A buffer is an aqueous solution that can resist significant changes in pH levels upon the addition of a small amount of acid or alkali.
There are two types of buffer solutions: acidic buffer and alkaline buffer.
Acidic buffer
Acid buffer solutions have a pH less than 7. It is generally made from a weak acid and one of its salts. Commonly used acidic buffer solutions are a mixture of ethanoic acid (a weak acid) and sodium ethanoate in solution (a weak basic salt), which have a pH of 4.76 when mixed in equal molar concentrations.
Alkaline buffer
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 (weak alkali) and ammonium chloride solution (a weak acidic salt). 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), by producing a weak acid or a weak alkali, respectively, hence maintaining the required pH.
For example, 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+, which is a weak acid. 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 NH4+ ion donates a proton to the base to become ammonia and water, which is a weak alkali, thus neutralizing the base without any significant pH change.
H2O (l) + NH3 (g) ⇌ OH– (aq) + NH4+ (aq)
Example:
Answer is A. The mixture of ammonia and ammonium chloride solution acts as an alkaline buffer solution, maintaining the pH at about 10.