Weak base conjugate acid

A buffer solution of pH 3.00 is needed. From Table 5.1, select a weak acid-conjugate base combination that would give that pH and calculate the ratio of acid concentration to conjugate base concentration that would give that pH. 

Recall, from Chapter 8, that a buffer consists of a weak acid/conjugate base mixture or a weak base/conjugate acid mixture. One buffer that you examined previously contains acetic acid and sodium acetate. The common-ion effect applies to this buffer. The equilibrium of the acetic acid is affected by the common acetate ion from sodium acetate. 

Thus, the H30+ concentration in a buffer solution has a value close to the value of Ka for the weak acid but differs by a factor equal to the concentration ratio [weak acid]/[conjugate base]. In the 0.10 M acetic acid-0.10 M sodium acetate solution, where the concentration ratio is unity, [H30 + ] equals Ka ... 

Adding 0.01 mol of NaOH changes [H30 + ] by only a small amount because the concentration ratio [weak acid]/[conjugate base] changes by only a small amount, from unity to 9/11 (Figure 16.3a). The corresponding change in pH, from 4.74 to 4.82, is only 0.08 pH unit. 

The initial rise in pH is greater for the titration of the weak acid, but the curve then becomes more level in the region midway to the equivalence point. Both effects are due to the buffering action of the weak acid-conjugate base mixture. The curve has minimum slope exactly halfway to the equivalence point, where the buffering action is maximized and the pH = pKa for the weak acid. 

The amount of acid or base that a buffer is able to neutralize before changing pH, the buffering capacity, is related to the amount of weak acid/conjugate base or weak base/conjugate acid present in the buffer solution. The greater the concentration of the conjugate pairs, the more resistant to a change in pH the buffer will be. In the next section, well look at some quantitative aspects of buffer solutions. 

Weak acid/conjugate base pairs In the Brpnsted-Lowry view, solute pairs that differ from one another by one proton. 

PHOSPHATE BUFFER Phosphate buffer consists of the weak acid-conjugate base pair H2P04 /HP042 (Figure 3.19) ... 

The shaded band indicates the pH range over which the weak acid-conjugate base pair H2P047HP042 functions effectively as a buffer. 

If no weak acid-conjugate base pair were present, derivation by the above procedure would have yielded the buffer intensity attributable to water ... 

Consider a buffered solution where [weak acid] > [conjugate base]. How is the pH of the solution related to the pK value of the weak acid If [conjugate base] > [weak acid], how is pH related to piCa ... 

Referring to Figure 16.5, at the half-equivalence point, [weak acid] = [conjugate base]. Using the Henderson-Hasselbalch equation ... 

A solution that contains significant concentrations of both members of a conjugate acid-base pair (weak acid—conjugate base or weak base—conjugate acid) is a buffer solution or simply a buffer. 

Weak acid Conjugate base Weak acid Conjugate base... 

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