Question

Question: The text states that “the pKa of the weak acid selected for the buffer should be as close as possible to the desired pH.” What if the pKa is not close to the desired pH? What is the problem with choosing such a weak acid used in the buffer?

Short answer

Answer

Choosing a weak acid that has pKa more or less than pH will not be an effective buffer. It will not be able to resist the change.

Step-by-step solution

pH close to pKa

Both the conjugate acid and conjugate base concentrations will be high if the pH and pKa are equal. To maintain the pH, a weak base will react with additional acid, and an additional base will react with the weak acid.

What happens when pKa is not close to pH

If the pH is very different from the pKa, one of the two species i.e., conjugate acid or conjugate base will be at a very low concentration. The value of [HA] and [A^-] will vary a lot and under this condition that the ratio [A^-]/ [HA] will not be very resistant to change when H⁺ or OH^- is added.