Ligands acidity constants

Conditional Metal—Ligand Formation Constants Recognizing EDTA s acid-base properties is important. The formation constant for CdY in equation 9.11 assumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, then equation 9.11 provides an adequate description of the formation of CdY . for pH levels less than 12, however, K overestimates the stability of the CdY complex. 

Soil particles were found to have a capacity for ligand binding of 2 1(H mol g-1 these surface functional groups are characterized by an apparent "mono-protic" acidity constant... 

Effect of Ligand. The hematite is characterized by the acidity constants... 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

Hence, the Pi ligand parameter reflects, in an overall way, the combined a- and Tt -electronic properties of the coordination M—L bond. It is noteworthy to mention that it relates to the variation of the free-energy difference of the redox processes (consider the known expression AG = —nFE, in which n is the number of electrons transferred and F is the Faraday constant). It has analogies with the Hammett Up constant [11, 12] defined as og[Kx/Kh), that is, log Kx - log K, in which Kx and ATh are the acidic constants of the p-substituted benzoic acid HOOCCg H4X-4 and of benzoic acid itself, respectively [13] (consider also the known relationship AG = —RT nK). 

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. 

A number of recent studies have shown that under certain conditions, FABMS indeed can very accurately measure the balance of ionic species in ongoing chemical reactions in solutions. These studies include the determination of acid dissociation constants (2), equilibrium constants for enzyme catalyzed reactions (1), metal-ligand association constants 03), and measurements of... 

Table 1. Acidity Constants of the Aqua Ligand in Various Pp-Nucleobase Complexes...

Many organic ligands are the conjugate bases of comparatively weak acids and their metal stability constants are readily obtained by pH-metric titrations after a preliminary determination of the important hydrogen-ion stability constants of the ligand acid itself... 

In the case of a polyprotic acid for which the individual ionizations are well separated (ideally, by at least 3 log units), values for the individual constants can be calculated from data points in the appropriate regions of the titration curve. If the individual ionizations overlap, the Bjerrum fi (n-bar) method may be used. This mathematical approach was introduced by Bjerrum for the calculation of stability constants of metal-ligand complexes, but it can also be applied to the determination of proton-ligand equilibrium constants. 

In simple models a combination of different ligands (typically two to five) is used each ligand is described in terms of a concentration, a metal-ion complexation constant, and an acidity constant. An example of this modeling approach is given in Table 6.6. This model fits best the titration data shown in Figure 6.17. 

The principles considered so far for (hydr)oxides and carbonates can of course be applied to salts containing other anions. In the case of sulfides, we also have to consider the formation of complexes with sulfur ligands (SH", S ). This tendency is especially pronounced with B metals. A difficulty is that there is some uncertainty on the value of the second acidity constant of H2S... 

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