Properties of Buffers
Introduced| English |
|---|
| weak acid/wiːk ˈæsɪd/ |
Choosing the right protector
- Not every buffer holds every pH.
- Each is tuned to a particular target near its acid's strength.
- The ratio of its two ingredients fine-tunes it.
- Pick the pair and the ratio, and you set the pH.
Set by pKa and ratio
- A buffer's pH sits near the weak acid 弱酸's $\text{p}K_a$.
- The exact pH depends on the ratio of base to acid.
- Equal amounts give $\text{pH} = \text{p}K_a$.
A buffer with equal amounts of acid and conjugate base has a pH equal to...
Equal amounts give $\text{pH} = \text{p}K_a$.
Changing the base-to-acid ratio shifts a buffer's pH.
The ratio fine-tunes the pH around $\text{p}K_a$.
Strongest when balanced
- A buffer works best when acid and base are about equal.
- Then it resists added acid and added base equally well.
- That is near $\text{pH} = \text{p}K_a$.
A buffer resists pH change best when the acid and conjugate base are about equal.
A 1:1 ratio handles added acid and base equally well.
Picking the pair
- To buffer at a target pH, choose an acid with $\text{p}K_a$ near it.
- Then adjust the ratio to fine-tune.
- The closer $\text{p}K_a$ is to the target, the better it works.
To make a buffer at pH 9, choose a weak acid with a $\text{p}K_a$...
Match the $\text{p}K_a$ to the target pH.
For the best buffer, match the acid's $\text{p}K_a$ to the target ____.
The $\text{p}K_a$ should be close to the desired pH.
You need a buffer at pH 5. Which weak acid should you pick?
- One with a $\text{p}K_a$ close to 5.
- Then set roughly equal amounts of the acid and its conjugate base.
How a buffer resists
In a CH3COOH / CH3COO- buffer, sort each addition by which component neutralises it.
A buffer is effective roughly within how much of its $\text{p}K_a$?
Buffering fades more than about one unit from $\text{p}K_a$.
A buffer works best near its $\text{p}K_a$ (within about $\pm1$ pH unit) -- far from $\text{p}K_a$ it barely buffers. Changing the ratio shifts the pH, but only over a limited range. And to buffer at a target pH, match the acid's $\text{p}K_a$ to that pH, not to 7.
A buffer's pH sits near the weak acid's $\text{p}K_a$, tuned by the base-to-acid ratio (equal amounts give $\text{pH} = \text{p}K_a$). It buffers best when the two are balanced, so choose an acid whose $\text{p}K_a$ is close to your target pH.