Dynamic approach to predict pH profiles of biologically relevant buffers

Abstract

Recently, dynamic approach has been applied to determine the steady state concentrations of multiple ionic species present in complex buffers at equilibrium. Here, we have used the dynamic approach to explicitly model the pH profiles of biologically relevant phosphate buffer and universal buffer (a mixture of three tri-protic acids such as citric acid, boric acid and phosphoric acid). The results from dynamic approach are identical to that of the conventional algebraic approach, but with an added advantage that the dynamic approach, allow for the modelling of complex buffer systems relatively easy compared to that of algebraic method.

Keywords: Dynamic approach; Phosphate buffer; Universal buffer; pH prediction.

Graphical abstract

Fig. 1

The simulation of pH profile obtained for titration of mono-protic (A), di-protic (B), tri-protic (C), mono-alkaline (D), Phosphate buffer (E), Universal buffer (F). NaOH was considered as a titrant for (A), (B), (C), (F). HCl was considered as titrant for (D). Na₂HPO₄ was considered as titrant for (E). For simulation, pKa was set to 4 for mono-protic acid, 4,9 for di-protic acid, 3,6,9 for tri-protic acid, 4.6 or pK_b=9.245 for mono-alkaline, 2.12,7.21,12.32 for phosphate buffer. For universal buffer the pKa values were set to 2.12, 7.21, 12.32 for phosphoric acid, 3.08, 4.72, 5.4 for citric acid, 9.23, 12.4, 13.3 for Boric acid. The simulation of phosphate buffer assumes titration of NaH₂PO₄ with incremental volume of Na₂HPO₄. Both the algebraic and dynamic approach yielded identical profiles for all the simulations.

Fig. 2

The experimental (•) and model fitted (—) pH profile of phosphate buffer prepared by mixing different volumes of equimolar concentrations of NaH₂PO₄ and Na₂HPO₄. (A) Shows the pH profile of 0.1 M phosphate buffer prepared by mixing mono and di sodium salt of phosphate. The experimental data was fitted to phosphate buffer model and optimized pKa values such as 2.239, 6.864, and 11.627 were obtained. (B) Shows the pH profile of 1 M phosphate buffer prepared by mixing mono and di sodium salt of phosphate. The experimental data was fitted to phosphate buffer model and optimized pKa values such as 2.15, 6.8537 and 8.768 were obtained.

Fig. 3

The experimental (•) and model fitted (—) pH profile of universal buffer prepared by titrating Na₃PO₄ (0.1 M) against a mixture of citric acid (0.05 M) and boric acid (0.2 M). The optimized pKa values for phosphoric acid were 3.259, 7.029, 12.233, citric acid were 2.628, 4.431, 5.620 and boric acid were 9.770, 12.507, 12.920.