STANDARD FORMATION ENTHALPIES OF HYDROXYETHYLIDENEDIPHOSPHONIC ACID AND PRODUCTS OF ITS DISSOCIATION IN AQUEOUS SOLUTION

This study aimed the measuring the standard enthalpies of formation of hydroxyethylidenediphosphonic acid and the products of its dissociation in aqueous solutions at 298.15 K. In the literature there is a large amount of data on the constants of stepwise dissociation of the acid. The works were done at different temperatures and the values of the ionic strength of the solution on the background of the different nature of the supporting electrolytes. In order to compare the values of the constants of stepwise dissociation of the acid, obtained by different authors, we recalculated the values of pK(1), pK(2), pK(3), pK(4), pK(5) at zero ionic strength by the Davies equation. The most probable values of thermodynamic constants of dissociation of hydroxyethylidenediphosphonic acid were the following: pK(1)(0) = 1.7 +/- 0.03; pK(2)(0) = 2.47 +/- 0.05; pK(3)(0) = 7.78 +/- 0.05; pK(4)(0) = 10.29 +/- 0.05 and pK(5)(0) = 11.13 +/- 0.05. Calorimetric measurements were performed on an ampoule calorimeter equipped with an isothermal shell and a KMT-14 thermistor temperature sensor, and the automatic recording the temperature changes over time. The operation of installation was verified with the integral enthalpies of dissolution in water of crystalline potassium chloride and was considered suitable for measurements, if certain it Delta H-sol(298.15) (KCl infinity H2O)=17.234 +/- 0.018 kJ/mol, corresponding to the standard SRM 1655NBS. The heat value of the calorimeter water was 87 +/- 12 sicsicsic/mol virtually coincided with the thermal value of the calorimeter for potassium hydroxide. The volume of the calorimetric liquid was 43.12 ml. The hydroxyethylidenediphosphonic acid samples were weighed on a VLR-200 balance with 2.10(-4) g accuracy. The confidence range of the average Delta H value was determined at 0.95 probability. The equilibrium compositions of solutions were calculated using RRSU software. The standard enthalpies of formation of hydroxyethylidenediphosphonic acid and the products of its dissociation in aqueous solutions were calculated.