of ammonia on acids are known as the ammonium salts and all contain the ammonium ion (). Although ammonia is well known as a weak base, it can also act as an extremely weak acid. It is a protic substance and is capable of formation of amides (which contain the ion). For example, lithium dissolves in liquid ammonia to give a blue solution (solvated electron) of lithium amide: Self-dissociation Like water, liquid ammonia undergoes molecular autoionisation to form its acid and base conjugates: Ammonia often functions as a weak base, so it has some buffering ability. Shifts in pH will cause more or fewer ammonium cations () and amide anions () to be present in solution. At standard pressure and temperature, K = = 10−30. Combustion Ammonia does not burn readily or sustain combustion, except under narrow fuel-to-air mixtures of 15–25% air. When mixed with oxygen, it burns with a pale yellowish-green flame. Ignition occurs when chlorine is passed into ammonia, forming nitrogen and hydrogen chloride; if chlorine is present in excess, then the highly explosive nitrogen trichloride () is also formed. The combustion of ammonia to form nitrogen and water is exothermic: , ΔH°r = −1267.20 kJ (or −316.8 kJ/mol if expressed per mol of ) The standard enthalpy change of combustion, ΔH°c, expressed per mole of ammonia and with condensation of the water formed, is −382.81 kJ/mol. Dinitrogen is the thermodynamic product of combustion: all nitrogen oxides are unstable with respect to and , which is