Buffer solutions resist changes in pH. In a normal (unbuffered) solution, the introduction of a few drops of acid or base could dramatically alter the pH. Adding just 1 oz. of concentrated (31 percent) hydrochloric acid to a gallon of water, for example, would change the pH of the water from 7 to less than 1. Adding the same amount of acid to a buffered solution, in comparison, would likely lower the pH by only a few tenths of a pH unit. Understanding the exact mechanism by which buffers function requires a basic understanding of acid-base chemistry.
pH is a 0-to-14 scale that measures the acidity or basicity of a water-based solution. A pH of less than 7 indicates an acidic solution; a pH of greater than 7 indicates a basic solution; and a pH of exactly 7 indicates a neutral solution.
Acids release protons (H?) into solution, and bases create or release hydroxide ions (OH?). Acids and bases are further characterized as being strong or weak. A strong acid splits into its respective ions in a process referred to as “dissociation” and does so completely. Hydrochloric acid (HCl), for example, dissociates into protons and chloride ions (Cl?). “Complete” dissociation means that every HCl molecule breaks apart.
A weak acid also dissociates, but the process is not complete. Acetic acid (CH?COOH), for example, partially dissociates into protons and acetate ions (CH?COO?), which represent the “conjugate base” of the acetic acid. Because acetic acid is a “weak” acid, only a small percentage of the acetic acid molecules dissociate.
Buffer solutions resist changes in their pH, even when strong acids or bases are added to the buffer solution. Chemically, a buffer is a combination of a weak acid and its conjugate base, or a weak base and its conjugate acid. An example would be acetic acid combined with a water-soluble acetate compound, such as sodium acetate (CH?COONa). This combination creates an equilibrium system in which the acetic acid, the protons and the acetate ions are all present simultaneously in the same solution.
LeChatelier’s principle states that any stress placed on a system at equilibrium will result in the system adjusting to alleviate the stress. Thus, the introduction of any compound that consumes one of the products of the equilibrium reaction will result in the generation of additional products (in this case, acetic acid) by consuming reactants (protons and acetate ions). Or, a compound that introduces additional products will result in the generation of more reactant (by consuming products).
Buffer + Base
When a base is introduced into a buffer solution, it will react with the most acidic species, which is the proton (H?). Thus, if sodium hydroxide (NaOH) were introduced into the acetic acid/sodium acetate buffer described previously, then the hydroxide and proton would react to become water (i.e., the acid and base would neutralize each other).
The equilibrium will respond by generating additional acid by consuming additional product. That is, more CH?COOH molecules will release a proton into solution to replace the protons consumed by the base. This process will continue as more base is added, up to the point where no more acetic acid molecules remain. This limit is referred to as the “buffer capacity” of the solution. Additional base added beyond this point will then dramatically alter the pH of the solution.