Figure 10.

Fraction of protonated species as a function of pH in the presence of 0.5 M Na⁺ (◆) and 10 mM Ca²⁺ (■) as determined by the NMR method. The fraction of protonated species at a given pH was generated using eq 6. The pK_a values were determined by fitting δ_obs vs pH plots to eq 4, as shown in Figure S9 in the Supporting Information, and were found to be 13.2 and 11.9 in the presence of 0.5 M Na⁺ and 10 mM Ca²⁺_, respectively.