Temperature dependence of dissociation constants for formic acid and 2,6-dinitrophenol in aqueous solutions up to 175 °C

Abstract

The temperature dependence of the dissociation constants for formic acid and 2,6-dinitrophenol (2,6-DNP) has been determined in aqueous solutions from 25 to 175 °C using spectrophotometric measurements. The optical indicator 2,6-DNP was employed to measure the pH of a buffer solution by deconvolution of absorption spectra of 2,6-DNP at various temperatures. First, using the temperature dependence of pKa for acetic acid, pKi of 2,6-DNP were determined in acetate buffer solutions as a function of temperature. The results were then used to study the ionization reaction of formic acid at elevated temperature. Measured values of pKa for formic acid were fitted to a function of temperature, which was derived by assuming that ΔCop is a non-zero constant. The result for formic acid was given by pKa=– 57.528 + 2773.9/T+ 9.1232 ln T where T is absolute temperature. Additionally, thermodynamic functions of formic acid were determined from the temperature dependence of pKa and compared with those obtained from other methods. The isocoulombic reactions for formic acid and 2,6-DNP were considered to predict the temperature dependence of pKa by the extrapolation of – log KOH values to higher temperatures.