Where nM
and nL = number of moles of the metal cation and the ligand, respectively; nT
= total number of moles (= nM + nL
) and XM
and XL = moles fraction of the metal and the ligand,
respectively.
Under ideal reaction conditions, the maximum amount of the complex [Fex
(o-phen)y
]
2+
will form when the mole fraction of Fe(II) and o-phen are in the correct stoichiometric ratio. All
other combinations result in the formation of less amounts of the complex. Since the product
complex is colored, the absorbance or the solution mixture indicates the amount of the complex
that has formed. A plot of absorbance versus the mole fraction of the metal ion (XM
) and the
ligand (XL
) generate a graph (Figure 2.1) whose maximum indicates the stoichiometric
composition. In Figure 2.1, the absorbance maximum occurs at about XL = 0.75 and XM = 0.25.
The ratio between them
XL
/XM = 0.75/0.25 = 3, indicating that the ligand – to – metal mole ratio is 3 :1.