buffer capacity
/ BUF-er kuh-PAS-ih-tee /
A small sponge mops up a little spill before it is soaked through; a big sponge handles a much larger one. Buffer capacity is the chemical version of sponge size: it measures how much acid or base a buffer can absorb before its pH finally gives way.
Quantitatively, buffer capacity is the amount of strong acid or strong base that must be added to a litre of buffer to change its pH by one unit. It depends on two things: the total concentration of the buffer components, since more of them means more reserve to draw on, and how evenly the acid and base forms are matched, since a buffer is at its most resilient when the two forms are present in roughly equal amounts.
Capacity matters because a buffer at the right pH is still useless if it runs out the moment you challenge it — biological and industrial buffers are sized for the load they must absorb. The honest caveat is that capacity always peaks at the pKa and falls off on either side, so a buffer can hold a target pH and yet have little reserve there if that pH sits far from its pKa.
A 1 mole-per-litre acetate buffer and a 0.01 mole-per-litre one can both hold pH 4.76, but the concentrated one absorbs a hundred times more added acid before its pH slips — same pH, vastly different capacity.
Higher total concentration means more reserve, hence greater buffer capacity.
Capacity and pH are independent settings. The Henderson-Hasselbalch ratio fixes the pH, while the total concentration sets the capacity, so you tune them separately when designing a buffer.