University:
Kentucky State University
Course:
CHE 102 | General Chemistry II
Academic year:

2021
Views:

177

Pages:

9
Author:

Arielle Armstrong

Chapter 9 BUFFER SOLUTIONS A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH of a solution. Buffers are used in all types of chemistry whenever it is desirable to maintain the pH of a solution at a constant and predetermined level What Are the Unique Properties of Buffer Solutions? Buffers do not maintain pH at an absolutely constant value, but changes in pH are relatively small when small amounts of acid or base are added. Calculating the pH of Buffer Solutions Buffers Formed from a Weak Acid and Its Conjugate Base A solution containing a weak acid, HA and its conjugate base A-, may be acidic, neutral, or basic, depending on the position of two competitive equilibria: HA + H2O H3O+ A - + H 2O OH- + HA + A-  H 3O+  A -  Ka   HA - Kw  OH  HA Kb    A-  Ka If the first equilibrium lies farther to the right than the second, the solution is acidic. If the second equilibrium is more favorable, the solution is basic. Properties of Buffer Solutions • The Effect of Dilution The pH of a buffer solution remains essentially independent of dilution until the concentrations of the species it contains are decreased to the point where the approximations used to develop equations [HA] ≈ CHA and [A-] ≈ CA- become invalid • The Effect of Added Acids and Bases Resistance to pH change after addition of small amounts of strong acids or bases.(Ex. 9-13) Buffer Capacity (β) A solution containing a conjugate acid/base pair possesses remarkable resistance to changes in pH. The ability of a buffer to prevent a significant change in pH is directly related to the total concentration of the buffering species as well as to their concentration ratio. The buffer capacity (β) of a solution is defined as the number of moles of a strong acid or a strong base that causes 1.00 L of the buffer to undergo a 1.00 unit change in pH. The capacity of a buffer depends not only on the total concentration of the two buffer components but also on their concentration ratio. Buffer capacity falls off moderately rapidly as the concentration ratio of acid to conjugate base departs from unity. The pKa of the acid chosen for a given application should lie within: ±1 unit of the desired pH for the buffer to have a reasonable capacity. Preparing Buffers • In principle, a buffer solution of any desired pH can be prepared by combining calculated quantities of a suitable conjugate acid/base pair. In practice, however the pH values of buffers prepared from theoretically generated recipes differ from the predicted values. We prepare buffers by making up a solution of approximately the desired pH and then adjust by adding acid or conjugate base until the required pH is indicated by a pH meter. • Buffers are of tremendous importance in biological and biochemical studies where a low but constant concentration of hydronium ions (10-6 to 10-10 M) must be maintained throughout experiments.