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

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.

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

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 0.100 M NH3 with 40 mL of 0.100 M NH4C1. What is the pH of this buffer? pKb for NH3 is 4.76. Solution:

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 + 1 [base]/[acid] =0.1, pH=pKa -1 Buffer Range : pKa±1 2011-4

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

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

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