Ammonium alum is an important powder material for producing high purity alumina. It is usually obtained by crystallization from aqueous solution, but impurity ions in solution have a great effect on the quality of final product particles. Effects of sodium, potassium and iron ions on the primary nucleation of ammonium alum were studied by measuring induction time and metastable zone width using focused beam reflectance measurement (FBRM). It was found that potassium ion or iron ion enhanced the nucleation, whereas sodium ion had no obvious effect. Analysis about the mechanism of impurity incorporation showed that, due to the isomorphous substitution, potassium ion or iron ion were easily incorporated into ammonium alum crystals. Moreover, potassium ion was more easily incorporated into crystal than iron ion. (C) 2020 Elsevier B.V. All rights reserved.