Selecting proper components for desired pH

For the weak base ammonia (NH3), the value of Kb is 1.8x10-5 , implying that the Ka for the dissociation of its conjugate acid, NH4 + , is Kw/Kb=10-14/1.8x10-5 = 5.6x10-10. Thus, the pKa for NH4 + = 9.25, so buffers using NH4 + /NH3 will work best around a pH of 9.25. (It's always the pKa of the conjugate acid that determines the approximate pH for a buffer system, though this is dependent on the pKb of the conjugate base, obviously.)

When the desired pH of a buffer solution is near the pKa of the conjugate acid being used (i.e., when the amounts of conjugate acid and conjugate base in solution are within about a factor of 10 of each other), the HendersonHasselbalch equation can be applied as a simple approximation of the solution pH, as we will see in the next section.