Video: Similarities of the ammonium ion to the alkali metal ions

As described in video clip DIC4-05-100, the larger alkali metal ions form precipitates with large, low-charge anions. The standard test for the larger alkali metal cations is the hexanitritocobaltate(III) ion:

3 K+(aq) + [Co(NO2)6]3-(aq) ® K3[Co(NO2)6](s)

Ammonium ion behaves much like an alkali metal of between the sizes of potassium and cesium. Thus the ammonium ion, too, gives a precipitate with the same anion reagent:

3 NH4+(aq) + [Co(NO2)6]3-(aq) ® (NH4)3[Co(NO2)6](s)