Question

sketch a Job's method plot below for a complex where the ratio of ligand to metal is 3:1.

Answer 1

Job's Method Plot:

A Job plot is also known as the method of continuous variation or Job's method which is named after P. Job is used to determine the stoichiometry of a metal-ligand complex. This method is widely used in analytical chemistry, instrumental analysis. In 1928 In 1928, Paul Job published a paper entitled Formation and Stability of Inorganic Complexes in Solution. He studied associations of ion. Part of his research focused on relating UV absorption to the mole fraction of Tl(NO₃) in a Tl(NO₃)/NH₃ solution. By plotting the absorbance versus mole fraction gives the first Job Plot, which provids information about equilibrium complexes and thus complexes present in solution.

In a solution containing two species, one species (A) binds with another species (B). In some cases, more than one A can binds to B. Job's plot is a way used to determine  the amount of A binding to B .

In this method, the molar concentrations of two species e.g. metal and ligand are kept constant, but their mole fractions are varied. He plotted proportional to complex formation against mole fraction i.e. X_A is the mole fraction of A and P is the physical property being measured during complex formation.

A simple Job Plot showing how a physical property changes upon changing the mole fraction of compound A, is shown in fig. 1 in image below.

Fig. 2 shows the Job's plot for 3:1 ligand to metal e.g. the metal–ligand complex between Fe²⁺ and o-phenanthroline.