Finding the pH of Polyprotic Acid Solutions

Finding the pH of a polyprotic acid solution is less difficult than we might first imagine because, for most polyprotic acids, is much larger than (or for triprotic acids). Therefore the amount of H30 formed by the first ionization step is much larger than that formed by the second or third ionization step. In addition, the formation of HsO in the first 

To find the pH, you must find the equilibrium concentration of H30. Treat the problem as a weak acid pH problem with a single ionizable proton. The second proton contributes a negligible amount to the concentration of H30 and can be ignored. Follow the procedure from Example 15.6, shown in condensed form here. Use for ascorbic acid from Table 15.10. 

Confirm that the x is small approximation is valid by calculating the ratio of x to the number it was subtracted from in the approximation. The ratio should be less than 0.05 (or 5%). 

Sulfuric acid is strong in its first ionization step and weak in its second. Begin by writing the equations for the two steps. As the concentration of an H2SO4 solution becomes smaller, the second ionization step becomes more significant because the percent ionization iuCTeases (as discussed in Section 15.6). Therefore, for a concentration of 0.0100 M, you can t neglect the H30 contribution from the second step, as you can for other polyprotic acids. You must calculate the H30 contributions from both steps. 

The [HsO ] that results from the first ionization step is 0.0100 M (because the first step is strong). To determine the [H30 ] formed by the second step, prepare an ICE table for the second step in which the initial concentration of H3O is 0.0100 M. The initial concentration of HS04 must also be 0.0100 M (due to the stoichiometry of the ionization reaction). 

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