Determining the Phosphoric Acid Content in Soft Drinks

Recommended for grades 9–12.

Figure from experiment 35 from Chemistry with Vernier

Introduction

Phosphoric acid is one of several weak acids that exist in carbonated beverages. It is a component of all cola soft drinks. Phosphoric acid has a much higher concentration than other acids in a container of soft drink, so its concentration can be determined by a simple acid-base titration.

In this experiment, you will titrate a sample of a cola soft drink with sodium hydroxide solution and determine the concentration of phosphoric acid, H₃PO₄. Hydrogen ions from the first dissociation of phosphoric acid react with hydroxide ions from the NaOH in a one-to-one ratio in the overall reaction:

${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}{\text{(aq) + O}}{{\text{H}}^{\text{ - }}}{\text{(aq)}} \to {{\text{H}}_{\text{2}}}{\text{O(1) + }}{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{4}}}^{\text{ - }}{\text{(aq)}}$

In this experiment, you will use a pH Sensor to monitor pH as you titrate. The region of most rapid pH change will then be used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the H₃PO₄.

Objectives

In this experiment, you will

Use a pH Sensor to monitor pH during the titration of phosphoric acid in a cola soft drink.
Using the titration equivalence point, determine the molarity of H₃PO₄.

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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List of materials and equipment

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