Stepwise protonation constants

The equations that follow use protonation constants rather than dissociation constants. The stepwise protonation constant of a weak base species B is the equilibrium constant for the following reaction (charges are omitted for clarity) ... 

K.K. Govender and I. Cukrowski, Density functional theory in prediction of four stepwise protonation constants for nitrilotripropanoic acid (NTPA), J. Phys. Chem. A 113 (2009), pp. 3639-3647. 

The basicity constants (Table 5) of the stepwise protonation of methylated tetramines (trienMeq and trienMeg) have been considered and compared with those of the unsubstituted parent triethylenetetramine (trien). 

The basicity constants (Table 6) of the stepwise protonation of polyazacycloalkanes have been determined. 

The experimentally determined evolution of N-N force constants upon stepwise protonation thus indicates a successive decrease in N-N bond order initiating bond cleavage, whereas the evolution of metal-N force constants reflects an increase of metal-ligand covalency, indicative of a successive strengthening of the metal-N bond. Besides... 

Similar spectrophotometric studies were also carried out for the ligand substitution reactions with SO [54] [56] and Cl- [56] [59]. However, the reaction rates were so fast that only the spectra at the respective equilibrated states could be observed. The gradual displacement of N02 discussed above is possible only because most of the N02 exists in its protonated form in the strongly acidic solution. Hydrolysis of axial aqua ligand was also examined spectrophotometrically [56][57]. The stepwise formation constants... 

Ion exchange data indicated the presence of two complexes of the trivalent ion, Pu(S04)" " and Pu(S0i )2, with respective stepwise stability constants of 44.7 and 43.7 at I = 2 M and 25° (12). It is unlikely and K2 would have such similar values, and hence K2 must be regarded with suspicion. Because these values were the same in both 1 M and 2 M HClOi, it was concluded that no proton-ated species were formed. This value of takes precedence over... 

That is, the acid involved in each successive step of the dissociation is weaker. This is shown by the stepwise dissociation constants given in Table 7.4. These values indicate that the loss of a second or third proton occurs less readily than the loss of the first proton. This result is not surprising the greater the negative charge on the acid, the more difficult it becomes to remove the positively charged proton. 

The acid dissociation constant of the free ligand, when the proton attached to the nitrogen dissociates, is approximately 10 14 B, and the effect of metal chelation is to increase this dissociation constant by a factor of 10B to 108. The values of Ka, and Ka the stepwise dissociation constants, were determined spectrophotometrically and potentiometrically, and the acid-strengthening effect on the NH group was found to depend markedly on the chelated metal ion (95). The order of increasing acidity is Mn(II) < Cd(II) < Zn(II) < Ni(II) < Fe(II). The values of pKa, and pKat for any one metal were found to be within 1.3. It therefore appears that these... 

In addition, the stepwise stability constant i MLH is frequently used when protonation of the metal complex is examined (301) ... 

The Principal Species assessment in this work differs from previous presentation formats in its objective of providing a simple quantitative means of assessing Principal Species variations with changes in pH. Stepwise equilibrium constants provide a simple means of assessing species concentration ratios as a function of pH. In the case of equilibria involving simple protonation of complex anions, MOx(OH)yn-, stepwise equilibrium constants are expressed in the form... 

Rossotti et al [Fa 70] found that the ratio of stability constants obtained by equilibrium measurements in solvent mixtures similar to the above was independent of the composition of the solvent. McBryde et al showed that in most of the systems they examined, the ratio of the first two stepwise complex stability constants did not depend on the solvent. In the case of the stepwise acid dissociation constants this correlation was approximately true only for ethylenediamine for glycine the ratio of the protonation constants increase with increase in the concentration of the organic component. However, if the formation of the glycine complexes is defined as a reaction between the protonated ligand and the metal ion, accompanied by the liberation of a hydrogen ion, the ratio of the equilibrium constants is approximately independent of the composition of the solvent mixture. 

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