Chapter 9: Q31P (page 209)

Why does buffer capacity increase as a solution becomes very acidic (pH ≈ 1) or very basic (pH ≈ 13)?

Short Answer

Expert verified

At a very low pH, i.e., very acidic (pH ≈ 1) or at a very high pH, i.e., very basic (pH ≈13), the buffer capacity increases as it resists changes in pH by either absorbing or desorbing $H^{+}$ and ${OH}^{-}$ ions.

Step by step solution

Definition of buffer capacity

Buffer capacity is a quantitative measure of how much a solution is reluctant to pH changes when strong acid or base is added. The more resistant the solution is to pH change, the greater the buffer capacity.

Impact on buffer capacity when pH is very acidic or very basic

At very low or very high pH values, there are high acid or base concentrations in a solution. So, adding a little bit of acid or base in that solution would have nearly no effect.

Also, we can consider the following:

At low pH $\Rightarrow$ buffer is $H_{3} O^{+} / H_{2} O$
At high pH $\Rightarrow$ buffer is $H_{2} O / {OH}^{-}$

Conclusion

Buffer capacity increases as a solution become very acidic or very basic. A buffer is most effective in resisting changes in pH when pH ≈ 1 or ≈ 13. Buffer capacity increases at high pH (and at low pH), simply because there is a high concentration of OH- at high pH (and H+ at low pH). Hence, adding a small amount of acid or base to a large amount of OH- (or H+) will not have a large effect on pH.