The ssoichiometric ratios and the equilibrium constants describing the extent of formation of complexes are the basic information required in studies of complex formation between two species. Some of this information can be obtained by measuring the apparent solubility, the absorption spectrum, or the reactivity of one species (the substrate) as a function of the concentration of the other species (the ligand). The stability constant evaluated by these methods, assuming that a single complex of 1 : 1 substrateligand~ratio is present, can be related to the actual constants of the system. Analyses of some of the complex systems most like\y to be encountered show that the three experimental approaches may not always yield the same numerical result and that comparative studies with several techniques may yield valuable information concerning the natures of the complexes. The solubility, spectral, and kinetic methods for studying complexes are, in general, subject to about the same degree of nonselectivity in their responses to multiple complexes and interactions.
HE CONCEPT of complex formation has been Tadopted
as a simple hypothesis that can account for nonadditive behavior in the physical and chemical properties of solutions of two or more species. With this hypothesis i t becomes possible to utilize quantitative measures of these properties to describe the extent of interaction between the species and to investigate the nature of the interaction product, or complex. Many definitions of "a complex" have been proposed, but for the purpose of this papcr i t will not be necessary t o limit sharply the chemical nature of the species, and the techniques to be discussed may be applicable to the study of reactions that may not he accepted a q complexation reactions. Throughout this paper complex formation is considered to be a reversible chemical reaction in which the rate of attainment of equilibrium is much greater than any rates involved in the measuring processes. The system is, therefore, considered to be at equilibrium.
The basic purpose of studies of complex formation is to provide a comprehension of the properties of complexes, including their structure and reactivity. Since the reversibility of complex formation is the fundamental aspect relating all of these processes, thc general reaction may be written
where S represents the substrate and L is the