Topic 12.2.2 Buffer Solutions

• A buffer solution is formed from a weak acid and its salt OR a weak acid and a base. This produces a mixture containing H+ ions and a large reservoir of OH- ions, which helps to resist any changes in pH.
• A buffer solution can be defined as a solution which is able to resist changes in pH when small volumes of acid or base are added.
• The large reservoir of OH- ions in a buffer solution allows the ratio of acid to base in the mixture to be kept almost constant.
• Exemplar buffer solution : NH3 + H2O ⇌ NH4+ + OH-. If more base is added, the concentration of OH- will increase, making the solution more basic. The extra OH- ions will react with the NH4+ ions to form the original reactants. Therefore, the equilibrium will shift to the left to remove the OH- ions and stop large changes in pH.
• There are two types of buffer calculations: acid + base and acid + salt reactions.
• To calculate the pH of an acid + base buffer solution, firstly find the moles of each species. Next, calculate the concentration of the acid and base at equilibrium (using the total volume). Finally, use Ka to find [H+], and then pH.
• To calculate the pH of an acid + salt reaction, firstly find the moles of the salt. Then use Ka to find the pH of the solution.
• The pH of a buffer solution doesn't change much, but will change in the order of 0.1 or 0.01 units of pH when a small volume of acid or base is added.
• When a small amount of acid (H+) is added to a buffer solution, this increases the concentration of acid in the solution, making it slightly more acidic.
• When a small amount of base (OH-) is added to a buffer solution, this decreases the concentration of acid in the buffer solution, meaning the overall solution will become slightly more basic.
• Buffer solutions are common in nature to regulate pH. This is because enzymes and metabolic reactions often require a specific pH, which can be maintained using a buffer solution.
• One buffer in nature is found in the human circulatory system. The pH of human blood is maintained in a buffer between carbonic acid and bicarbonate ions. These ions neutralise any acidic substances that enter the bloodstream, converting them into carbonic acid and water.