A buffer is an aqueous solution that can resist significant changes in pH levels upon the addition of small amount of acid or alkali. Each buffer is characterized by a set ‘capacity’ which is defined as the quantity of strong acid or base that must be added to change the pH of one liter of solution by one pH unit. In other words, buffer capacity is the amount of acid or base that can be added before the pH begins to change significantly.
A buffer range is the specific pH range in which a buffer effectively neutralizes the added acid or base, while maintaining nearly constant pH. Capacity and range of a particular buffer ensures that the added small amount of acid/base is neutralized and the chemical reaction keeps going without giving a wrong outcome for the experiment/process. Simply put, a buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.
There are two types of buffer solutions…
Acid buffer solutions have a pH less than 7. It is generally made from a weak acid and one of its salts (often called conjugate*). Commonly used acidic buffer solutions are a mixture of ethanoic acid and sodium ethanoate in solution, which have a pH of 4.76 when mixed in equal molar concentrations. You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts.
Alkaline buffer solutions have a pH greater than 7 and are made from a weak base and one of its salts. A very commonly used example of an alkaline buffer solution is a mixture of ammonia and ammonium chloride solution. If these were mixed in equal molar proportions, the solution would have a pH of 9.25.
How do buffers work?
Buffers work by neutralizing any added acid (H+ ions) or base (OH- ions) to maintain the moderate pH, making them a weaker acid or base. Let’s take an example of a buffer made up of the weak base ammonia, NH3 and its conjugate acid, NH4+. When HCl (strong acid) is added to this buffer system, the extra H+ ions added to the system are consumed by the NH3 to form NH4+. Now, because all the extra H+ ions are locked up and have formed a weaker acid, NH4+, thus the pH of the system does not change significantly. Similarly when NaOH (strong base) is added to this buffer system, the ammonium ion donates a proton to the base to become ammonia and water thus neutralizing the base without any significant pH change.
Now there is a term we call, ‘Breaking of the buffer solution’ which arises when the entire base and its conjugate acid (in the above case NH3 and NH4+) are consumed to neutralize the added acid or base. The further addition of an acid or base to the buffer will change its pH quickly. Thus the breaking of the buffer is its capacity, or in other words, it is the amount of acid or base, a buffer can absorb before breaking its capacity. It is to be noted that a solution with a weak base has a higher buffer capacity for addition of a strong acid and a solution of weak acid has higher buffer capacity for the addition of strong base.
*A conjugate acid is a species formed by the gain of a proton by a base while in reverse, a conjugate base is a species formed by the removal of a proton from an acid.
We can understand this by the following equation-
Acid + Base ⇌ Conjugate Base + Conjugate Acid
H2O (l) + NH3 (g) → OH− (aq) + NH4+ (aq)
In this equation H2O is the conjugate acid and its corresponding conjugate base is OH− while NH3 and NH4+ represent the base and its conjugate acid pair. Here it is to be noted that the stronger the acid or base, the weaker the conjugate, and the weaker the acid or base, the stronger the conjugate.