Titration polyfunctional base

The derivation of a titration curve for a polyfunctional base is similar to the previous one for acid. 

The construction of a titration curve for a polyfunctional base involves no new principles. To illustrate, consider the titration of a sodium carbonate solution with standard hydrochloric acid. The important equilibrium constants are... 

This chapter describes polyfunctional acid and base systems, including buffer solutions. Calculations of pH and titration curves are also described. 

The pH of polyfunctional systems, such as phosphoric acid or sodium carbonate, can be computed rigorously through use of the systematic approach to multiequilibrium problems described in Chapter 11. Solution of the several simultaneous equations that are involved is difficult and time consuming, however. Fortunately, simplifying assumptions can be invoked when the successive equilibrium constants for the acid (or base) differ by a factor of about 10 (or more). With one exception, these assumptions make it possible to compute pH data for titration curves by the techniques we have discussed in earlier chapters. 

Thus far, we have not considered how to calculate the pH of solutions of salts that have both acidic and basic properties—that is, salts that are amphiprotic. Such salts are formed during neutralization titration of polyfunctional acids and bases. For example, when 1 mol of NaOH is added to a solution containing 1 mol of the acid... 

Taking all of these considerations together, we can deduce that the possibility of the satisfactory sequential titration of the different functions of a polyfunctional compound is, above all, based on the values of the standard potential differences E° 2 — E°n, E°i2 —E°i2. These differences permit us to neglect some exponentials according to the stages of the titration. In the case of the titration of multifunctional redox derivatives, it experimentally turns out that a difference amounting to 0.24 V is sufficient to detect the successive functions. The difference AE° = 0.24 V has already been encountered. It was during the study of the titration of a monofimctional redox compound, but the difference concerned the standard potentials of the titrand and the titrant, and not only that of the titrand as here. 

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