|MoBio||pH and pKa||Chapter 2|
A molecule, or an atom group in a molecule, may lose or gain a proton when the molecule is placed in an aqueous solution. The exact probability that a molecule will be protonated or deprotonated depends on the pKa of the molecule and the pH of the solution.
Let AH be an atom group in a molecule. AH may be neutral or charged. After AH loses a proton, it is denoted by A-. The protonation/deprotonation reaction may be written as
From the last equation, we can see that half of molecules will lose protons if they are in a solution with pH = pKa. The higher the pH value, the more likely a molecule will lose a proton. On the other hand, from the definition of pH, we find that the proton concentration of a solution becomes lower at higher pH, thereby capable of accepting more protons. This physical concept is consistent with the result we just obtained from the equation.
This information tells us that in a neutral solution: