The test for an ammonium compound is the Nessler's test. This test involves adding Nessler's reagent, which contains potassium pyroantimonate and mercury(II) chloride, to a solution of the ammonium compound. If ammonium ions are present, a yellow-brown precipitate of ammonium hexachloroantimonate will form.

The reaction that occurs during the Nessler's test is as follows:

NH4+ + 2 [HgCl4]2- + 4 OH- -> Hg2NCl + 2 HgO(s) + 2 H2O + 6 Cl-

The Nessler's test is a sensitive test for ammonium ions, and it can be used to detect ammonium ions in concentrations as low as 0.1 ppm.