How to prepare a buffer solution?

How do we prepare a buffer solution?

In the previous article, we explained the factors affecting the buffer capacity of a solution. To prepare an effective buffer (e.g. an acidic buffer), we need to:

      1. Select a weak acid, which has a pKa at a particular temperature that is as close as possible to the pH that the buffer is supposed to maintain at that temperature.
      2. Choose a corresponding conjugate base, i.e. salt of the weak acid.
      3. Use the Henderson-Hasselbalch equation to calculate the ratio of the conjugate base concentration to the weak acid concentration, which should be as close to unity as possible to achieve maximum buffer capacity.
      4. Calculate the volumes and concentrations of the weak acid and its conjugate base for mixing.



How to prepare 500.0 ml of a buffer to maintain a pH of 4.6?


Step 1: Use 1.0 M acetic acid (\(pK_a=4.757\))

Step 2: Choose sodium acetate (Ar = 82.03)

Step 3: Substitute values in the Henderson-Hasselbalch equation

4.6=4.757+log\frac{[CH_3COO^-]}{[CH_3COOH]}\; \; \; \; \; \Rightarrow \; \; \; \; \;\frac{[CH_3COO^-]}{[CH_3COOH]}=0.697

Step 4: Since [CH3COOH] = 1.0,  [CH3COO] = 0.697 M

The mass of solid sodium acetate m needed to dissolve in 500.0 ml of 1.0 M acetic acid to achieve a concentration of 0.679 M is:

\frac{m}{82.03}/0.5=0.697\; \; \; \; \; \Rightarrow \; \; \; \; \; m=28.6\: g


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