Spectrophotometric measurement of the first dissociation constants of carbonic acid at elevated temperatures

Using 2,5-dintrophenol as an optical indicator, the pH of a carbonate buffer solution was spectrophotometrically measured at temperatures from 25 to 175°C. The pH values measured in this study were utilized to determine the first dissociation constant of carbonic acid at various temperatures. The result was pK₁ = -1234.4 + 68608/T + 193.37 ln T - 0.16423T - 3746800/T² where T is in Kelvin. By using this fitted function, the thermodynamic functions for carbonic acid at 25°C were calculated. When the dissociation reaction of carbonic acid was rewritten as an isocoulombic reaction, the plot of log K_OH against 1/T was approximately linear over the temperature ranges studied, as in other isocoulombic reactions.