δ conversion parameter between pH scales (WSpH and SSPH) in acetonitrile/water mixtures at various compositions and temperatures

Abstract

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°C. In this paper, the δ parameter has been determined for acetonitrile/water mixtures from 0 up to 90% acetonitrile at different temperatures from 15 to 60°C, and the values were fitted to a very simple simultaneous function of composition and temperature. The δ values allow conversion of the WSpH scale (pH measured in acetonitrile/water with electrodes calibrated in water) to the S SpH scale (pH measured in acetonitrile/water with electrodes calibrated in the same acetonitrile/water mixture). The practical determination of WSpH 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 WSpH and the corresponding δ value.