Method of continuous variations

Of the three methods, the method of continuous variations, also called Job s method, is the most popular. In this method a series of solutions is prepared such that the total moles of metal and ligand, tot> in each solution is the same. Thus, if ( m)( and ( l)( are, respectively, the moles of metal and ligand in the i-th solution, then... 

In essence, the corrected absorbance gives the change in absorbance due to the formation of the metal-ligand complex. An example of the application of the method of continuous variations is shown in Example 10.7. 

Both the method of continuous variations and the mole-ratio method rely on an extrapolation of absorbance data collected under conditions in which a linear relationship exists between absorbance and the relative amounts of metal and ligand. When a metal-ligand complex is very weak, a plot of absorbance versus Ay or n-J m may be curved, making it impossible to determine the stoichiometry by extrapolation. In this case the slope ratio may be used. 

In this experiment the method of continuous variations is used to determine the stoichiometry and equilibrium constant for the organic complex of 3-aminopyridine with picric acid in CHCI3, and the inorganic complex of Fe +with salicylic acid. 

Data from the spectrophotometric titrations of Fe + with SCN , and of Cu + with EDTA are used to determine the stoichiometry of the resulting complexes using the method of continuous variations. 

This experiment describes the use of FIA for determining the stoichiometry of the Fe +-o-phenanthroline complex using the method of continuous variations and the mole-ratio method. Directions are also provided for determining the stoichiometry of the oxidation of ascorbic acid by dichromate and for determining the rate constant for the reaction at different pH levels and different concentration ratios of the reactants. 

Another method for determining the composition of a complex in solution is that known as Job s method or the method of continuous variations. Suppose that the formation of a complex between metal A and m ligands B can be shown as... 

Determination of the stoichiometry of a complex by the method of continuous variations (Job s method)... 

A plotting protocol (also known as the method of continuous variation) that provides useful information about protein-ligand and protein-protein interactions. Mole fractions Xa and Xb of two interacting substances, say A and B, are varied such that the total molarity remains constant. Note that Xa = [A]/([A] + [B]) and Xb = [B]/([A] + [B]), such that (Xa + Xb) = 1. If an enzyme prefers to bind AB as a one-to-one complex, then the enzymatic activity will be maximal at a mole fraction Xa of 0.5 (Le., the point at which A and B are present in a one-to-one stoichiometry). Similarly, if AB2 is the active species, then the enzyme will be most active at a Xa value of 0.33. In this manner, the stoichiometry of binding can be readily determined, and the technique can yield information concerning the affinity of the enzyme for the active species. 

Huang has provided an excellent account of the application of this method of continuous variation in studies... 

In this case, X + Xb = 1, and this formulation allows one to analyze binding interactions between two such components by the method of continuous variation. See... 

METHIONYL-tRNA SYNTHETASE METHIONYL-tRNA FORMYLTRANSFERASE METHIONYL-tRNA SYNTHETASE AMINO-ACYL tRNA SYNTHETASES Method of continuous variation,... 

METHOD OF CONTINUOUS VARIATION MOLYBDENUM COFACTOR (MoCo) Molybdenum-dependent reactions, ALDEHYDE OXIDASE MOLYBDOPTERIN NITRATE REDUCTASE NITROGENASE SULFITE OXIDASE XANTHINE DEHYDROGENASE MOLYBDOPTERIN... 

Furthermore, the apparent extinction coefficient for the complex increases steadily with increasing concentration of nickel chloride. On the basis of this evidence an attempt was made to determine the nature of the equilibrium involved and the extent to which dissociation takes place. Such an equilibrium has also been suggested by Jicha and Busch (17) on the basis of the results obtained from the method of continuous variations. The appropriate equilibrium appears to be that given in Equation 22. 

If one complex (say, PX2) predominates, the method of continuous variation (also called Job s method) allows us to identify the stoichiometry of the predominant complex. 

The method of continuous variation can be carried out with many separate solutions, as in Table 19-1. However, a titration is more sensible. Figure 19-9a shows a titration of EDTA with Cu2+. In Figure 19-9b, the abscissa has been transformed into mole fraction of Cu2+(= [moles of Cu2+]/[moles of Cu2+ + moles of EDTA]) instead of volume of Cu2+. The sharp maximum at a mole fraction of 0.5 indicates formation of a 1 1 complex. If the equilibrium constant is not large, the maximum is more curved than in Figure 19-9b. The curvature can be used to estimate the equilibrium constant.7... 

Isosbestic (crossing) points are observed when a solution contains variable proportions of two components with a constant total concentration. A Scatchard plot is used to measure an equilibrium constant, and the method of continuous variation allows us to determine the stoichiometry of a complex. In flow injection analysis,... 

Method of continuous variation. Make a graph of absorbance versus mole fraction of thiocyanate for the data in the table. 

It has been reported112 on the basis of a spectrometric study, using the method of continuous variations, that U02+ ions form a soluble 1 2 complex [U02(PhN202)2] initially, but with excess of cupferron an insoluble product, NH4[U02(PhN202)3], is obtained which has a solubility product of 5.8 2.5 x 10-10. 

Gal and Nikolic22) have shown that Job s method of continuous variations could be successfully used for finding out the composition of the species responsible for synergism and also for calculating the equilibrium quotients. In this method the distribution ratio data are obtained using a fixed composition of the aqueous phase and employing mix-... 

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