In pure water at 25o C, the concentration of H+ is always equal to the concentration of OH-. Both have a concentration of 1.0 x 10-7 Molar. (Placing the symbol for a chemical in brackets [H+] is chemical shorthand for “concentration of.” Therefore, [H+] is read “the concentration of hydrogen ion.”) If we add a substance that results in an increase in [H+], we say that substance is an acid. If we add a substance that results in a decrease in [H+], we say that substance is a base. An acid is any substance that, when added to an aqueous solution, increases the [H+] of the solution, and a base is any substance that, when added to an aqueous solution, decreases the [H+] of the solution. A common acid, for example, is hydrochloric acid, HCl. When HCl reacts with water, it dissociates into an H+ and a chloride ion (Cl-), thus increasing the [H+]. HCl is considered a strong acid because when placed in water, it completely dissociates into its two ions.
HCl → H+ + Cl-
A weak acid, such as acetic acid (CH3COOH), dissociates into H+ and CH3COO- (acetate). However, most remains intact as acetic acid, and there is a chemical equilibrium between the CH3COOH and the H+ + CH3COO-. The conjugate base is the name given to the now unprotonated compound (acetate). Thus, the disassociation of acetic acid (weak acid) produces acetate (conjugate base) and a hydrogen ion:
CH3COOH ←→ H+ + CH3COO-
An example of a base is ammonia (NH3), which will combine with H+ to form an ammonium ion (NH4+), thus removing H+ from the solution.
NH3 + H+ → NH4+
Another common base is sodium hydroxide (NaOH). How is this a base? When it dissolves, it dissociates into a sodium ion (Na+) and OH-, no change in [H+], right? However, the OH- will combine with H+ to form water, thus removing H+ from the solution.
NaOH → Na+ + OH-
H+ + OH- → H2O