Chlorine Analyzers

Chlorination is one of sterilization method in drinking water and industrial water, and chlorine measurement is very important to secure water quality. Yokogawa started chlorine analyzer business since 1958, our analyzers have been contributed to safety drinking water around the world. We provides two type of chlorine analyzers, non reagent type chlorine analyzer for free available chlorine measurement and reagent type chlorine analyzer for free chlorine or residual chlorine (total chlorine) measurement. You can find the best solution thanks to unique Yokogawa technologies proven by experiences.

About Chlorine

Application for new Chlorine Analyzers

Target application Application detail FC800D RC800D
Water purification plant Tap water/municipal water/drinking water/potable water
Raw water N/A N/A
Industrial water Same level of drinking water N/A
Acceptable application Application detail    
Food & Beverage Water measurement for beer/soft drinks/juice N/A
Disinfectant / Sanitizer N/A N/A
Wastewater treatment Discharge point N/A N/A
Treatment plant, coagulation N/A N/A
Process industries as Bleaching agent Chemicals, Pharma, Textiles, Pulp and  Paper N/A N/A
Desalination plant After RO membrane N/A
Drinking water N/A




What is Residual Chlorine?

Definition of Residual Chlorine

Residual chlorine is defined by the Standard Methods for Examination of Water (2001), issued by Japan Water Works Association, and the Testing Methods for Industrial Water (Japan Industrial Standards, JIS K0101-1998), as follows:

Residual chlorine = free available chlorine + combined available chlorine; or
Residual chlorine = free residual chlorine + combined residual chlorine.

Sometimes free available chlorine is also called free chlorine, and residual chlorine is also called total chlorine.


What is Free Available Chlorine?

Free Available Chlorine

When chlorine gas (Cl2) is added to water, it reacts to form hypochlorous acid (HClO) and hydrochloric acid (HCl) and the reaction reaches equilibrium, as shown in chemical equation (1). If the pH of the solution is 4 or higher, the reaction shifts to the right with predominant production of HClO.

Free Available Chlorine

HClO dissociates in equilibrium to hydrogen ion (H+) and hypochlorite ion (ClO- ), as shown in equation (3). The equilibrium constant is dependent on the temperature and pH of the solution

Free Available Chlorine

Figure 1 shows the relationship between HClO and ClO- to pH. At pH 5 or less, only HClO exists. Chlorine water has strong bactericidal properties on the acid side and weak bactericidal properties on the alkaline side. Therefore, HClO is considered to be effective as a disinfectant. Free available chlorine, in general, refers to chlorine that is present in forms of Cl2, HClO, ClO- , etc. in water.

Figure1 Dissociation of Hypochlorous Acid (HClO) vs. pH (20 °C)
Figure 1 Dissociation of Hypochlorous Acid (HClO) vs. pH (20 °C)


What is Combined Available Chlorine?

Combined Available Chlorine

If water contains ammonia nitrogen, such as ammonium ion (NH4+) caused by contamination of river or other source water, chloramines are generated by reactions shown in equations (4), (5) and (6).

Combined Available Chlorine

Chloramines are one of combined available chlorine and exist in different forms: monochloramine (NH2Cl); dichloramine (NHCl2) and trichloramine (NCl3). The predominant form is dependent on pH. At pH 8.2 or higher, only NH2Cl exists, where the pH is between 5.0 and 8.0, NH2Cl and NHCl2 coexist, and at pH 4.4 or lower, only NCl3 exists. The relationship between chloramine forms and pH is shown in Figure 2.

Figure 2 Chloramine Forms Depending on pH
Figure 2 Chloramine Forms Depending on pH

Chloramines are far less effective as disinfectants than free available chlorine. To kill bacteria completely in a certain period of time, combined available chlorine is required about 25 times more than free available chlorine in terms of quantity. It is also said that when combined available chlorine and free available chlorine have the same concentration, disinfection with combined available chlorine will take 100 times longer than the one with free available chlorine.


What is the Measurement Method?

Polarographic Method Using a Rotating Electrode

The FC800D No-reagent Type Free Available Chlorine Sensor Unit employs the polarographic method using a rotating electrode, as the measuring principle to measure free available chlorine existing in the form of chlorine (Cl2), hypochlorous acid (HClO), and hypochlorite ions (ClO- ).
The FC800D applies external voltage between measurement electrodes and reference electrodes to electrolytically reduce free available chlorine, and then measures the diffusion current during the above voltage application to obtain the concentration of free available chlorine. The voltage to be applied is selected from among values in the area where current value remains stable even if the voltage value changes, i.e., the area (plateau area) where concentration polarization occurs in polarography. Since this applied voltage changes according to the diffusion current value, the FC800D performs compensation complying with this change, to apply suitable voltage. Further, this obtained diffusion current value also changes with temperature, and the current is automatically corrected using signals from the RTD (Pt 1000 Ω) built in the reference electrode.

(Note) The FC800D was designed to require minimal maintenance and uses no reagent. However, it must be understood that there are restrictions on the conditions of the water to be measured (sample) in comparison with the RC800D Residual Chlorine Analyzer that uses reagent. If combined chlorine is present in sample, FC800D performance is affected.

Basic Polarography Diagram
Basic Polarography Diagram

Current-Voltage Diagram at Different Chlorine Concentration
Current-Voltage Diagram at Different Chlorine Concentration

Working Curve of Diffusion Current vs. Chlorine Concentration
Working Curve of Diffusion Current vs. Chlorine Concentration


RC800D is based on the rotational electrodes polarographic method as well.
The amount of free chlorine or total residual chlorine-containing combined chlorine in free chlorine is measured by selectively using reagents, electrodes, and applied voltages.


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