The Buffer in Lemonade

Recommended for grades 9–12.

Figure from experiment 32 from Chemistry with Vernier

Introduction

One important property of weak acids and weak bases is their ability to form buffers. A buffer is the combination of a weak acid and a salt of the weak acid. Acetic acid and sodium acetate are an example of this kind of buffer pair. Buffers resist changes in pH upon the addition of small amounts of H⁺ or OH^– ions. The dissociation equation for acetic acid contains both of the buffer components, HC₂H₃O₂ and C₂H₃O₂^–:

${\text{H}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}{\text{(aq)}} \leftrightarrow {{\text{H}}^{\text{ + }}}{\text{(aq) + }}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}^{\text{ - }}{\text{(aq)}}$

When a small amount of an HCl solution is added to the buffer solution, most of the H⁺ ions are removed when they react with acetate ions:

${{\text{H}}^{\text{ + }}}{\text{(aq) + }}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}^{\text{ - }}{\text{(aq)}} \leftrightarrow {\text{H}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}{\text{(aq)}}$

When a solution of NaOH is added to the buffer, most of the OH^– ions are removed when they react with acetic acid molecules:

${\text{O}}{{\text{H}}^{\text{ - }}}{\text{(aq) + H}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}{\text{(aq)}} \leftrightarrow {{\text{H}}_{\text{2}}}{\text{O(l) + }}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}^ - {\text{(aq)}}$

Buffers are incorporated into various consumer products to help control the effects of varying pH. The popular powdered drink mix used in this experiment uses a citric acid–sodium citrate buffer to “control acidity,” according to its label.

Objectives

In this experiment, you will

Use a pH Sensor to monitor pH as you titrate a given volume of the commercial brand of lemonade drink.
Use a pH Sensor to monitor pH as you titrate an unbuffered solution of 0.010 M citric acid.
Compare the results of the unbuffered solution with the lemonade buffer system.

Sensors and Equipment

This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):

pH Sensor

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

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Step-by-step instructions for computer-based data collection
List of materials and equipment

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