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 degrees 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 pK(1) = - 1234.4 + 68608/T + 193.37 In T - 0.16423T - 3746800/T-2 where T is in Kelvin. By using this fitted function, the thermodynamic functions for carbonic acid at 25 degrees 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.