The hydronium ion is very acidic: at 25 °C, its p"K"a is approximately 0. It is the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike hydronium in neutral solutions that result from water's autodissociation, hydronium ions in acidic solutions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.