Buffer region

Before the equivalence point, and for volumes of titrant in the titration curve s buffer region, the concentrations of HA and A are given by the following equations. 

Suppose we are titrating the triprotic acid H P04 with a solution of NaOH. The experimentally determined pH curve is shown in Fig. 11.13. Notice that there are three stoichiometric points (B, D, and F) and three buffer regions (A, C, and E). In pH calculations for these systems, we assume that, as we add the hydroxide solution, initially NaOH reacts completely with the acid to form the diprotic conjugate base... 

At point A, the system is in the first buffer region and pH = pKa). Once all the acid H,P04 molecules have lost their first acidic protons, the system is at B and the primary species in solution are the diprotic conjugate base and sodium ion—we have a solution of NaH2P04(aq). Point B is the first stoichiometric point, and to reach it we need to supply 1 mol NaOH for each mole of H P04. 

At point C, the system is in the second buffer region and pH = pKa2- Enough base takes us to the second stoichiometric point, D. The primary species in solution are now the monoprotic anions HP042 and sodium ions, which form a solution of... 

At point E, the system is in the third buffer region and pH = pXa3. When this reaction is complete, the primary species in solution are phosphate ions and sodium ions, which form a solution of Na,P04(aq). To reach this stoichiometric point (F in the plot), we have to add another mole of OH for each mole of H3P04 initially present. At this point, a total of 3 mol OH has been added for each mole of H,P04. Notice that the third stoichiometric point (point F) is indistinct, largely because Ka3 is comparable to Kn.. As a result, it is not detected in titrations. 

Figure 11.14 shows the pH curve of a diprotic acid, such as oxalic acid, H2C204. There are two stoichiometric points (B and D) and two buffer regions (A and C). The major species present in solution at each point are indicated. Note that it takes twice as much base to reach the second stoichiometric point as it does to reach the first. 

This buffer region contains the midpoint of the titration, the point at which the amount of added OH" is equal to exactly half the weak acid originally present. In the current example, the solution at the midpoint contains 0.0375 mol each of acetic acid and acetate. Applying the buffer equation reveals the key feature of the midpoint ... 

Beyond the buffer region, when nearly all of the acetic acid has been consumed, the pH increases sharply with each added drop of hydroxide solution. The titration curve passes through an almost vertical region before leveling off again. Recall from Chapter 4 that the stoichiometric point of an acid titration (also called the equivalence point) is the point at which the number of moles of added base is exactly equal to the number of moles of acid present in the original solution. At the stoichiometric point of a weak acid titration, the conjugate base is a major species in solution, but the weak acid is not. 

In the long, flat buffer region, both B and its conjugate acid B are major species. The midpoint of the titration occurs in the buffer region. At this point, the pH of the solution is equal to the p. a of the conjugate acid of the base. 

Point A iies aiong the section of the titration curve known as the buffer region. Buffering action comes from the presence of a weak acid and its conjugate base as major species in solution. Moreover, Point A iies beyond the midpoint of the titration, which teiis us that more than half of the weak acid has been consumed. We represent this soiution with two moiecuies of H four ions of A, and four H2 O moiecuies ... 

Within the first buffer region, both H2 A and RA are major species in solution, and we can apply the buffer equation to caicuiate the pH. Haifway to the first equivalence point of the titration [H2 A] — [H A ] and pH pTai = 1.82. 

Moving beyond the first stoichiometric point, the titration enters the second buffer region. Here, the major species are H A and its conjugate base,. The pH in this region is given by the buffer equation, using the p of H j4.". At the second midpoint, [H j4" ] = [j4 " ], and pH = p. Ta 2 For the titration of maleic acid, pH = 6.59 at the second midpoint. 

C18-0020. Glycolic acid (HOCH2 CO2 H), a constituent of sugar cane juice, has a p Zg of 3.9. Sketch the titration curve for the titration of 60.0 mL of 0.010 M glycolic acid with 0.050 M KOH. Indicate the stoichiometric point, the buffer region, and the point of the titration where pH- p. S a. Sketch the curve qualitatively without doing any quantitative calculations. 

When are the simplified results valid If the work path has buffer regions at its beginning and end during which the work parameter is fixed for a time > Tshort, then the subsystem will have equilibrated at the initial and final values of p in each case. Hence the odd work vanishes because TL 0, and the probability distribution reduces to Boltzmann s. 

In the titration curves shown in Fig. 23-5, you start with the fully protonated form of the amino acid. Notice that at pH s that are not near the pKa of any functional group, the pH changes more when base is added. Also notice that there are multiple buffer regions (where the pH doesn t change rapidly when base is added) when there are multiple acid and base groups present. If the pAVs of two groups are close to each... 

As a good generalization, the buffer region extends over the range of p a 1. 

As a further permutation, adding a strong acid to a weak base also yields a buffer solution, this time with a buffer region centred on the pKa of the base. The pH at the end point will be lower than 7. 

For the buffer region, 5 turbulent stresses are of comparable magnitude. The data can be fitted by an equation of similar form ... 

FIGURE 5.15 A drawing of a titration curve for a weak acid pointing out the buffer region and midpoint. 

Table 5.1 gives commonly used examples of conjugate acid-base pair combinations and the pH range for which each is useful. This range corresponds to the pH range defined by the buffer region in the titration curve for each, and the middle of the range corresponds to the midpoint of each titration. 

Define buffer solution, conjugate acid, conjugate base, conjugate acid-base pair, buffer capacity, and buffer region. 

Buffer region—The region of a titration curve leading up to the inflection point. 

D) After the equivalence point, the pH becomes constant because this is the buffer region. 

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