Acids and Bases: Buffers
| Home |
| Table of Contents |
| Introduction |
| Molecular
Structure |
| Ionization
Constants |
| Salts |
| Buffers |
| Lewis Theory |
Why did a NH4+ / NH3 buffer work for the water hardness test?
Would a buffer made from acetic acid (CH3CO2H) and its conjugate base (acetate ion, CH3CO2-) work in this analysis?
Using
the Henderson-Hasselbalch eqn, what must the base to acid ratio be to
acheive a pH of 10.0? (Ka of CH3CO2H
is 1.8 x 10-5)
Good!
This means that there would have to be almost 200,000 times more base
present in the solution than acid!
To
solve for the base to acid ratio, first subtract the pKa
from the desired pH:
10.0 - 4.74 = log ( [ CH3CO2- ] / [ CH3CO2H ] )
[ CH3CO2- ] / [ CH3CO2H ] = 10(10.0 - 4.74)
Do
you think a CH3CO2H/CH3CO2-
buffer with a pH of 10 would be able to withstand pH changes when a strong
base is added?
| Yes | No |
 |
|
If
the base to acid ratio is almost 200,000, there is almost no acid
present. In order to maintain a pH of 10, acid is needed to neutralize
the added base.
Good!
With a base to acid ratio of close to 200,000, there will not be a
significant amount of acid to neutralize the added base.
The key to a buffer is the presence of significant amounts of both an acid and its conjugate base. A good rule to use is to keep the ratio of base to acid between 0.10 and 10. This means neither the base nor acid concentration will be more than 10 times greater than the other. Beyond these boundaries, it is difficult to have significant amounts of both acid and base.
Using
the above guidelines, what are the minimum and maximum pH's a CH3COO-/CH3COOH
buffer could have?
Good!
This shows that the pH range of a buffer is 
1 (where pKa is the negative log of the acid ionization
constant of the weak acid).
Try
substituting the two ratios given in the guideline (10 and 0.1) into
the Henderson-Hasselbalch equation and solving for pH.
If
the following acids were available along with their conjugate bases, which
could you use in the hard water analysis (requires a pH of 10)? What would
be the base to acid ratio in the buffer?
