Buffer solutions

2 MAIN POINTS ABOUT BUFFER SOLUTIONS

1. A buffer solution is a solution containing an acid and its conjugate base.
2. Buffer solutions are used to minimise/negate any changes in pH.

Let’s make a buffer solution using ethanoic acid…

Ethanoic acid (CH3COOH) is a weak acid, hence it only partially dissociates.

 

Now let’s add an alkali such as sodium hydroxide (NaOH).

The OH- ions from NaOH reacts with the H+ ions, lowering its concentration.

Le Chatelier’s principle states that if the dynamic equilibrium of a system is disturbed/changed, the equilibrium position will shift to counteract the change.

Since [H+] decreases, the equilibrium shifts to the right to produce more H+ ions. At the same time, more CH3COO- ions are formed.

Now the solution contains both the acid (CH3COOH) and its conjugate base (CH3COO-).

HOW DOES A BUFFER SOLUTION WORK?

Now that we’ve got our buffer solution, let’s put it to use.

Upon addition of an acid, the pH of the solution is lowered, and the [H+] increases. This causes the conjugate base to react with the excess H+, and the equilibrium shifts to the left (refer to above equation). This in turn lowers [H+] and the pH is raised again.

Upon addition of an alkali, the pH of the solution is raised. The OH- ions produced by the alkali reacts with the H+ ions, thus lowering [H+]. The equilibrium position shifts to the right, to produce more H+ ions. When [H+] increases, the pH is lowered again.

WHY ARE BUFFER SOLUTIONS IMPORTANT?

Buffer solutions can be found in our blood. Our blood needs to have a consistent pH level for many of our functions to work (i.e. enzyme activity). Our blood needs to have a pH between 7.35 and 7.45. A condition called acidosis occurs when blood pH < 7.35. Another condition called alkalosis occurs when blood pH > 7.45.

Buffer solutions can also be found in shampoo, baby lotion, eye drops, washing powder etc.