The pH of the solution resulting from the topical application of ammonium alum with perspiration is typically in the slightly acid range, from 4 to 5. It is a popular deodorant because of its high availability and low cost. A 120 gram stone lasts for at least a year of daily usage - much longer than other deodorants and antiperspirants. It is also hypoallergenic and non-staining. Potassium alum is also used for this purpose.