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Determination of temperature effecton the measurements of pKa of some weak acids and weak bases using capillary zone electrophoresis
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Abstract(s)
This study was done to determine the pKa values of a number of weak acidic and basic compounds at different temperatures and to be able to identify the effects of temperature variations in the measured pKa using capillary zone electrophoresis. The method employed was adopted from Fuguet et al [12] in which internal standard is used. Running buffers of different pH were prepared. The maximum voltage was chosen based from Ohm’s law to reduce the Joule heating effects. Considering the linearity in the relationship between voltage and current to be with an r2≥0.999, the maximum voltage was found to be 18 kV and applied to the determination of pKa values of several compounds at 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C. For the determination of the pKa values of the analytes using internal standards with comparable pKa values at different temperatures, generally the calculated values did not vary significantly from the literature values under all the observed temperatures. For the investigation of the effects of temperature changes during the measurement of the pKa of the analyte at a specific temperature, internal standard with ΔH similar or different with the analyte was utilized. Results showed a small difference between the literature and experimental pKa values of the analyte at a specified temperature despite the temperature variations in the instrument when the internal standard and analyte have similar ΔH, which follows that in this case temperature control is not critical.
Description
Keywords
Acidity constant pKa Capillary electrophoresis Internal standard Joule heat