The most comprehensive collection of rigorously measured pH values of mobile phases in reversed-phase liquid chromatography

During recent years, we have been engaged in extensive investigations of the unified pH (pH_abs) values of reversed-phase liquid chromatography (RPLC) mobile phases. The pH_abs scale has the advantage over the conventional pH scale because pH_abs values express acidity in terms of the thermodynamic activity of the solvated proton. Therefore, pH_abs values are directly comparable between solvents/media of different compositions. At the same time, pH_abs is convenient to use, as pH_abs values of aqueous solutions are equal to the respective conventional pH values.

This comparability is especially useful in RPLC, as mobile phases are mixtures of water with organic solvents in different ratios. Thus, it can be said that pH_abs is the best way of expressing pH if a rigorous comparison of pH between solutions in different solvents is needed.

As a result of our work, we have carefully measured the pH_abs values of 78 mobile phases commonly used in RPLC, using around 300 individual ΔpH_abs measurements between different mobile phases (see the “ladder” scheme below). This is, to the best of our knowledge, the most comprehensive collection of rigorous pH_abs values of RPLC mobile phases and has now been published as A. Heering, M. Lahe, M. Vilbaste, J. Saame, J. P. Samin, I. Leito. Improved pH measurement of mobile phases in reversed-phase liquid chromatography. Analyst 2024.

The ΔpH_abs values were measured by differential potentiometry, using potential differences in a symmetric cell with two glass electrode half-cells (see figure above) and almost ideal ionic liquid triethylamylammonium bis((trifluoromethyl)sulfonyl)imide [N₂₂₂₅][NTf₂] salt bridge with multiple overlapping measurements. The system of altogether 300 ΔpH values, pictured in the “ladder” scheme below, was anchored to the pH value of standard pH 7.00 aqueous buffer solution.

In addition, a simpler measurement method that uses double junction reference or double junction combined electrodes was tested and was found suitable for routine laboratories. The results show that the design of the junction is an important factor in deciding if the electrode can be used for unified acidity measurements. This is the first successful use of double junction combined electrodes filled with ionic liquid for the measurement of pH_abs values.

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