Lanthanide complexes competitive experiments

The same kind of experiments can be applied to test varions conditions in the medium (e.g., pH effect, competition with anions, cations, etc.). It is of great importance to guarantee the integrity of the lanthanide complex in the composite enviromnent of the biological medium. Any decom-position/exchange reaction of the Ln ions would result in a loss of the luminescence intensity and a possible toxic exposme of the cells to the free Ln ions. This means that the stability of the complex has to be checked under conditions as close as possible to those encountered within cells. 

The competition titrations presented in the previous paragraph are interesting to establish the stability of a given complex compared to another. They are also useful tools to ensure the kinetic inertness of the complex over time. Any decrease of intensity would iudicate a modificatiou of the coordination sphere. On release of any ligand, the lanthanide ion would be exposed to water molecules in its vicinity, thus causing a decay of its luminescence intensity. Nevertheless, in snch long experiments, one has to ensure that the lamp is stable over time, that no evaporation of the solution or precipitation takes place over time, etc. 

Throughout this time is was appreciated that the helical structure of Ho, and indeed the rich diversity of complex structures that were discovered in the same period in many of the other lanthanides, resulted from a competition between the crystal-field and exchange interaction. It was first suggested by Vigren (1976) that this finely balanced competition could give rise to a structure with spin discommensurations or spin slips , but it was not until the first X-ray magnetic scattering experiments were performed that the full importance of this proposal was realized. 

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