Chlorine Analyzer

Chlorine Analyzer

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

The following are available in the HaloSense range;

  • Online free and total chlorine analyzers 0-2 ppm, 0-5 ppm, 0-10 ppm, 0-20 ppm
  • Online residual chlorine in seawater analyzers (free or total bromine) 0-2 ppm, 0-5 ppm, 0-10 ppm, 0-20 ppm
  • Online zero chlorine (designed to measure the absence of free chlorine) 0-2 ppm, 0-5 ppm, 0-10 ppm for applications such as post activated carbon and pre-RO monitoring.

The HaloSense 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 color display, remote access controller (CRIUS) – and all with the same great sensors! The CRATOS HaloSense is now available with up to 12 sensor inputs, significantly reducing the cost per point. Chlorine dosing control is now simpler and cheaper than ever! All three analyzers can have multiple sensor and multiple sensor types, saving money on the requirement for one sensor and one transmitter per measurement.

Chlorine Analyzer Sensor Tip

Chlorine Analyzer Sensor Tip

The membraned amperometric sensors are enhanced with a third, reference electrode which eliminates zero drift. (NB. These chlorine sensors are often known as polarographic sensors although this is a misuse of the word polarographic). Its unique design means that pH compensation is not usually required at all, completely eliminating reagents.

The sensors used by the HaloSense analyzers are largely pH independent meaning that the measurements are bufferless and reagentless. They are amperometric sensors and show remarkable sensitivity and stability. For those making a free chlorine measurement at high pH (>pH 8.5) on variable pH water it is possible to provide pH compensation from either a pH sensor connected to the transmitter or from an external pH meter.

The sensors work by separating the electrodes that perform the measurement from the sample, by a membrane. This membrane allows the free residual chlorine (HOCl and OCl) or the total residual chlorine (HOCl and OClplus chloramines) through the membrane. Inside the sensor the dissolved chlorine meets the electrolyte which is at a low pH. This converts the majority of the OCl to HOCl. The HOCl is reduced at the gold working electrode and the current generated is proportional to the chlorine present, and the instrument gives a reading in ppm or mg/l.

This technique is the most advanced method of continuous chlorine measurement and has many benefits to the user including a very stable online measurement and better dosing control.

The HaloSense range is bufferless and reagent free, meaning that is has a low total cost of ownership and with maintenance intervals at 3 or even 6 months. HaloSense is fast becoming the instrument of choice for the engineer who wants the best instrument at the best price.

  • Low purchase cost
  • Low cost of ownership
  • Reduced pH dependency (largely pH independent)
  • Stable and reliable
  • Bufferless
  • Reagentless

Many water companies want to measure free chlorine residuals without the need for chemical buffers traditionally associated with such measurements. Acetate and phosphate buffers are expensive and environmentally unfriendly. Buffer delivery systems are maintenance intensive and have fairly costly consumables and there are health and safety considerations in the handling of the acids and high disposal costs if the acid treated water is unable to be fed back into the water supply.

Amperometric cells and most polarographic probes only respond to hypochlorous acid, (HOCl). HOCl dissociates into hypochlorite (OCl-) in a pH dependent manner. This is why most chlorine monitors need acid buffers in most applications. The typical pH of water measured on a water treatment works may range from 7 to 9.2. Chemical buffering reduces the pH to between 5 and 6 and ensures that the majority of the residual chlorine is present as HOCl (see graph below).

Graph showing typical probe response to pH (unbuffered).


The HaloSense Free Chlorine Sensor measures all the HOCl and the majority of the OCl- present (blue line on graph). This results in a vastly reduced pH effect and means that most monitoring applications require no buffer and no pH compensation.

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  • Continuous online monitoring for chlorine in any water
  • Water treatment plant residual chlorine dosing control
  • Secondary chlorination free chlorine dosing control
  • Distribution monitoring
  • Cooling tower monitoring and control
  • Pasteurizer dosing control
  • Seawater chlorination control
  • Bromine monitoring in seawater
  • Food washing
  • Chloramination control

Anywhere you have a requirement to measure residual free or total chlorine is a suitable application for the HaloSense chlorine monitor. The HaloSense chlorine monitor range is particularly suited to working in sites where reliability and ease of use are most important.

The HaloSense sensors can come equipped to automatically clean themselves at user defined intervals, with all the benefits of no operator intervention for up to 6 months. The autoflush is particularly useful in food preparation, pulp and paper, and many applications where there is likely to be a build up of solids in the sample. For more information about autoflush click here.

For some free chlorine applications with high and variable pH, pH compensation can improve the accuracy of the analyzer. For pH compensation to be valid it must be done with the highest quality pH sensors and with chlorine sensors that have a reduced susceptibility to varying pH, such as those used in the HaloSense range.

The graph below shows the errors on a real HaloSense free chlorine sensor when a sample of 1ppm free chlorine has the pH changed from pH9 to more than pH10, down to pH7.5 and back again. The graphs show that the vast majority of applications won’t need pH compensation at all and for those that do that free chlorine sensor is the most appropriate sensor available to have that compensation applied.


The CRONOS, CRIUS and CRATOS free and total residual dosing controllers can be equipped with four PID process control options, data-logging, relay outputs, analog outputs and serial communications such as: Ethernet, Modbus and Profibus. Remote monitoring of the instruments (including remote access to all control options) is available via the internet over GPRS and via a LAN. In fact the CRIUS and CRATOS HaloSense monitors have all the options you could want, whilst the CRONOS provides a low cost alternative and is particularly great value for money!

Each Chlorine Analyser from Pi has the capability to be an extremely capable Chlorine Controller. The controllers can have multiple control channels which can utilise chemical control (usually a relay (switch) turns dosing on when the chlorine is too low or off when it is too high) or PID control.

PID stands for Proportional Integrated Derivative and it is a mathematical manipulation of the sensor signal to give an output that will control a pump and managing a constant chlorine level in the water. All the features are adjustable and there are safety features build in such as overfeed protection. For a future discussion of PID control please see our technical notes here.

Pi’s chlorine controllers has been used in many control applications such as in pasteuriser, water treatment, cooling towers, swimming pools etc.

What does ‘Residual Chlorine’ means?
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.

What is ‘Free Chlorine’?
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’.

What is ‘Combined’ 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’.

What is ‘total’ chlorine?
Total chlorine is the combination of free chlorine and combined chlorine.

What range of sensors are available?
Pi offers Free and Total Chlorine sensors in the range 0.05-2 ppm, 0.05-5 ppm, 0.05-10 ppm, 0.05-20 ppm and 0.05-200 ppm.

Can I measure Chlorine in Seawater?
Yes, but when you add chlorine to seawater there is a displacement reaction to form Residual Bromine. For more information see our Technical Note on measuring chlorine in seawater.

How often do I calibrate my sensor?
This depends on the application. The online chlorine sensor has a very low drift so most people calibrate it either once a week, once a month or even every three months.

How often do I change the chlorine electrolyte?
Every 3-6 months.

How often do I change a chlorine membrane?
Every 12-18 months.