Difference Between Free Chlorine and Total Chlorine

The key difference between free chlorine and total chlorine is that the free chlorine is the amount of chlorine that can inactivate pathogenic microorganisms present in the water whereas total chlorine is the sum of combined chlorine and free chlorine. The value of the free chlorine is always a lower than the value of the total chlorine.

Combined chlorine is the amount of chlorine that reacts with nitrogen-containing compounds present in the water. Therefore, this chlorine is not available for the disinfection process of water. This means the term total chlorine gives the total amount of chlorine that is available and unavailable for the inactivation of pathogenic (disease-causing) microorganisms in water.

CONTENTS

1. Overview and Key Difference
2. What is Free Chlorine
3. What is Total Chlorine
4. Side by Side Comparison – Free Chlorine and Total Chlorine in Tabular Form
5. Summary

What is Free Chlorine?

Free chlorine is the amount of chlorine that can inactivate pathogenic microorganisms present in water. It is the concentration of chlorine in the form of dissolved chlorine gas (Cl₂), hypochlorous acid (HOCl) and hypochlorite ions (OCl^–). Moreover, hypochlorous acid mainly contributes to this measurement because hypochlorous acid has an oxidizing effect that is ten times higher than hypochlorite ions. We can measure this concentration in swimming pool water because it is the amount of chlorine that is available to sanitize the water in the pool. Ex: the free chlorine content in swimming pool water is suitable in the range of 1-3 ppm.

We need to measure the free chlorine in water because we need to monitor the chlorine content in water (due to the disinfection effect, chlorine is consumed by water). There are two ways of measuring free chlorine; colorimetric tests and amperometric tests. Colorimetric tests are automated systems; it requires samples and specific reagents that can cause a color development in the sample. amperometric tests are also automated systems that have a rugged chlorine sensor that automatically compensates for pH.

What is Total Chlorine?

Total chlorine is the sum of combined chlorine and free chlorine. Combined chlorine is the amount of chlorine unavailable for the sanitization process of water. In other words, it is the measurement of the concentration of chlorine that combines with nitrogen-containing compounds in water. Therefore, this chlorine is unavailable for the disinfection process.

The nitrogen-containing compounds in water are ammonia and organic amines. These compounds are present in either natural or polluted waters. However, water treatment plants sometimes deliberately add ammonia into the water in order to create chloramines (this process is chloramination), which are also disinfectants.

What is the Difference Between Free Chlorine and Total Chlorine?

Summary – Free Chlorine vs Total Chlorine

Chlorine-containing compounds are very common as bleaching agents and disinfectants. It can act as an oxidizer to oxidize many harmful compounds in water. In addition, it can improve the taste of drinking water. Free chlorine and total chlorine are two types of measurements we take in order to ensure the quality of water. The difference between free chlorine and total chlorine is that free chlorine is the amount of chlorine that can inactivate pathogenic microorganisms present in water whereas total chlorine is the sum of combined chlorine and free chlorine.

Reference:

1. “Free Chlorine Measurement in the Process Industries.” WaterWorld. Available here  
2. Hydro Instruments. Available here
3. “The Difference Between Free and Total Chlorine.” Swim University®, 24 Mar. 2018. Available here 

Image Courtesy:

1.’Liquid Pool Chlorine’ By Maksym Kozlenko – Own work, (CC BY-SA 4.0) via Commons Wikimedia