formation of complex in solution is often accompanied by the color appearance. Measurement of the absorbance of such a solution will afford a measure of the amount of complex ion in solution. For a reaction of the type,
Mn+ + yL [MLy]n+
the amount of complex ionic solution can be determined colorimetrically for various ratios of [Mn+] to [L]; the total concentration of metal ion and ligand is kept constant. Measurements of the absorbance at a suitable wavelength will show a maximum when the ratio of ligand to metal is equal to that in the complex. The method is known as ‘Job’s Method’, after the originator, and is often referred to as ‘The Method of Continuous Variation’. Measurement may be made at any wavelength where the complex shows appreciable absorption. A portion of maximum absorption is preferred.
If the measurements are made at only one wavelength then the system must such that only one complex is formed. This may be verified by making measurements at a number of wavelength within the absorption spectrum of the complex(es). If measurements at all wavelengths give the same result, it may be concluded that only a single compound is formed. Note that concurrent formation of a colorless compound may be overlooked by this method.
In general, transition metal ions form a large number of complex compounds. For example, the almost colorless iron(II) cation reacts with 1,10-phenanthroline (o-phen, colorless) to form a red complex cation:
xFe2+(aq) + y(o-phen) [Fex(o-phen)y]2+