Retention of ionisable compounds on high-performance liquid chromatography: XIX. pH variation in mobile phases containing formic acid,piperazine and tris as buffering systems and methanol as organic modifier

In previous works a model to estimate the pH of methanol–aqueous buffer mobile phases from the aqueous pH and concentration of the buffer and the fraction of organic modifier was developed. This model was successfully applied and validated for buffers prepared from ammonia, acetic, phosphoric and citric acids. In the present communication this model has been extended to formic acid, piperazine and tris(hydroxymethyl)aminomethane buffers. Prior to the modelling work, the pK_a values of the studied buffers at several methanol–water compositions were determined.