As the pH moves further into the basic range, the -NH3 + group begins to lose its proton, forming the A -species. The HA and A -concentrations are equal at pH = pK a 2 . Beyond this point, the A -form predominates, and at a very high pH, the molecule exists almost entirely in the A -form. Consider the possible set of interconnected acid-base equilibria that may occur in solution at pI.