# Am I limited by the pKa value of an weak acid when preparing a buffer solution of it (and its salt)?

If I was to make an acidic buffer system with the ph of, for instance, 4, would I be limited with my choice of weak acids? That is, how big does the difference between pH and pKa have to be in order for the weak acid not to be usable? Or is that not the case? I am very well aware that the optimal weak acid for has to have a pKa value quite close to the pH of the wanted buffer because they offer the most buffering capacity. Also - Can someone hook me up with some equations applicable for making/dealing with buffers other than the Henderson Hasselbach equation? Are there any equations that can help me predict the buffer capacity?

• There is a pretty good treatment of this topic at chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity – iad22agp Nov 8 '16 at 12:58
• The gist is that the further the pH is from the pKa then the less buffer capacity the solution will have. If the absolute difference between the pH and the pKa is more than 2 pH units then there is essentially no buffer capacity. – MaxW Nov 8 '16 at 16:52