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Free Chlorine

Free chlorine refers to chlorine present as hypochlorous acid (HOCl) or hypochlorite (OCl-) ion. When the chlorine demand of the water is met, the remaining free chlorine is available for oxidation of contaminants, which is measured by a free chlorine sensor.

Like all halogens, chlorine is a very reactive element. In solution, chlorine occurs in many different molecular forms. When chlorine is added to water, it reacts with organic materials and metals to form combined chlorine. Since combined chlorine is not available for disinfection, this effect is called the chlorine demand of water.

The measurement of free chlorine is one of the most important measurements in the water industry today. These measurements ensure that a sufficient amount of chlorine is available for disinfection purposes and in some cases that the chlorine has been removed.

How is free chlorine measured?

Many are familiar with DPD test kits for measuring chlorine. DPD is a reagent that reacts with chlorine to produce a red color. While DPD test kits may be sufficient for simple applications, this method does not distinguish between free chlorine and combined chlorine in solution. For applications where the concentration of free chlorine is critical for disinfection, the most robust measurement technique is to use an amperometric free chlorine sensor.

An amperometric sensor design consists of two electrodes (anode and cathode), a membrane and a filling solution. These sensors measure a change in current caused by the chemical reduction of hypochlorous acid (HOCl) at the cathode. The current flow of this reduction is proportional to the available chlorine. A membrane and electrolyte help control the reaction. Constant flow rate and pressure are controlled to ensure accurate measurement.


Chlorine Dioxide

Chlorine dioxide is a neutral chlorine compound. It differs greatly from elemental chlorine both in its chemical structure (ClO2) and in its behavior. One of the most important properties of chlorine dioxide is its high solubility in water, especially in cold water. Chlorine dioxide does not hydrolyze when it enters water; it remains a dissolved gas in solution. Chlorine dioxide is about 10 times more soluble in water than chlorine. It is not affected by pH and has excellent residual activity that remains active for hours to days. It does not interact with ammonia or ammonium. It is effective at cold temperatures.

Chlorine dioxide is one of the most effective disinfection methods in the water industry today. These measurements ensure effective use of chlorine dioxide for disinfection purposes as well as other applications. Underdosing can be ineffective and overdosing results in excess chlorite due to product formation. It is extremely effective in the control of Legionella bacteria.

How is Chlorine dioxide measured?

An amperometric sensor design consists of two electrodes (anode and cathode), a membrane and a filling solution. These sensors measure a change in current caused by chemical reduction at the cathode. A membrane and electrolyte help control the reaction. Constant flow rate and pressure are controlled to ensure accurate measurement.