Lecture 11 summary

As a further example of a triprotic amino acid, we consider the basic amino acid arginine. As the structures in the figure below show, at low pH the a-amino and a-carboxyl groups are protonated, and the positively charged guanidinium group is in its protonated form as well. Raising the pH (equivalent to titrating arginine dihydrochoride with strong base) converts some of the fully-protonated (H₃B²⁺) form to H₂B⁺ by loss of the proton of the a-carboxyl group as the pH approaches the pK_a of the a-carboxyl group. The carboxyl and carboxylate forms are in equlibrium when pH = pK_a1. The concentrations of the other forms are still negligible. However, when the titration reaches its first equivalence point, the intermediate species H₂B⁺ is the predominant species, with smaller (and approximately equal) amounts of H₃B²⁺ and HB. The pH calculation for the intermediate species case is roughly concentration-independent (for our typical applications), and here consists of averaging pK_a1 and pK_a2.