Acid-base equilibria with polyprotic acids

We can predict the pH at any point in the titration of a polyprotic acid with a strong base by using the reaction stoichiometry to recognize what stage we have reached in the titration. We then identify the principal solute species at that point and the principal proton transfer equilibrium that determines the pH. 

We can predict the pH at any point in the titration of a polyprotic acid with a strong base (see Toolbox 11.1). First, we have to consider the reaction stoichiometry to recognize what stage we have reached in the titration. Next we have to identify the principal solute species at that point and the proton transfer equilibrium that determines the pH. We then carry out the calculation appropriate for the solution, referring to the previous worked examples if necessary. In this section, we see how to describe the solution at various stages of the titration our conclusions are summarized in Tables 11.3 and 11.4. 

In the 1920s, Johannes Bronsted and Thomas Lowry recognized that acids can transfer a proton to bases regardless of whether an OH" ion accepts the proton. In an equilibrium reaction, the direction of proton transfer depends on whether the reaction is read left to right or right to left, so Bronsted acids and bases exist in conjugate pairs with and without a proton. Acids that are able to transfer more than one proton are called polyprotic acids. 

We saw in the following section how polyprotic acids are capable of providing more H ions in several steps and we saw how pH ay be calculated in solutions of polyprotic acids. In connection with this we looked briefly at acid and base properties of salt and on how pH may be calculated in such salt solutions. In connection with the influence of foreign ions on the equilibrium conditions in chapter 4 we looked briefly at ion effects and its influence on pH conditions. 

Vt (aq) in a polyprotic acid solution comes from the first dissociation step, the pH can usually be estimated satisfactorily by considering only Weak bases include NH3, amines, and the anions of weak acids. The extent to which a weak base reacts with water to generate the corresponding conjugate acid and OH is measured by the base-dissociation constant, JCj. Kh is the equilibrium constant for the reaction B(aq) -H H20(/) HB (aq) -I- OH aq), where B is... 

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