Vernier Software & Technology

## 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 features the following Vernier sensors and equipment.

### Option 2

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

## Chemistry with Vernier

See other experiments from the lab book.

 1 Endothermic and Exothermic Reactions 2 Freezing and Melting of Water 3 Another Look at Freezing Temperature 4 Heat of Fusion of Ice 5 Find the Relationship: An Exercise in Graphing Analysis 6 Boyle's Law: Pressure-Volume Relationship in Gases 7 Pressure-Temperature Relationship in Gases 8 Fractional Distillation 9 Evaporation and Intermolecular Attractions 10 Vapor Pressure of Liquids 11 Determining the Concentration of a Solution: Beer's Law 12 Effect of Temperature on Solubility of a Salt 13 Properties of Solutions: Electrolytes and Non-Electrolytes 14 Conductivity of Solutions: The Effect of Concentration 15 Using Freezing Point Depression to Find Molecular Weight 16 Energy Content of Foods 17 Energy Content of Fuels 18 Additivity of Heats of Reaction: Hess's Law 19 Heat of Combustion: Magnesium 20 Chemical Equilibrium: Finding a Constant, Kc 21 Household Acids and Bases 22 Acid Rain 23 Titration Curves of Strong and Weak Acids and Bases 24 Acid-Base Titration 25 Titration of a Diprotic Acid: Identifying an Unknown 26 Using Conductivity to Find an Equivalence Point 27 Acid Dissociation Constant, Ka 28 Establishing a Table of Reduction Potentials: Micro-Voltaic Cells 29 Lead Storage Batteries 30 Rate Law Determination of the Crystal Violet Reaction 31 Time-Release Vitamin C Tablets 32 The Buffer in Lemonade 33 Determining the Free Chlorine Content of Swimming Pool Water 34 Determining the Quantity of Iron in a Vitamin Tablet 35 Determining the Phosphoric Acid Content in Soft Drinks 36 Microscale Acid-Base Titration

### Experiment 32 from Chemistry with Vernier Lab Book

#### Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.