The 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,
M
n+ + 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+
Job’s method of continuous variation will be used in conjunction with visible
spectroscopy to establish the formula of the complex formed by Fe(II) and 1,10-phenanthroline.