Preparing a pH 5.0 Buffer from Sodium Acetate and HCl Using the Henderson-Hasselbalch Equation

Introduction

Buffers are essential in maintaining stable pH levels in various applications, such as biological research, chemical reactions, and analytical processes. In this article, we will guide you through the process of preparing a pH 5.0 buffer from sodium acetate and hydrochloric acid (HCl), using the famous Henderson-Hasselbalch equation. Follow step-by-step instructions for a successful and accurate buffer preparation.

The Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a fundamental equation used in biochemistry and chemistry to determine the pH of a buffer solution. The equation is as follows:

pH pKa log10( []{[Acetate]} { [Acetic , Acid]} )

Understanding the Key Components

pH: The desired pH of the buffer, in this case, 5.0 Acetate and Acetic Acid (HA): Reactants in the buffer solution, where sodium acetate provides acetate ions and HCl converts some of these ions to acetic acid pKa: The acid dissociation constant of acetic acid, which is approximately 4.76

Step-by-Step Guide to Prepare the pH 5.0 Buffer

Calculate the Required Ratio of Acetate to Acetic Acid

Using the Henderson-Hasselbalch equation:

Rearrange the equation to find the ratio:

log10( []{[Acetate]} { [Acetic , Acid]} ) pH - pKa

Substitute the values:

log10( []{[Acetate]} { [Acetic , Acid]} ) 5.0 - 4.76 0.24

Calculate the ratio:

[]{[Acetate]} { [Acetic , Acid]} 10^{0.24} ≈ 1.74

Set Up the Concentrations

Since we are preparing the buffer in 50 mL (0.050 L), and the concentration of sodium acetate is 0.1 M, we can calculate the required concentration of acetic acid (HA).

Acetate (from sodium acetate): 0.1 M Acetic Acid (HA): To be determined Using the ratio we calculated:

0.1}{[Acetic , Acid]} ≈ 1.74

Solving for Acetic Acid (HA):

[Acetic , Acid] 0.1}{1.74} ≈ 0.0575 M

Calculate the Moles of Acetic Acid Needed

Using the concentration and volume:

moles of HA [Acetic Acid] × Volume 0.0575 M × 0.050 L ≈ 0.002875 moles

Calculate the Amount of HCl Needed

To achieve the desired moles of acetic acid, we need to add HCl, which converts acetate ions to acetic acid:

moles of HCl needed moles of HA 0.002875 moles

Convert Moles of HCl to Volume

A typical HCl concentration is 1 M. Assuming a 1 M HCl solution:

Volume of HCl moles}{concentration} 0.002875}{1} ≈ 0.002875 L 2.875 mL

Conclusion

To prepare a pH 5.0 buffer using 50 mL of 0.1 M sodium acetate, you need to add approximately 2.875 mL of 1 M HCl. This method ensures an accurate pH determination, and adjustments can be made if your HCl concentration differs.