analysis methods

Methods of Analysis
A determination of the stoichiometric ratio of ligand to metal or donor to acceptor (n) and a quantitative expression of the stability constant for complex formation are important in the study and application of coordination compounds. A limited number of the more important methods for obtaining these quantities is presented here.

Method of Continuous Variation
Job suggested the use of an additive property such as the spectrophotometric extinction coefficient (dielectric constant or the square of the refractive index may also be used) for the measurement of complexation. If the property for two species is sufficiently different and if no interaction occurs when the components are mixed, then the value of the property is the weighted mean of the values of the separate species in the mixture. This means that if the additive property, say dielectric constant, is plotted against the mole fraction from 0 to 1 for one of the components of a mixture where no complexation occurs, a linear relationship is observed, as shown by the dashed line in Figure 10-7. If solutions of two species A and B of equal molar concentration (and hence of a fixed total concentration of the species) are mixed and if a complex forms between the two species, the value of the additive property will pass through a maximum (or minimum), as shown by the upper curve in Figure 10-7. For a constant total concentration of A and B, the complex is at its greatest concentration at a point where the species A and B are combined in the ratio in which they occur in the complex. The line therefore shows a break or a change in slope at the mole fraction corresponding to the complex. The change in slope occurs at a mole fraction of 0.5 in Figure 10-7, indicating a complex of the 1:1 type.








When spectrophotometric absorbance is used as the physical property, the observed values obtained at various mole fractions when complexation occurs are usually subtracted from the corresponding values that would have been expected had no complex resulted. This difference, D, is then plotted against mole fraction, as shown in Figure 10-8. The molar ratio of the complex is readily obtained from such a curve. By means of a calculation involving the concentration and the property being measured, the stability constant of the formation
If the magnitude of the measured property, such as absorbance, is proportional only to the concentration of the complex MAn, the molar ratio of ligand A to metal M and the stability constant can be readily determined. The equation for complexation can be written as