Retention of ionisable compounds on high-performance liquid chromatography XVII: Estimation of the pH variation of aqueous buffers with the change of the methanol fraction of the mobile phase

The use of methanol–aqueous buffer mobile phases in HPLC is a common election when performing chromatographic separations of ionisable analytes. The addition of methanol to the aqueous buffer to prepare such a mobile phase changes the buffer capacity and the pH of the solution. In the present work, the variation of these buffer properties is studied for acetic acid–acetate, phosphoric acid–dihydrogenphosphate–hydrogenphosphate, citric acid–dihydrogencitrate–hydrogencitrate–citrate, and ammonium–ammonia buffers. It is well established that the pH change of the buffers depends on the initial concentration and aqueous pH of the buffer, on the percentage of methanol added, and on the particular buffer used. The proposed equations allow the pH estimation of methanol–water buffered mobile phases up to 80% in volume of organic modifier from initial aqueous buffer pH and buffer concentration (before adding methanol) between 0.001 and 0.01 mol L−1. From both the estimated pH values of the mobile phase and the estimated pKa of the ionisable analytes, it is possible to predict the degree of ionisation of the analytes and therefore, the interpretation of acid–base analytes behaviour in a particular methanol–water buffered mobile phase.