Equilibria of Acid-Base Buffer Systems

Why do some lakes become acidic when showered by acid rain, while others remain unaffected How does blood maintain a constant pH in contact with countless cellular acid-base reactions How can a chemist sustain a nearly constant [H30 ] in reactions that consume or produce HsO or OH The answer in each case depends on the action of a buffer. 

After the addition of 1 mL of 1 M HCi left) or of 1 M NaOH right), the pH changes by severai units. 

Buffers work through a phenomenon known as the common-ion effect. An example of this effect occurs when acetic acid dissociates in water and some sodium acetate is added. As you know, acetic acid dissociates only slightly in water. 

From Le Chatelier s principle (Section 17.6), we know that if some CHaCOO ion is added (from the soluble sodium acetate), the equilibrium position shifts to the left thus, [HaO ] decreases, in effect lowering the extent of acid dissociation  

Similarly, if we dissolve acetic acid in a sodium acetate solution, acetate ion and H3O ion from the acid enter the solution. The acetate ion already present in the solution acts to suppress as much acid from dissociating, which lowers [HaO ]. Thus, the effect again is to lower the acid dissociation. Acetate ion is called the common ion in this case because it is common to both the acetic acid and sodium acetate solutions that is, acetate ion frc n the acid enters a solution in which it is already present. The common-ion effect occurs when a given ion is added to an equilibrium mixture that already contains that ion, and the position of equilibrium shifts away from forming more of it. 

Consider what happens to a solution containing high [CH3COOH] and high [CH3C00 ] when we add small amounts of strong acid or base. The expression for HA dissociation at equilibrium is 

---

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

Consider just a few cases of aqueous equilibria. The magnificent formations i n limestone caves and the vast expanses of oceanic coral reefs result from subtle shifts in carbonate solubility equilibria. Carbonates also influence soil pH and prevent acidification of lakes by acid rain. Equilibria involving carbon dioxide and phosphates help organisms maintain cellular pH within narrow limits. Equilibria involving clays in soils control the availability of ionic nutrients for plants. The principles of ionic equilibrium also govern how water is softened, how substances are purified by precipitation of unwanted ions, and even how the weak acids in wine and vinegar influence the delicate taste of a fine French sauce. In this chapter, we explore three aqueous ionic equilibrium systems acid-base buffers, slightly soluble salts, and complex ions. 

Applications of Aqueous Equilibria Blood as a buffer system. Reactivity of strong and weak acids and bases. 

Hydrogen ion regulation in natural waters is provided by numerous homogeneous and heterogeneous buffer systems. It is important to distinguish in these systems between intensity factors (pH) and capacity factors (e.g., the total acid- or base-neutralizing capacity). The buffer intensity is found to be an implicit function of both these factors. In this chapter, we discuss acid-base equilibria primarily from a general and didactic point of view. In Chapter 4 we address ourselves more specifically to the dissolved carbonate system. 

The pH of the micellar mobile phase is usually buffered using the phosphoric or citric acid-base systems. Potassium ion caimot be used with SDS, as potassium dodecyl sulfate precipitates from aqueous solutions due to its high Krafft point (see Chapter 2). The column should be equilibrated by purging with the mobile phase until the pH before and after the column is identical. Only one peak is observed in the chromatograms of weak acids and bases because prototropic equilibria are much faster than the solute-micelle or solute-stationary phase dynamics. 

The major buffer system that is used to control the pH of blood is the carbonic add-bicarbonate buffer system. Carbonic acid (H2CO3) and bicarbonate ion (HC03 are a conjugate acid-base pair. In addition, carbonic add can decompose into carbon dioxide gas and water. The important equilibria in this buffer system are... 

Several aspects of these equilibria are notable. First, although carbonic acid is a diprotic acid, the carbonate ion is unimportant in this system. Second, one of the components of this equilibrium, CO2, is a gas, which provides a mechanism for the body to adjust the equilibria. Removal of CO2 via exhalation shifts the equilibria to the right, consuming H ions. Third, the buffer system in blood operates at a pH of 7.4, which is fairly far removed from the pK i value of H2CO3 (6.1 at physiological temperatures). In order for the buffer to have a pH of 7.4, the ratio [base]/[acid] must have a value of about 20. In normal blood plasma the concentrations of HCOs and H2CO3 are about 0.024 M and 0.0012 M, respectively. As a consequence, the buffer has a high capacity to neutralize additional acid, but only a low capacity to neutralize additional base. 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

---