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

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 pK_a for acetic acid, pK_i 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 pK_a for formic acid were fitted to a function of temperature, which was derived by assuming that ΔC°_p is a non-zero constant. The result for formic acid was given by pK_a = -57.528 + 2773.9/T + 9.1232 In T where T is absolute temperature. Additionally, thermodynamic functions of formic acid were determined from the temperature dependence of pK_a 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 pK_a by the extrapolation of -log K_OH values to higher temperatures.