Free Chlorine vs. Total Chlorine: What Is the Difference?

Overview

This article explains the practical difference between free chlorine and total chlorine, and how this affects sensor selection and reference testing.

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Definitions

• Free chlorine: chlorine available for immediate disinfection (mainly HOCl and OCl^-, and in some contexts dissolved Cl₂).
• Combined chlorine: chlorine bound to nitrogen compounds, mainly chloramines (NH₂Cl, NHCl₂, NCl₃) and organic chloramines.
• Total chlorine: sum of free chlorine and combined chlorine.

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How They Are Related

When free chlorine reacts with ammonia or organic nitrogen compounds, combined chlorine forms.

Therefore: Total chlorine = Free chlorine + Combined chlorine

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What Changes in Measurement

• Zirkon® DIS / ZIRKON® ONE DIS: for free chlorine measurement.
• Zirkon® DIS Total / ZIRKON® ONE TCL BIOCIDES: for total chlorine measurement.

The key difference is not only the displayed parameter, but also the sensor chemistry and matching calibration/reference method.

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Reference Testing and Calibration

• Always use a reference method configured for the same parameter you measure (free or total chlorine).
• Use the correct reagents for the selected parameter.
• A mismatch between sensor parameter and DPD method leads to incorrect calibration and unstable operation.

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Application Notes

• Free chlorine control is typically preferred where immediate disinfecting activity is the main focus.
• Total chlorine monitoring is typically preferred where chloramines/combined chlorine are relevant to the process.
• As a rule of thumb, free chlorine measurement is commonly applied below pH 8.0, while total chlorine applications can extend to higher pH ranges (up to approx. pH 10, depending on conditions).

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Related Articles

• ZIRKON® ONE Sensors Overview
• DPD as a Reference Measurement
• What Does the Zirkon® DIS Sensor Measure?
• What Does the Zirkon® DIS Total Sensor Measure?