The isosbestic point presence is an argument of complexation in your case. Besides, you have a decrease in optical density when add increasing concentration of  organic ligand that help you to calculate the number of coordinated ligands and stability constant of europium complex. During complexation process several protons are released and this phenomenon also will give an information about complexing process. There are a lot of methods described in old books devoted this problem that were write by Beck M.T.; Rosotti H.S.; Ringbom A. and many other in 60-70-th years of 20th century. They described spectrophotometric and potentiometric methods for this purpose. May be you can find information about your system in old reference-books.

Would a ratiometric method work? 

I know that FURA and INDO dyes are used for fluorescent determination of calcium.

Here is a reference for adsorption: http://www.sciencedirect.com/science/article/pii/S1386142515004242

Dear Dr Dumpala,

I do not understand why you have a problem. With modern spectrophotometers, there is no difference between the UV and visible regions. You mention an isobestic point and the Job method. By varying the concentrations ratios, these methods should give you all the information necessary to calculate the metal/ligand ratio and the formation constant of the complex. If the similarity of the spectra does not allow it, you could perhaps use an electrochemical method like a potentiometric titration.

Mrs/Miss/Mr.  Dumpala ,

Only with quantum chemistry. Experimentally is difficult an unambiguous assignment.

Models are useful but are never the reality. The results of calculations can always be questioned, only well-established experimental results are definitive.

Thank you all for your informative responses. I do have the data determined by potentiometry but i do just want to use the spectrophotometric data also for such calculations.