Amino acids generally have two pKa values, one in the range of 2 to 3 and the other in the range of 9 to 10. Serine, for example, has pKa values of 2.19 and 9.21. Using species such as acetic acid and ammonia as models, suggest the origin of the two pKa values.

Ka=1.8×10−5Ka=1.8×10−5, Kb=1.8×10−5