∆G prot = RT log (10) (pH -pK a ) . (1) This relationship suggests that the free energy is linearly dependent on the solution pH; as the pH moves farther away from pK a , the free energy required to (de)protonate also shifts. A purely linear relationship between pH and ∆G implies a joint relationship with the probability of finding a protonatable group i in a given state; this follows from the rearranged Henderson-Hasselbalch (HH) equation: pK i a = pH + log 10