Residual Chlorine Monitoring

Pi has three residual chlorine analysers for free and total chlorine. These are the HaloSense based on the electrochemical technology, the DPDSense (an online DPD analyser) and the ChloriBrid (hybrid, innovative and patented XXXXX. Both technologies that bring the benefits of both to a single Chlorine analyser.

Residual chlorine analysers from Pi are used in many applications requiring the measurement and control of online residual chlorine levels in water. The range is suitable for total or free residual chlorine monitoring or control applications in potable water, seawater, process water, swimming pool water, wastewater, food washing, paper and pulp, etc.

The Pi range of controllers/transmitters means that you get exactly what you need and nothing that you don’t. From a low cost no-frills chlorine dosing controller (CRONOS®) to a highly sophisticated colour display, remote access controller (CRIUS®4.0) – and all with the same great sensors! Chlorine dosing control is now simpler and cheaper than ever! Both controllers can have multiple sensors and multiple sensor types, saving money on the requirement for one sensor and one transmitter per measurement.

Online electrochemical is an online DPD method of measuring residual choline has different strengths and weaknesses, so depending on your application you maybe better off with an electrochemical solution (HaloSense) or online DPD solution (DPDSense) or if you need the benefits of both the hybrid ChloriBrid will be the instrument of choice for you.





Instant start up


Continuous measurements


No chemical reagent



Instant recovery from power outage


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When chlorine is added as a disinfectant to water it oxidises material in the water thereby killing any organisms. The ‘Residual Chlorine’ is the chlorine left over at the end of the process and is usually what we measure.

Free chlorine is the chlorine in the water that exists as HOCl or OCl. When chlorine is added to pure water between pH 4 and pH 11

Cl2 + OH ↔ HOCl + Cl
HOCl ↔ OCl + H+

so if chlorine is added to water you get HOCl (Hypochlorous acid) and OCl (Hypochlorite), which together make ‘free chlorine’.

If water contains both ammonia and hypochlorite it will react to form monochloramine.

NH3 + OCl– → NH2Cl + OH

In an acidic solution Monochloramine disproportionates to form Nitrogen Trichloride.

2NH2Cl + H+ → NHCl2 + NH4+
3NHCl2 + H+ → 2NCl3 + NH4+

In solution where there are low concentrations of chlorine it is often Chloramines that can be smelled not ‘chlorine’. The three Chloramines above are collectively known as ‘Combined Chlorine’.

Total chlorine is the combination of free chlorine and combined chlorine.

No. Total Chlorine is the sum of Free Chlorine and Combined Chlorine, therefore Total Chlorine is always equal to or greater than both Combined Chlorine or Free Chlorine.

Free Chlorine is made when a chlorine species is added to water. The chlorine hydrolyses and forms OCL and HOCl, the relative amount of both depends on the pH and to a lesser extent on the temperature and ionic strength of the water. Typical additive to form  free chlorine are Chlorine (gas), Sodium Hypochlorite (liquid), Calcium Hypochlorite (solid).
Chlorine is the name of the element. In its elemental form Chlorine is a highly toxic gas, that has been used as a weapon. When Chlorine is dissolved in water it forms two species, OCl (the hypochlorite ion) and HOCl (hypochlorous acid), the sum of these two species is Free Chlorine which is often used as a disinfectant in, for example, a swimming pool.

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Commercial Pi France

Franck Cussemane
Chef d’Equipe

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