Delta conversion parameter between pH scales (SWpH and SSpH) in acetonitrile/water mixtures at various compositions and temperatures.

The SSpH in acetonitrile/water mixtures at different temperatures cannot be directly measured because of the lack of calibration buffers in these hydroorganic media at most temperatures different from 25 degrees C. In this paper, the delta parameter has been determined for acetonitrile/water mixtures from 0 up to 90% acetonitrile at different temperatures from 15 to 60 degrees C, and the values were fitted to a very simple simultaneous function of composition and temperature. The delta values allow conversion of the SWpH scale (pH measured in acetonitrile/water with electrodes calibrated in water) to the SSpH scale (pH measured in acetonitrile/water with electrodes calibrated in the same acetonitrile/water mixture). The practical determination of SWpH is direct because the calibration of the electrodes is carried out with commercial aqueous standard buffers. Thus, the SSpH value of any buffered acetonitrile/water mobile phase used in reversed-phase liquid chromatography, which is directly related to the ionized fraction of analyte and, therefore, to its average retention, can be easily known at any temperature from the measured SWpH and the corresponding delta value.