Vernier Software & Technology

# Determining the Free Chlorine Content of Swimming Pool Water

## Introduction

Physicians in the nineteenth century used chlorine water as a disinfectant. Upon the discovery that certain diseases were transmitted by water, it became common for municipalities to chlorinate public water supplies. It is now standard practice to add chlorine to swimming pools and hot tubs. Chlorine reacts with water to form hypochlorous acid (HOCl) and hypochlorite ion (ClO).

${\text{C}}{{\text{l}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{HOCl + }}{{\text{H}}^{\text{ + }}}{\text{ + C}}{{\text{l}}^{\text{ - }}}$ ${\text{HOCl}} \leftrightarrow {{\text{H}}^{\text{ + }}}{\text{ + OC}}{{\text{l}}^{\text{ - }}}$

The chlorine that exists in water as HOCl and OCl is known as free chlorine. Free chlorine can kill bacteria, prevent algae growth, and oxidize iron to form a precipitate that can be removed from a pool by the filtering system. Swimming pool operators try to maintain a desired range of 1.0 to 1.5 mg/L of free chlorine for proper sanitation.

In this experiment, you will use a Colorimeter to determine the amount of free chlorine in a sample of swimming pool or hot tub water. You will first measure the absorbance of green light (565 nm) by aqueous solutions of known chlorine concentration. From the resulting graph of absorbance vs. free chlorine concentration (Beer’s law), you will be able to determine the free chlorine content of your swimming pool sample.

A compound called DPD (N, N-diethyl-p-phenylenediamine) is reacted with the chlorine in each sample. The DPD is oxidized, forming a magenta (red) product. The intensity of the sample’s color is directly proportional to its chlorine concentration.

## Objectives

In this experiment, you will

• Prepare free chlorine standard solutions.
• Use a Colorimeter to measure the absorbance of each standard solution.
• Plot a graph of absorbance of free chlorine vs. concentration.
• Use a Colorimeter and your Beer's law plot to determine the amount of free chlorine in a sample of swimming pool or hot tub water.

## Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

### Option 4

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 33 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.