1. 2. The pH of a buffer solution
Example: the HB-NaB buffer solution

2. For weak electrolytes: [HB] ≈ c(HB) , [B-] ≈c(B-)
The volume of the buffer solution is constant.
Notice:
1. Influencing factors: pKa and [B-]/[HB] (buffer ratio).
2. Buffer pairs that find extensive application are HAc and Ac-, NH3 and NH4+, H2CO3 and HCO3-, H2PO4- and HPO42-. Note Ka for the conjugate acids.

3. 2. The pH of a buffer solution
Notice:
Influencing factors:
pKa (deciding factor)
and [B-]/[HB] (buffer ratio).
2. In order to prepare a buffer solution, the first step is to select a weak acid with a pKa near the pH range of interest.
2011-4

4. Example 1 Instructions for making up a buffer say to mix 60 mL of 0
Example 1 Instructions for making up a buffer say to mix 60 mL of M NH3 with 40 mL of M NH4C1. What is the pH of this buffer? pKb for NH3 is 4.76.
Solution:

5. 3. Buffer capacity and buffer range
If the [base]/[acid] ratio is larger than 10 or smaller than 0.1, the solution will not be particularly effective at keeping the pH constant.
If this ratio is between 1:10 and 10:1, the solution will be effective at keeping the pH constant.
[base]/[acid] =10, pH=pKa [base]/[acid] =0.1, pH=pKa -1
Buffer Range : pKa±1
2011-4

6. Buffers (Acid-Base) pKa Buffer Range HAc-Ac- pKa = 4.76 3.76 ～ 5.76
NH3 - NH4+
pKa = 9.25
8.25 ～ 10.25
H2CO3 -HCO3-
pKa1= 6.37
5.37 ～ 7.37
HCO3- - CO32-
pKa2= 10.32
9.32 ～ 11.32
2011-4

7. the total buffer concentrations: c The buffer ratio: [HB]/[B-]
Buffer Capacity
Definition: the amount of strong acid or base needed to change the pH of one liter of buffer by 1 unit.
Influencing factors:
the total buffer concentrations: c
The buffer ratio: [HB]/[B-]
ratio→same,β∝c (c (total)= [HB] + [B-])
c (total)→same, [base]/[acid] =1, βmax =0.576c
2011-4

8. KaKb = Kw Acid If: Ca/Ka≥400, Base If: Cb/Kb≥400, Buffers (two forms)
Summary
Acid
If: Ca/Ka≥400,
Base
If: Cb/Kb≥400,
Buffers (two forms)
1.pH
2. Buffer range: pKa±1
3.β: the total buffer concentrations and buffer ratio
KaKb = Kw
2011-4