Which buffer solution is effective at maintaining pH?

Buffer solutions are designed to resist changes in pH when small amounts of acid or base are added to them. The most effective type of buffer consists of a weak acid and its conjugate base. This combination allows the solution to neutralize added acids or bases, maintaining the pH within a narrow range.

When an acid is introduced to a buffer solution made of a weak acid and its conjugate base, the weak acid can react with the additional hydrogen ions, minimizing the impact on the overall pH. Conversely, if a base is added, the conjugate base can react with the hydroxide ions, again helping to stabilize the pH. This balance is what makes such buffer solutions effective for maintaining a relatively constant pH.

In contrast, other choices do not serve this buffering purpose. A solution of water and salt lacks the necessary components to resist pH changes effectively. Concentrated acids or highly diluted bases will not stabilize pH in the same manner, as they can lead to significant changes in hydrogen ion concentration when diluted or concentrated, respectively. Thus, the combination of a weak acid with its conjugate base stands out as the most effective in maintaining pH in various chemical contexts.