Hydronium ion

Hydronium ion H₃O+ is the conjugate acid of water. Its pK_a value is often reported as -1.7, but it has recently been suggested that the correct value of its pK_a is zero (0). The hydronium ion is a considerably weaker acid than the hydrohalic acids (HCl, HBr, HI) and undergoes complete protonation in their presence.