Question

Which of the following combinations would be the best to buffer the pH of a solution at approximately \(9 ?\) (a) HCl and NaCl (b) \(\mathrm{NH}_{3}\) and \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) and \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\)

Short answer

(b) NH3 and NH4Cl would be the best buffer for pH 9.

Step-by-step solution

Understanding a Buffer System

A buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. Its pH remains stable upon the addition of small amounts of acid or base.

Identifying pH Range for Buffering

To buffer a solution at pH 9, we should use a buffer system with a pKa close to 9. This ensures that the buffer can efficiently resist changes in pH around this point.

Evaluating Given Options

(a) HCl and NaCl do not form a buffer because HCl is a strong acid, not a weak acid or base. (b) NH3 (a weak base, pKb ≈ 4.76) and NH4Cl can create a buffer with a pKa around 9.25. (c) CH3CO2H (acetic acid) and NaCH3CO2 form a buffer with a pKa of about 4.76, which is too low for pH 9.

Selecting the Best Option

The best choice is (b) NH3 and NH4Cl, as they form a basic buffer system where the pKa ( ext{pKa} = 14 - ext{pKb} = 14 - 4.76 = 9.24 ext{pKa} = 9.24 ) is close to the desired pH 9.

Key concepts

Weak Acids and Bases

In the realm of chemistry, weak acids and bases play a vital role in buffer solutions. Unlike strong acids or bases, which completely dissociate in water, weak acids and bases only partially ionize. This means they release protons (H⁺) or accept protons less readily, allowing the solution to maintain a relatively stable pH balance.
For instance, acetic acid ( CH₃COOH ) is a common weak acid that partially dissociates in solution. As a result, a weak base like ammonia ( NH₃ ) can fulfill a similar function, accepting some protons without fully dissociating.
These partial ionizations are crucial for forming buffer solutions, as they enable the solution to neutralize small quantities of additional acids or bases. Therefore, when selecting components for a buffer solution, ensure at least one is a weak acid or base to retain this stabilizing effect.

pH Buffering

Buffering is the ability of a solution to resist drastic changes in pH when small amounts of acid or base are added. This critical property is essential in many chemical and biological processes. To achieve effective buffering, the chosen buffer components should have a pKa close to the target pH of the solution.
The specifics of buffering involve balancing between a weak acid and its conjugate base, or a weak base and its conjugate acid. For example, in the case of NH₃ and NH₄Cl , the basic buffer formed resists pH changes around 9 due to its suitable pKa value (around 9.24).
Consequently, a well-formed buffer solution can absorb excess protons or hydroxide ions, maintaining a stable pH in fluctuating conditions. This is why choosing the correct acid/base pair is crucial for creating an efficient pH buffer.

Conjugate Acid-Base Pairs

The concept of conjugate acid-base pairs is integral to understanding buffer solutions. When a weak acid donates a proton (H⁺), it becomes its conjugate base, and conversely, when a weak base accepts a proton, it becomes its conjugate acid.
For example, when ammonia ( NH₃ ) accepts a proton, it transforms into ammonium ( NH₄⁺ ), making NH₃ and NH₄⁺ a conjugate acid-base pair. Similarly, acetic acid ( CH₃COOH ) and acetate ion ( CH₃COO¯ ) form another such pair.
In a buffer system, these pairs work together dynamically to mitigate changes in pH. The conjugate base neutralizes added acids, while the conjugate acid counters additional bases.

• This reversible interaction is why buffer systems are so effective in maintaining a specific pH range.
• Picking the right conjugate acid-base pair, with ideally a pKa close to your desired pH, ensures the buffer solution performs optimally under variable conditions.