Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry249nificantly smaller than [NH 3 ]. Our third assumption that [Ag(NH 3 ) 2 + ] issignificantly smaller than [NH 3 ] also is reasonable.6HBuffer SolutionsAdding as little as 0.1 mL of concentrated HCl to a liter of H 2 O shifts thepH from 7.0 to 3.0. Adding the same amount of HCl to a liter of a solutionthat is 0.1 M in acetic acid and 0.1 M in sodium acetate, however, resultsin a negligible change in pH. Why do these two solutions respond so differentlyto the addition of HCl?A mixture of acetic acid and sodium acetate is one example of an acid–base buffer. To understand how this buffer works to limit the change inpH, we need to consider its acid dissociation reaction+ −CH COOH( aq) + HO() l HO ( aq) + CH COO ( aq )3 2 3 3and its corresponding acid dissociation constant[ CH COO ][ H O ]−= = 175 . × 10 5 6.52[ CH COOH]− +3 3K a3Taking the negative log of the terms in equation 6.52 and solving for pHleaves us with the result shown here.−CH COOCH CO33pH = pK + log [ ] = . + log [ O− ]476a[ CH COOH][ CH COOH]336.53Buffering occurs because of the logarithmic relationship between pH andthe ratio of the concentrations of acetate and acetic acid. Here is an exampleto illustrate this point. If the concentrations of acetic acid and acetate areequal, the buffer’s pH is 4.76. If we convert 10% of the acetate to acetic acid,by adding a strong acid, the ratio [CH 3 COO – ]/[CH 3 COOH] changesfrom 1.00 to 0.818, and the pH decreases from 4.76 to 4.67—a decreaseof only 0.09 pH units.6H.1 Systematic Solution to Buffer ProblemsEquation 6.53 is written in terms of the equilibrium concentrations ofCH 3 COOH and CH 3 COO – . A more useful relationship relates a buffer’spH to the initial concentrations of the weak acid and the weak base. Wecan derive a general buffer equation by considering the following reactionsfor a weak acid, HA, and the salt of its conjugate weak base, NaA.+ −NaA( aq) → Na ( aq) + A ( aq)+−HA( aq) + HO() l HO ( aq ) + A ( aq)2 3+ −2HO() l HO ( aq) + OH ( aq )2 3