Solutions of Weak Acids Their Salts Buffer Changes in pH

Solutions of weak acids or bases and their conjugates exhibit buffering, the ability to resist a change in pH following addition of strong acid or base. Since many metabolic reactions are accompanied by the release or uptake of protons, most intracellular reactions are buffered. Oxidative metabolism produces CO2, the anhydride of carbonic acid, which if not buffered would produce severe acidosis. Maintenance of a constant pH involves buffering by phosphate, bicarbonate, and proteins, which accept or release protons to resist a change

Figure 2-4. Titration curve for an acid of the type HA. The heavy dot in the center of the curve indicates the pKa 5.0.

Figure 2-4. Titration curve for an acid of the type HA. The heavy dot in the center of the curve indicates the pKa 5.0.

in pH. For experiments using tissue extracts or enzymes, constant pH is maintained by the addition of buffers such as MES ([2-^-morpholino]ethanesulfonic acid, pKa 6.1), inorganic orthophosphate (pKa2 7.2), HEPES (^-hydroxyethylpiperazine-A<" -2-ethanesulfonic acid, pKa 6.8), or Tris (tris [hydroxymethyl] amino-methane, pK 8.3). The value of pK relative to the desired pH is the major determinant of which buffer is selected.

Buffering can be observed by using a pH meter while titrating a weak acid or base (Figure 2-4). We can also calculate the pH shift that accompanies addition of acid or base to a buffered solution. In the example, the buffered solution (a weak acid, pK = 5.0, and its conjugate base) is initially at one of four pH values. We will calculate the pH shift that results when 0.1 meq of KOH is added to 1 meq of each solution:

Initial pH

5.00

5.37

5.60

5.86

[A ]initial

0.50

0.70

0.80

0.88

[HA]initial

0.50

0.30

0.20

0.12

([A-]/[HA]) initial

1.00

2.33

4.00

7.33

Addition of 0.1 meq of KOH produces

[A-]final

0.60

0.80

0.90

0.98

[HA]final

0.40

0.20

0.10

0.02

([A-]/[HA])final

1.50

4.00

9.00

49.0

([A-]/[HA])final

0.176

0.602

0.95

1.69

Final pH

5.18

5.60

5.95

6.69

ApH

0.18

0.60

0.95

1.69

to the pK(. A solution of a weak acid and its conjugate base buffers most effectively in the pH range pKa ±1.0 pH unit.

Figure 2-4 also illustrates the net charge on one molecule of the acid as a function of pH. A fractional charge of -0.5 does not mean that an individual molecule bears a fractional charge, but the probability that a given molecule has a unit negative charge is 0.5. Consideration of the net charge on macromolecules as a function of pH provides the basis for separatory techniques such as ion exchange chromatography and elec-trophoresis.

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