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Define buffering capacity. How do the following
buffers differ in buffering capacity? How do they differ in pH?
Buffer a: 0.01 M Na2HPO4 and 0.01 M NaH2PO4
Buffer b: 0.10 M Na2HPO4 and 0.10 M NaH2PO4
Buffer c: 1.0 M Na2HPO4 and 1.0 M NaH2PO4
The Correct Answer and Explanation is:
Buffering Capacity and Buffer Comparison
Definition of Buffering Capacity:
Buffering capacity refers to the ability of a buffer solution to resist changes in pH when small amounts of an acid or a base are added. It depends primarily on the concentrations of the weak acid and its conjugate base in the buffer. Higher concentrations result in a greater capacity to neutralize added H⁺ or OH⁻ ions without significantly changing the pH.
Comparison of Buffers A, B, and C:
All three buffers—A, B, and C—are made from the same buffering system: Na₂HPO₄ (the base form) and NaH₂PO₄ (the acid form), which form a phosphate buffer. The pKa₂ of phosphoric acid (relevant here) is approximately 7.2, making this buffer system effective near neutral pH.
- Buffer A: 0.01 M Na₂HPO₄ and 0.01 M NaH₂PO₄
- Buffer B: 0.10 M Na₂HPO₄ and 0.10 M NaH₂PO₄
- Buffer C: 1.0 M Na₂HPO₄ and 1.0 M NaH₂PO₄
All three have equal concentrations of acid and base components, so the pH of each buffer will be the same, and close to the pKa value:
pH ≈ pKa₂ ≈ 7.2
However, their buffering capacities differ significantly:
- Buffer A (0.01 M) has low buffering capacity. It can neutralize only small amounts of added acid or base before the pH changes noticeably.
- Buffer B (0.10 M) has a moderate buffering capacity, capable of neutralizing more added acid or base compared to A.
- Buffer C (1.0 M) has the highest buffering capacity, resisting pH changes even with relatively large additions of acid or base.
Conclusion:
Although all three buffers have the same pH (about 7.2) due to their 1:1 ratio of acid and base, their ability to resist pH changes—buffering capacity—increases with concentration. Thus, Buffer C > Buffer B > Buffer A in buffering strength.
