Parameter / Chlorine 

Chlorine is used in Germany as disinfectant in swimming pools and household detergents. Due to its reactivity it is an important reagent in the chemical industry for synthetic materials such as PVC or Polycarbonate. Chlorine is an approved disinfectant for drinking water in Germany. 



Whenever Chlorine is used in water applications, a distinction is made between free Chlorine, organically bound Chlorine, and total Chlorine.  Organically bound Chlorine comprises organic molecules containing Chlorine, the most prominent being Chloramines, with Chlorine bound to the Nitrogen. Free Chlorine comprises products of the reaction of Chlorine with water in the absence of organic matter. Total Chlorine is the sum of free and bound Chlorine. For the disinfection activity, the amount of free Chlorine is relevant. The activity is strongly affected by the pH – at high pH values the activity is drastically reduced, due to the formation of different compounds at different pH values.  

chlorine in water

At pH 0, Chlorine in water is present as Chlorine gas – Cl2. With rising pH, Chlorine reacts with water to form Hypochloric acid - HOCl. In the range of pH 2-6, this is the prominent compound. At higher pH values, Hypochloric acid is neutralized to form its corresponding salt ion, the Hypochlorite ion – ClO-. At approx. pH 9, the neutralisation is complete. The disinfection activity of Hypochlorite is only 1% of that of the Hypochloric acid. As a result of its reaction with water, Chlorine remains in solution for several hours, depending on temperature, circulation etc. 


Chlorine measurement

diagram sensor

Our Chlorine measurement is based on the potentiostatic method with a three-electrode sensor. Between measuring and reference electrode, a potential is applied that corresponds to a specific reaction of the disinfectant to be measured. At the measuring electrode, Chlorine molecules or more precisely hypochloric acid molecules are reduced, thus removing part of its electrical charge. The potential is being constantly readjusted, and the required current is a direct measure for the Chlorine concentration.The Hypochlorite ion is too inactive to be measured. This means that in the range of pH 6-8, the measured signal decreases with increasing pH. Between pH 7 and 8, the decrease is approx. 38%.If the pH of your water is constant, the pH influence on the signal strength is accounted for during calibration and shows only in the resulting slope. If the pH changes it has to be measured simultaneously to compensate ist influence on the Chlorine measurement. Both I possible only as long as the signal strength is still strong enough to be measured, which is the case only up to approx. pH 8. 

You are looking for a Chlorine measurement?

You can measure Chlorine with handheld instruments  photometrically, or continuously with an online measuring and control instrument.

We recommend one of our ready-to-use systems for the online Chlorine measurement: at constant pH values, our Krypton K, and at fluctuating pH values, our Krypton K Multi with integrated pH measurement and pH-compensation of the Chlorine measurement.

Our systems contain all components necessary for the measurement and a variety of safety measures. Click here for further information.

For the online measurement you need a comparison method for calibration. Our photometer Radon FM is ideally suited for such purposes. Click here for further information.

Why is a comparison method necessary? Because all Chlorine molecules that reach the electrode surface contribute to the measuring signal. This leads to a flow influence of the measurement. Without flow, the signal drops to zero. Special flow-cells are required to optimise the water flow, and with these a reduction of flow from 50l/h to 40l/h reduces a measured value of 0.30 mg/l to 0.28 mg/l. To take into account the influence of flow and other measuring conditions, you calibrate the measurement after installation. And since Chlorine sensors are wearing parts similar to pH sensors, the signal output can decrease with time, making recalibration necessary. 


Krypton K
Krypton K Multi


You would like to monitor the Chlorine concentration in ambient air?

We recommend our K 100 (W) Gas. You can connect one or two gas sensors type GE 710 CL2. The diagram shows a typical measuring set-up to monitor the ambient air in storage rooms.

gas measuring point

For each sensor two limit values can be defined, which operate separate relays. The first is supposed to activate an acoustic signal: by overstepping the limit value the alarm is set off. Manual acknowledgement switches off the relay. The second is supposed to activate the sprinkling system. A dela ycan be adjusted so that the sprinkling system starts only if the limit value is exceeded for a certain time. This relay can be switched off by digital input: if you connect a door contact, the sprinkling system will be switched off as soon as the maintenance personnel opens the door.


Plug & Play...
Our measuring systems for disinfectants contain all neccessary components and are equipped with safety features that allow a smooth measurement.


You don´t need to discard your measurement water any more. Thanks to our pressure-resistant sensors and assemblies you can redirect the water into the piping.


Maintenance is greatly reduced by our automatic sensor cleaning ASR. ASR keeps the electrode surfaces clean simply and environmental-friendly by electrolysis of water. More Information about ASR