Polyprotic acids ionization constants

Polyprotic acids ionize in steps, each having its own value for the dissociation constant. 

Certain weak acids are polyprotic they contain more than one ionizable hydrogen atom. Such acids ionize in steps, with a separate equilibrium constant for each step. Oxalic acid, a weak organic acid sometimes used to remove bloodstains, is diprotic ... 

The secondary ionization constant of polyprotic acids is always smaller than the primary (K2 < K1) the tertiary, K3, is even smaller and so on. 

The dependence of the equilibrium constants of polyprotic weak acids and weak bases on ionic strength allows them to be treated in exactly the same manner as monoprotic weak acids and weak bases. The first, second, and third ionization constants for triprotic weak acids in terms of activity coefficient are given by ... 

For polyprotic acids such as H3PO4 or H3As04, there is usually a factor of approximately 105 difference in successive Ka values. Phosphoric acid has dissociation constants that have the values Kal = 7.5 x 10-3, Ka2 = 6.2 x Itr8, and Ka3 = 1.0 x 10-12. This is because the first proton comes from a neutral molecule, the second from a -1 ion, and the third from a -2 ion. As a result of electrostatic attraction, it is energetically less favorable to remove H+ from species that are already negative. When considering the first and second ionization... 

Acids that contain more than one ionizable hydrogen atom per molecule are called polyprotic acids. These acids ionize in steps. The second (or third) proton has a much lower dissociation constant than does the prior proton because it is harder to remove a hydrogen ion the more negatively charged the Brpnsted acid (Table 19.4). Also, the prior ionization produces hydronium ions that repress the further ionization, in accord with LeChatelier s principle. Any acid ionizes less in the presence of a stronger acid (see Example 19.20). Thus, the hydronium ion in a solution of a polyprotic acid comes mainly from the first step in the ionization. 

Successive Ionizations of a Polyprotic Acid. A polyprotic acid has several acid constants, corresponding to dissociation of successive hydro-... 

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. 

Thus far we have considered only monoprotic weak acids. Acids that can furnish two or more hydronium ions per molecule are called polyprotic acids. The ionizations of polyprotic acids occur stepwise, that is, one proton at a time. An ionization constant expression can be written for each step, as the following example illustrates. Consider phosphoric acid as a typical polyprotic acid. It contains three acidic hydrogen atoms and ionizes in three steps. 

As a simple illustration of the importance of dipolar effects on acid strength, the pKa values of selected benzenecarboxylic acids are presented in Figure 3. Using benzoic acid as a reference, it is clear that the first ionization constants of all the other acids are greater, even though all the acids are uncharged. A small part of the increased acidity is due to statistical effects, but most of the effect is attributable to dipolar stabilization of the monoanion in the polyprotic acids. The dipolar effect is so pronounced in these molecules that the Ka values of several of the anions are greater than that of benzoic acid. 

The treatment of diprotic and polyprotic acids is more involved than that of mono-protic acids because these substances may yield more than one hydrogen ion per molecule. These acids ionize in a stepwise manner that is, they lose one proton at a time. An ionization constant expression can be written for each ionization stage. Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution. For example, for carbonic acid, H2CO3, we write... 

Table 15.5 shows the ionization constants of several diprotic acids and one polyprotic acid. For a given acid, the first ionization constant is much larger than the second ionization constant, and so on. This trend is reasonable because it is easier to remove a ion from a neutral molecule than to remove another H+ from a negatively charged ion derived from the molecule. 

TABLE 15.5 Ionization Constants of Some Common Diprotic and Polyprotic Acids in Water at 25°C... 

We see that phosphoric acid is a weak polyprotic acid and that its ionization constants decrease markedly for the second and third stages. Thus we can predict that, in a solution containing phosphoric acid, the concentration of the nonionized acid is the highest, and the only other species present in significant concentrations are and H2PO4 ions. 

Since, phosphoric acid is a weak polyprotic acid that has three dissociation constants, four species (PO - , HPO, H2PO4, and H PO ) will coexist in equilibrium with one another although the concentration of some may be negligible at a particular pH. Therefore the dominating species at a particular pH will differ at different pHs. Figure 5-24 shows, the % of the ionized forms of phosphoric acid plotted against the pH. 

Note that the acid-dissociation constants are labeled ol and K 2- The numbers on the constants refer to the particular proton of the acid that is ionizing. Thus, K 2 always refers to the equilibrium involving removal of the second proton of a polyprotic acid. 

The acid-dissodation constants for common polyprotic acids are listed in V TABLE 16.3, and Appendix D provides a more complete list. The structure of citric acid illustrates the presence of multiple ionizable protons FIGURE 16.12. 

For many polyprotic acids is much larger than subsequent dissodation constants, in which case the H (aq) in the solution comes almost entirely from the first ionization reaction. As long as successive values differ by a factor of 10 or more, it is possible to obtain a satisfactory estimate of the pH of polyprotic acid solutions by treating the adds as if they were monoprotic, considering only JC i. 

Polyprotic acids, such as H2SO3, have more than one ionizable proton. These acids have acid-dissociation constants that decrease in magnitude in the order > 1 h3. Because nearly all the H (flq)... 

---