Amino acids are an important group of compounds. At low pH, both the carboxylic acid group (—CO₂H) and the amine group (—NHR) are protonated. However, as the pH of the solution increases (say, by adding base), the carboxylic acid proton is removed, usually at a pH between 2 and 3. In a middle range of pHs, therefore, the amine group is protonated, but the carboxylic acid group has lost the proton. (This is called a zwitterion.) At more basic pH values, the amine proton is dissociated.

What is the pH of a 0.20 M solution of alanine hydrochloride, [NH₃CHCH₃CO₂H]Cl?

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0 || H₂N-CH-C-OH CH3 alanine cationic form pk₂ = 2.4 pK₂ = 9.7 0 H₂N-CH-C-0- CH3 zwitterionic form 0 || H₂N-CH-C-0- CH3 anionic form