Examples H3PO4 titration

A plot of the fractions of each phosphoric acid species as a function of pH is given in Figure 7.1. This figure illustrates how the ratios of the four phosphoric acid species change as the pH is adjusted, for example, in titrating H3PO4 with NaOH. While some appear to go to zero concentration above or below certain pH... 

How far apart must successive pK values be in order to have clearly distinguishable endpoints for titration curves of polyprotic acids Consider as examples H3PO4 and citric acid. What about equivalence points at very high or low pH How easy are they to locate ... 

Example 8.10 shows how the pH is calculated at the first equivalence point of the titration of phosphoric acid with sodium hydroxide, where the solution contains H2PO4-. Note that the pH of the solution does not depend on the concentration of H2P04 in the solution. Thus the pH is 4.67 at the first equivalence point in any typical titration of H3PO4. Likewise, at the second equivalence point, where the major species are HP042 and H20, the pH is calculated from the expression... 

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 pH found in Example 5-2 is quite low, showing that the mercuric ion has the property of a weak acid of approximately the same strength as acetic acid. The acid strength of some metal ions is quite considerable. For example, the pH of equimolar H3PO4 and solutions are similar. The strong acid properties of hydrated metal ions become evident in processes such as coagulation/flocculation, where hydrated aluminum sulfate (alum) or ferric chloride is added to a natural water that is buffered with bicarbonate. The metal ion "titrates" (or reacts with) a stoichiometric amount of alkalinity, just as the addition of an equal amount of strong mineral acid would. 

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