What is a positive test for ammonium?

A positive test for ammonium ions (NH4+) can be indicated by the formation of a colored complex when a solution containing ammonium is treated with sodium hydroxide (NaOH) and heated, producing ammonia gas, which can be detected by its characteristic smell or by using a pH indicator. Additionally, the presence of ammonium can be confirmed by adding a few drops of Nessler's reagent (potassium mercuric iodide), which produces a yellow to brown coloration if ammonium is present.