Using the values of the ionization constant as quantitative measures of acid strength is equivalent to the qualitative statement that a strong acid is an acid for which loss of the proton to water is essentially complete, while a weak acid is an acid for which loss of the proton is noticeably incomplete . Likewise, a strong base is a base for which acquisition of a proton from water is essentially complete while a weak base is a base for which acquisition of a proton is noticeably incomplete . These distinctions can be used to illustrate some of the terminology of acid-base reactions. Hydrogen cyanide is said to be the conjugate acid of cyanide ion, and cyanide ion is said to be the conjugate base of hydrogen cyanide. In this reaction ammonia and ammonium ion form a conjugate pair, as do water and hydronium ion.
Water can act either as an acid or a base.
The constant K w , the autoprotolysis constant or the autoionization constant of water, has the value 1.014 x 10 -14 at 25 o C when molar units are used. The autoprotolysis of water sets limits on the acid strength and base strength which can exist in aqueous solution . Any base significantly stronger than hydroxide ion, such as methoxide ion, will react quantitatively to deprotonate water producing hydroxide ion. Likewise, any acid significantly stronger than hydronium ion, such as hydrochloric acid, will react quantitatively to protonate water, producing hydronium ion. For thise reason the range of molar ionization constants observed in aqueous solutions is from about one to about 1 x 10 -14 in molar units.