COMPETITIVE COMPLEXATION OF LANTHANIDE CATIONS BY XYLITOL AND NITRATEANION IN AQUEOUS-SOLUTION - A MICROCALORIMETRIC INVESTIGATION

The apparent equilibrium constants and enthalpies of complexation of N d3+ Sm3+, Eu3+, and Gd3+ by xylitol in aqueous solutions containing Na NO3 at an ionic strength of 2.0 mol-kg(-1) have been determined by mic rocalorimetry at 25 degrees C. Since nitrate anion weakly complexes th e lanthanide cations, these values are analyzed in terms of competitio n between xylitol and NO3-. The method leads to the apparent equilibri um constants and enthalpies of complexation of the lanthanide cations by NO3- at this particular ionic strength. Despite the difficulties en countered in characterizing rather weak associations, the results are, whenever comparison is possible, in good agreement with those obtaine d by direct microcalorimetry. The advantage of this competition method is that it can be used when the enthalpic effects are too weak and in sufficiently concentration dependent for direct microcalorimetric dete rmination. In the present case, it allows us to thermodynamically char acterize the formation of SmNO32+ and EuNO32+, processes we have not b een able to study directly.