Polyprotic Acids and Their Salts

You may have wondered why, in buffer mixtures, the salt does not react with water to hydrolyze as an acid or base. This is because the reaction is suppressed by the presence of the acid or base. In Equation 7.28, the presence of appreciable amounts of either HA or OH will suppress the ionization to a negligible amount. In Equation 7.33, the presence of either B or H3O+ will suppress the ionization. 

Many acids or bases are polyfunctional, that is, have more than one ionizable proton or hydroxide ion. These substances ionize stepwise, and an equilibrium constant can be written for each step. Consider, for example, the ionization of phosphoric acid  

Recall from Chapter 16 that the overall ionization is the sum of these individual steps and the overall ionization constant is the product of the individual ionization constants  

The individual pKa values are 1.96, 7.12, and 12.32, respectively, for pKai, pKa2, and pKa3. In order to make precise pH calculations, the contributions of protons from each ionization step must be taken into account. Exact calculation is difficult and requires a tedious iterative procedure since [H ] is unknown in addition to the various phosphoric acid species. See, for example, Refs. 7 and 10 for calculations. 

In most cases, approximations can be made so that each ionization step can be considered individually. If the difference between successive ionization constants is at least 10 , each proton can be differentiated in a titration, that is, each is titrated separately to give stepwise pH breaks in the titration curve. (If an ionization constant is less than about 10-, then the ionization is too small for a pH break to be exhibited in the titration curve—for example, the third proton for H3PO4.) Under these conditions, calculations are simplified because the system can be considered as simply a mixture of three weak acids. 

---

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated ... 

Just as in the ionization of polyprotic acids, so in the hydrolysis of their salts, the reaction proceeds in successive stages. The extent of the second stage is generally very small compared with the first. This is particularly true in this case, where H2Cr04 is quite a strong acid with respect to its first ionization and much weaker in the second ionization. The equation of interest is... 

The only salts left to consider are those in which the the cation comes from a strong base and the anion comes from a polyprotic acid with one or more ionizable protons still attached. These anions are amphiprotic— they can act as an acid and release a proton to water or as a base and abstract a proton from water. As in the previous case, to determine the overall acidity of their solutions, we compare the magnitudes of and but here we compare both the and of the same species, the anion. 

---