Determination of the thermodynamic dissociation constant of capecitabine using spectrophotometric and potentiometric titration data

Capecitabine has been studied by a combined potentiometric and spectrophotometric titration at T = (298.15, 310.15) K at different ionic strengths. The thermodynamic dissociation constant was estimated by non-linear regression of (pK(a), I) data: pK(a)(T) = 8.97(1) at T = 298.15 K and pK(a)(T) = 8.74(0) at T = 310.15 K, where the figure in the brackets is the standard deviation in last significant digits. Thermodynamic parameters Delta H degrees/kJ.mol(-1) and Delta S degrees/J.K-1.mol(-1) have been determined from the temperature variation of thermodynamic dissociation constants. The value of enthalpy Delta H degrees = 33.9 kJ.mol(-1) shows the dissociation process is endothermic and the value of entropy Delta S degrees = -51.9J.K-1.mol(-1) indicates that the dissociation is not a spontaneous process. (c) 2011 Elsevier Ltd. All rights reserved.