Metal complexation

The theoretical study of the complexation equilibrium associated with the law of mass action can be described as shown in Eq 1, provided that only 1:1 complex can be formed:

where M is the free metal, L is the free ligand and ML is the ligand-metal complex.

The Chau’s method is widely used to estimate the number of available binding sites of the ligand [L]₀ [25]. After each addition of the metal ion in the dilute solution of the studied polymer, the current intensity i corresponding to the free metal ion concentration, [M]_f, was measured. Firstly, a part of the metal added to the solution was bound by γ-PGA. After the occupation of all the available binding sites of γ-PGA, the added metal remained in solution and the current intensity i showed a linear draw. The value of [L]₀ was determined from the representation of i vs the metal ion total concentration introduced [M]_t.

Another common method to determine the complexation was described by Ruzic [25]. In this approach, it is assumed that only 1:1 complex is formed once all the complexing sites of the ligand is saturated by the metal ions. The conditional stability constant (K) of the ML complex species formed can be expressed as follows:

where [L]₀ represents the number of available binding sites of the ligand. Further, Eq 2 can be redefined as:

If the model is valid, the representation of [M]_f / ([M]_t -[M]_f) vs [M]_f should be a straight line. The slope and intercept allow the calculation of [L]₀ and K, respectively.