Requerimientos a nivel de software

It is important for the jellies shown in Figure 18.27 that the aquarium water be kept within a narrow pH range. A constant pH is also impor- tant in your body. The pH of your blood must be maintained within the range of 7.1 to 7.7. The gastric juices in your stomach must have a pH between 1.6 and 1.8 to promote digestion of certain foods. Your body maintains pH values within such narrow limits by producing buffers. What is a buffer? Buffers are solutions that resist changes in pH when limited amounts of acid or base are added. For example, adding 0.01 mol of HCl to 1 L of pure water lowers the pH by 5.0 units, from 7.0 to 2.0. Similarly, adding 0.01 mol of NaOH to 1 L of pure water increases the pH from 7.0 to 12.0. However, if you add the same amount of either HCl or NaOH to 1 L of a buffered solution, the pH might change by no more than 0.1 unit.

How do buffers work? A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. The mixture of ions and molecules in a buffer solution resists changes in pH by reacting with any hydrogen ions or hydroxide ions added to the buffered solution.

Suppose that a buffer solution contains 0.1M concentrations of hydrofluoric acid (HF) and sodium fluoride (NaF). The NaF provides a 0.1M concentration of F - _{ions. HF is the acid, and F} - _{is its conjugate} base. The following equilibrium would be established.

HF(aq) ⇌ H +_{(aq) + F} -_(aq)

Adding an acid When an acid is added to this buffered solution, as shown in Figure 18.27b, the equilibrium shifts to the left. According to Le Châtelier’s principle, the added H + _{ions from the acid are a stress on} the equilibrium, which is relieved by their reaction with F - _{ions to form} additional undissociated HF molecules.

HF(aq) H +_{(aq) + F} -_(aq)

Equilibrium is established again with a larger amount of undissociated HF present. However, the pH of the solution has changed little because the shift to the left consumed most of the added H + _ion.

■Figure 18.27 To provide a healthy environment for these jellies, the pH of the aquarium water at the Monterey Bay Aquarium must be adjusted to stay within the range of 8.1 to 8.4.

Section 18.4 • Neutralization 667

Table 18.7

Buffer Systems with Equimolar Components

Buffer Equilibrium Conjugate Acid-Base Pair in _{Buffered Solution} Buffer pH

HF(aq) ⇌ H +_{(aq) + F} -_{(aq) HF/ F} - _3.20

C H 3 COOH(aq) ⇌ H +(aq) + C H 3 CO O -(aq) C H 3 COOH/C H 3 CO O - 4.76

H 2 C O 3 (aq) ⇌ H +(aq) + HC O 3 -(aq) H 2 C O 3 /HC O 3 - 6.35

H 2 P O 4 -(aq) ⇌ H +(aq) + HP O 4 2-(aq) H 2 P O 4 -/HP O 4 2- 7.21

N H 3 (aq) + H 2 O(l) ⇌ N H 4 +(aq) + O H -(aq) N H 4 +/N H 3 9.4

C 2 H 5 N H 2 (aq) + H 2 O(l) ⇌ C 2 H 5 N H 3 +(aq) + O H -(aq) C 2 H 5 N H 3 +/ C 2 H 5 N H 2 10.70

V

S

. C

Buffer

Science usage: a solution that resists

changes in pH when limited amounts of acid or base are added

The chemist decided to use a buffer consisting of equal molar amounts of formic acid and sodium formate.

Common usage: something that

serves as a protective barrier For the homes along the shore, the high seawall served as a buffer against stormy seas.

Adding a base When a base is added to the hydrofluoric acid/fluo- ride ion buffer system, the added O H - _{ions react with H} + _{ions to form} H 2 O. This decreases the concentration of H + ions, and the equilibrium

shifts to the right to replace the H + _ions.

HF(aq) H +_{(aq) + F} -_(aq)

Although the shift to the right consumes HF molecules and produces additional F - _{ions, the pH remains fairly constant because the H} + _ion concentration has not changed appreciably.

A buffer solution’s capacity to resist pH change can be exceeded by the addition of too much acid or base. The amount of acid or base a buffer solution can absorb without a significant change in pH is called the buffer capacity of the solution. The greater the concentrations of the buffering molecules and ions in the solution, the greater the solu- tion’s buffer capacity.

Choosing a buffer A buffer system is most effective when the con- centrations of the conjugate acid-base pair are equal or nearly equal. Consider the H 2 P O 4 -/HP O 4 2- buffer system made by mixing equal

molar amounts of Na H 2 P O 4 and Na H 2 P O 4 .

H 2 P O 4 -⇌ H ++ HP O 4 2-

What is the pH of such a buffer solution? The acid ionization constant expression for the equilibrium can provide the answer.

K a = 6.2 × 1 0 -8 =

[ H +_{][HP O} 4 2-]

__

[ H 2 P O 4-]

Because the solution has been made with equal molar amounts of N a 2 HP O 4 and N a 2 HP O 4 , [HP O 4 2-] is equal to [ H 2 P O 4 -]. Thus, the

two terms in the acid ionization expression cancel. 6.2 × 1 0 -8 ₌

__

[ H 2 P O 4 -]

= [ H +_] pH =-log [ H +_{] =-log (6.2 × 1 0} -8 _{) = 7.21}

Thus, when equimolar amounts of each of the components are present in the HP O 4 -/ H 2 P O 4 2- buffer system, the system can maintain a pH

close to 7.21. Note that the pH is the negative log of K a . Table 18.7 lists

Section 18.418.4 Assessment