On December 6, 2017 Agnes Heering successfully defended her PhD thesis titled Experimental realization and applications of the unified acidity scale.
Her work literally redefines the way the pH of non-aqueous and mixed aqueous solution is understood and measured. The main focus of the experiments was on validating the measurement approach and measuring the unified pH values, i.e. pHabs values, of HPLC mobile phases (eluents). Her work introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases and has been published in the Analytical Chemistry journal: Unified pH Values of Liquid Chromatography Mobile Phases. Anal. Chem. 2015, 87, 2623–2630.
Mobile phase pH is very important in LC, but its correct measurement is not straightforward and all commonly used approaches have deficiencies. The new and fundamentally correct approach developed by Agnes enables direct comparison of acidities of solutions made in different solvents, based on chemical potential of the proton in the solutions.
The work by Agnes represents the first experimental realization of the pHabs concept using differential potentiometric measurement for comparison of the chemical potentials of the proton in different solutions (connected by a salt bridge), together with earlier published reference points for obtaining the pHabs values (referenced to the gas phase) or pHabsH2O values (referenced to the aqueous solution). The liquid junction potentials were estimated in the framework of Izutsu’s three-component method.
She determined the pHabs values for a number of common LC and LC-MS mobile phases and formed a self-consistent pHabs scale. This scale enables for the first time direct comparison of acidities of any LC mobile phases: with different organic additives, different buffer components etc. Agnes has developed a possible experimental protocol of putting this new approach into chromatographic practice and has tested its applicability. She has demonstrated that the ionization behavior of bases (cationic acids) in the mobile phases can be better predicted by using the pHabsH2O values and aqueous pKa values than by using the alternative means of expressing mobile phase acidity. Description of the ionization behavior of acids on the basis of pHabsH2O values is possible if the change of their pKa values with solvent composition change is taken into account.
The defence was successful in every respect. Agnes presented very well, answered questions confidently and convincingly demonstrated to everyone that she is really on top of this whole matter.
(Photo: Agnes Heering and prof. Peeter Burk, the chairman of the defence committee, during defence)