Ligand titration curve

The formation of metal complexes is indicated by the considerable shift along the volume axis of the metal-ligand titration curve as compared to the ligand titration curve. The study assumes a negligible extent of hydrolysis of M(III), and the formation of polynuclear... 

Fig. 12. Titration curves for complexometric titrations. Titration of 60.0 ml of a solution that is 0.02 mol dm-3 in M curve A represents a 0.02 mol dm-3 solution of the tetradentate ligand D to give MD as product curve B represents a 0.04 mol dm-3 solution of the bidentate ligand B to give MB2 and curve C represents a 0.08 mol dm-3 solution of the un-identate ligand A to give MA4. The overall formation constant for each product is 1.0 x 1020. Adapted from ref. 115.

Figure 12 [115] shows a series of complex formation titration curves, each of which represents a metal ion-ligand reaction that has an overall equilibrium constant of 1020. Curve A is associated with a reaction in which Mz+ with a coordination number of 4 reacts with a tetradentate ligand to form an ML type complex. Curve B relates to a reaction in which Mz+ reacts with bidentate ligands in two steps, first to give ML complexes, and finally close to 100% ML2 complexes in the final stages of the titration. The formation constant for the first step is 1012, and for the second 108. Curve C refers to a unidentate ligand that forms a series of complexes, ML, ML2. .. as the titration proceeds, until ultimately virtually 100% of Mz+ is in the ML4 complex form. The successive formation constants are 108 for ML, 106 for ML2, 104 for ML3, and 102 for ML4 complexes. 

Example 2.4 Shift in the Alkali metric Titration Curve of an Oxide in the Presence of an Adsorbable Metal Ion or Ligand... 

In a similar way one can treat the shift (in the opposite direction) of an alkalimetric-acidimetric titration curve by the adsorption of a ligand (Sigg and Stumm, 1981), e.g.,... 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

In Fig. 3.5 we illustrated generally that an alkalimetric or acidimetric titration curve of a hydrous oxide dispersion becomes displaced by the adsorption of a metal ion or, - in opposite direction - by the adsorption of an anion (ligand). 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

Fig. 10. NMRD curves of free Gd (O) and its complexes with calix[4]arenes2(0) (at the maximum of the relaxivity titration curve), 3 ( ) (at the maximum of the relaxivity titration curve), 4 (A) (ligand to metal ratio = 2) in anhydrous acetonitrile at 25° C (63,66,67).

The pH dependence of could be due to changes in A-B loop disorder rates, perhaps the chemical exchange phenomenon observed for NPl-ImH (Section ll,E,2,b), or to changes in ligand bond strength. The change in lies in the off-rates (Tables I-Ill) consistent with the loop disorder model. Plots of vs pH display an excellent fit with the equation for a titration curve (Fig. 21), indicating that the transition... 

Scheme 3.4-1. Simulated titration curves for the catalytic model system described above. The change in the steady-state concentrations following the ligand association process is schematically depicted (the species present at relatively high concentration is underlined).

Solid lines standard titration curves, broken lines manifold systematic variations, arrows direction of the induced relative shift. F s. 1 and 2 simulate structural changes in the ligand-free complexes. Figs 3-6 inhibition and activation processes induced by the controlling ligand (kinetic control). Figs 7 and 8 simulate a variation of the catalytic concentration (see Scheme 3.3-4) or of the constants of association of L to M (thermodynamic control). 

To obtain information on the coupling of the various intermediates one has to analyze the relationship between the corresponding titration curves. Scheme 3.4-3 shows typical steady-state curves for the (1) stepwise twofold association of ligand L with metal complex M, (2) association of L with two metal complexes M and N at equilibrium and (3) association of L to two metal complexes M and N being not at equilibrium (kinetically separated). From these three types of coupling most of the partial maps can be easily interpreted. 

In [LJ-control maps the substitution of one ligand by another one results in a change of the range of existence of the manifold intermediates. This change can be expressed by the ligand-property imluced shift of the titration curves identified by the relative position of their inflection points Lq s on the log (lL o/[Ni)Q) scale. These characteristic shifts provide information on the thermodynamic selectivity governed by the association processes only. This type of analysis is designated by . 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

A difference plot, also called a Bjerrum plot, is an excellent means to extract metal-ligand formation constants or acid dissociation constants from titration data obtained with electrodes. We will apply the difference plot to an acid-base titration curve. 

Figure 1 Titration curves for H,edta. Curve 1 H4edta curve 2 H4edta + lithium curve 3 H4edta + magnesium curve 4 H4edta + copper. The quantity a is in moles of strong base per mole H4edta. Total ligand and metal concentration ...

FIGURE 4.9 Relaxation titration curves (left) and NMRD dispersion curves (right) of ligands CPw3 ( ) and CPwl7 ( ) in anhydrous acetonitrile. 

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