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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.

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

 

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).

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.

 

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.