 That its p"K"a is lower than −9 is evidenced by the fact that its monohydrate contains discrete hydronium ions and can be isolated as a stable, crystalline solid, formulated as [H3O+][ClO4–].The most recent estimate of its aqueous p"K"a is –15.2 ± 2.0.It provides strong acidity with minimal interference because perchlorate is weakly nucleophilic (explaining the high acidity of HClO4).