Determining the Dissociation Constant of N-(2-Acetamido)-Iminodiacetic Acid Monosodium (ADA) at Different Temperatures from 278.15 to 328.15 K

Abstract

The aim of the study is to know the Dissociation Constant of N-(2-Acetamido)-Iminodiacetic Acid Monosodium (ADA) at different temperatures. The acidic dissociation constant of N-(2-acetamido)-iminodiacetic acid monosodium (ADA) has been determined at 12 temperatures from 278.15 to 328.15 K by electromotive-force (EMF) measurements of hydrogen-silver chloride cells without liquid junction. The commercial sample of ADA was obtained from Research Organics, Inc. The analysis of the purified sample averaged 99.9%. At 298.15 K, the value of the dissociation constant (pK₂) is 6.8416 ± 0.0004. In response to the need for new physiological pH standards, buffer solutions of NaADA and its disodium salt, Na₂ADA would be useful for pH control in the biological region of pH 6.5 to 7.5. The pK₂ values over the experimental temperature range are given as a function of the thermodynamic temperature (T) by the equation pK₂ = 2943.784/T - 47.05762 + 7.72703 ln T. At 298.15 K, standard thermodynamic quantities for the dissociation process have been derived from the temperature coefficients; \(\Delta\)H° = 12,252 J·mol^-1, \(\Delta\)S° = -89.9 J·K^-1·mol^-1 and \(\Delta\)Cp°= -148 J·K^-1·mol^-1. The results are interpreted and compared with those of structurally related derivatives of GLYCINE. It is concluded that the precise EMF method yields very stable, accurate, and reliable data with accuracy better than 0.04 mV in the entire temperature range. The buffer solution of NaADA + Na₂ADA would be useful for the measurement of pH in biological specimens.