Diprotic and Polyprotic Acids

Diprotic and polyprotic acids undergo successive ionizations, losing one proton at a time [ M4 Sec- ti6n 4. 3]V md each ionization has a associated with it. Ionization constants for a diprotic acid are designated and We write a separate equilibrium expression for each ionization, and we may need two or more equilibrium expressions to calculate the concentrations of species in solution at equilibrium. For carbonic acid (H2CO3), for example, we write 

Sample Problem 16.18 shows how to calculate equilibrium concentrations of all species in solution for an aqueous solution of a diprotic acid. 

Oxalic acid (H2C2O4) is a poisonous substance used mainly as a bleaching agent. Calculate the concentrations of all species present at equilibrium in a 0.10 AT solution at 25 °C. 

Strategy Follow the same procedure for each ionization as for the determination of equilibrium concentrations for a monoprotic acid. The conjugate base resulting from the first ionization is the acid for the second ionization, and its starting concentration is the equilibrium concentration from the first ionization. 

Setup The ionizations of oxalic acid and the corresponding ionization constants are 

This result may seem strange at first, but if we add the two equations we see that the sum is simply the autoionization of water. 

This example illustrates one of the rules for chemical equilibria When two reactions are added to give a third reaction, the equilibrium constant for the third reaction is the product of the equilibrium constants for the two added reactions (see Section 14.2). Thus, for any conjugate acid-base pair it is always true that 

We can use Equation (15.12) to calculate the Ki, of the conjugate base (CH3COO ) of CH3COOH as follows. We find the value of CH3COOH in Table 15.3 and write 

Consider the following two acids and their ionization constants  

The treatment of diprotic and polyprotic acids is more involved than that of monoprotic 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 add solution. For example, for carbonic acid, H2CO3, we write 

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So far we have only considered monoprotic acids and bases. Fortunately, the corresponding relations for diprotic and polyprotic acids and bases are quite similar. The relations for diprotic acids and bases will be given here, while those for their polyprotic counterparts (with three or more dissociable protons) can be found in section 4.9. 

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

Diprotic and polyprotic acids have more than one proton to donate. They undergo stepwise ionizations. Each ionization has a Ky value associated with it. 

Ionization Constants of Some Diprotic and Polyprotic Acids at 25°C Table 16.8 p. 701... 

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