I'm going to plot a Beer's Law calibration curve by taking 0.05M, 0.1M and 1M Nickel chloride solution and measuring the absorbance, before plotting my results on a graph.
I reacted the metal ion solution (using unknown concentration of NiCl2) and the ligand together in different ratios, and I'm going to plot an average of the maximum absorbance (for each complex) on the graph.
My question is, what will this actually tell me the concentration of in the complex? Is it the Ni2+ ions, or NiCl2, because NiCl2 doesn't actually exist in any of the complexes?
Indeed - as when you dissolve NiCl2 it dissociates into it's respective ions - Ni2+ and Cl-, it is the Ni2+ you then react the ligands with (via ligand substitution) to give you a complex with n ligands NiLn(depending on what type of ligand it is):
Ni2+ + nL ----> NiLn so as you can see the ratio of complex to Ni2+ is 1:1
In dilute solution, the measured absorbance is a measure of the Ni-complex. But how the Ni is partitioned between free and complexed forms doesnt actually matter. This will vary depending on the equilbrium constant for the complexation reaction and the concentrations used. But you dont need to know these details and can use the callibration curve totally operationally. So having got a curve relating the measured absorbance to the total nickel in standards you can work out the total nickel in an unknown sample by reference to the callibration curve. However, if the curve is linear with a flat top you can assume that the reaction is essentialy irreversible so that limiting amounts of Ni are completely complexed (but equally it wont matter if that is not the case)