δ Conversion parameter between pH scales (swpH and sspH) in acetonitrile/water mixtures at various compositions and temperatures

The (s)(s)pH 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 (s)(w)pH scale ( pH measured in acetonitrile/ water with electrodes calibrated in water) to the (s)(s)pH scale (pH measured in acetonitrile/ water with electrodes calibrated in the same acetonitrile/ water mixture). The practical determination of (s)(w)pH is direct because the calibration of the electrodes is carried out with commercial aqueous standard buffers. Thus, the (s)(s)pH 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 (s)(w)pH and the corresponding delta value.