Buffer Solution Calculator

Because pH depends on the RATIO [A⁻]/[HA], not the absolute concentrations -- diluting both species by the same factor leaves that ratio unchanged, so the Henderson-Hasselbalch pH stays essentially the same. What dilution DOES reduce is buffer capacity (β): a more dilute buffer has fewer acid and base molecules available to absorb an added strong acid or base, so its pH will swing more for the same addition even though its resting pH barely moved.

At pH = pKa, the ratio [A⁻]/[HA] equals exactly 1, meaning the weak acid and its conjugate base are present in equal amounts. That's the point where both halves of the buffer pair are simultaneously most available -- to neutralize added base you need HA, and to neutralize added acid you need A⁻, so having equal reserves of both maximizes the total amount of either kind of addition the buffer can absorb before its pH shifts significantly.

Choose one whose pKa sits within about 1 pH unit of your target, ideally as close as possible -- that keeps the [A⁻]/[HA] ratio near 1, where buffer capacity is highest. Outside pKa ± 1, one of the two species becomes the minority component, so the buffer still technically works (the Henderson-Hasselbalch equation still applies) but resists pH change much less effectively.

No -- the effective range (pKa ± 1) is set entirely by the acid's pKa, which is a fixed chemical property of that acid, not by how much of it you dissolve. Concentration instead determines buffer capacity: a more concentrated buffer resists a larger absolute addition of acid or base before its pH exits that same pKa ± 1 window, but the window's boundaries themselves don't move.