Stepwise dissociation constants

42 Stepwise dissociation constants. The dissociation of a triprotic acid (e.g. H3P04) can be expressed as a series of stepwise dissociation constants  

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TABLE 7.4 Stepwise Dissociation Constants for Several Common Polyprotic Acids... 

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. 

Where is the total concentration of acid species and A, and K2 are the first and second stepwise dissociation constants of the acids. This equation can be used to compute the buffer index of a polyprotic acid as long as successive dissociation constants differ by at least 20 times (this assures a calculation error of 5% or less). In other words, for a diprotic acid K2fK should be less than 0.05 (cf. Butler 1964). Thus, for example, Eq. (5.114) may be used to compute the buffer index due to species of carbonic acid, for which A = 10 and K2 = 10" °, or/ for species of silicic acid, for which a , = lO- and K, = 10... 

H2S species. Schoonen and Barnes (1988) suggest the following stepwise dissociation constants for the polysulfide acids ... 

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... 

Successive stepwise dissociation constants become smaller. 

The stability constant given in Eq. 4 is an association constant. A dissociation constant is numerically equal to the reciprocal of the corresponding association constant. It is common practice to quote dissociation constants for acids (particularly organic acids), while association constants are used for complexes. For a monobasic acid, HL. there are no further complications. But for polybasic acids, the numerical order of the two sets is reversed. For example, for a dibasic acid, the first stepwise dissociation constant is equal to the reciprocal of the second stepwise association constant and vice versa. 

Water-soluble p-sulfonatocalix[7]arene derivative 24i was obtained [58] by treatment of p-H-calix[7]arene 24a with H2SO4 at 80 °C for 3 h. The following stepwise dissociation constants of 24i were obtained by potentiometric titration p/sTai = 3.19 pAT32 = 5.40, pAT = 9.41, pAT,4= 12.0 [58]. 

TABLE 14.4 Stepwise Dissociation Constants for Severai Common Poiyprotic Adds... 

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