Acid-base equilibrium problems with salts

In Chapter 7 we were concerned with calculating the equilibrium concentrations of species (particularly H+ ions) in solutions containing an acid or a base. In this section we discuss solutions that contain not only the weak acid HA but also its salt NaA. Although this case appears to be a new type of problem, it can be handled rather easily by using the procedures developed in Chapter 7. 

Very many problems in solution chemistry are solved with use of the acid and base equilibrium equations. The uses of these equations in discussing the titration of weak acids and bases, the hydrolysis of salts, and the properties of buffered solutions are illustrated in the following sections of this chapter. 

Type 2. What is the pH of a 0.01 M solution of NaA (pXa of HA = 5.0) The salt NaA completely dissociates in H20 to give Na+ and A". This is a type 2 problem because only the base form (A-) is initially present. HA is a weak acid, the A" will tend to combine with any available H+ to form HA. The only H+ available, however, comes from H20 dissociation, and H20 is such a weak acid that only a limited amount of H+ will become available. The two reactions below will proceed simultaneously until equilibrium is obtained. 

These can be illustrated by the use of emf measurements to find the equilibrium constants for weak acids and bases, for the self ionisation of water, for the formation of a complex or ion pair and for the solubility of sparingly soluble salts. This, taken with the situations described in the previous worked problems, illustrates the extreme versatility of emf studies. 

A more quantitative prediction of activity coefficients can be done for the simplest cases [18]. However, for most electrolytes, beyond salt concentrations of 0.1 M, predictions are a tedious task and often still impossible, although numerous attempts have been made over the past decades [19-21]. This is true all the more when more than one salt is involved, as it is usually the case for practical applications. Ternary salt systems or even multicomponent systems with several salts, other solutes, and solvents are still out of the scope of present theory, at least, when true predictions without adjusted parameters are required. Only data fittings are possible with plausible models and with a certain number of adjustable parameters that do not always have a real physical sense [1, 5, 22-27]. It is also very difficult to calculate the activity coefficients of an electrolyte in the presence of other electrolytes and solutes. Even the definition is difficult, because electrolyte usually dissociate, so that extrathermodynamical ion activity coefficients must be defined. The problem is even more complex when salts are only partially dissociated or when complex equilibriums of gases, solutes, and salts are involved, for example, in the case of CO2 with acids and bases [28, 29]. 

When lithiumdiisopropylamide (LDA) was used as base the yield of the oxidative dimerization of H—T2 H 2 to H—T4—H 4 was raised to 86%. In the same way, H-Tg-H 6 was obtained from a-terthiophene H—T3—H 3 in 65% [48] and 73% yield [43] after purification. Nevertheless, lithiation of oligothiophenes alwa includes the problem, that with the use of equimolar amounts of base a mixture of the desired product and its dimer is always found. Thus, the reaction of Li-Ti-H 9 with CuC gave 41% of H-T2-H 2 and 30% of H-T4-H 4. The equilibrium obtained in a mixture of H—T2—H 2 and Li—Tj—H 9 evidently favors the lithium salt of the L1-T2-H 15 [Eq. (5)]. This clearly indicates that the a-protons of a-oligothiophenes H-T -H exhibit greater acidity compared to those of H—Ti—H1. The use of half equimolar amount of base, however, led to the nearly exclusive formation of the desired oligomer. The excess of unreacted H—Tj—H 1 could mostly be recovered [43]. 

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