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 instruments can have multiple sensor and multiple sensor types, saving money on the requirement for one sensor and one transmitter per measurement.

Total Chlorine Analyzer

Total Chlorine Analyzer Sensor

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 free chlorine sensors used by the 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 needing to measure chlorine 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 it 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.

Free Chlorine Monitor

Free Chlorine Monitor

  • 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 chlorine monitoring applications require no buffer and no pH compensation.

Chlorine Monitor

Pi’s Chlorine Monitor at exhibition in China

Need help with an application? Click here!

  • Continuous online monitoring for residual 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 or control 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!

Residual Chlorine Analyzer

Residual Chlorine Analyzer

Each instrument from Pi has the capability to be an extremely capable Controller. The controllers can have multiple control channels which can utilize 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 built in such as overfeed protection. For a discussion of PID control please see our technical notes here.

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

What does ‘Residual Chlorine’ means?
When chlorine is added as a disinfectant to water it oxidizes 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.

Will changing pH affect my reading?
Yes, but only a very small amount and most users are happy to accept this.

What are the interferences?
Both ozone and chlorine dioxide will interfere with the measurement. For more information, click here.

What is the shelf life of the membranes and electrolyte?
If stored in a cool dry place, two years.

What are the materials of construction?
PVC, stainless steel and perspex.

What is the Temperature range of the sensor?
>3 °C – < 50 °C.

Why is there no zero adjustment?
The sensor operates at a positive voltage all of the time so any drift on the zero is negligible compared to the positive operating voltage so no zero is necessary.

What will happen if the temperature varies?
Nothing! The sensor has a thermistor that measures the temperature and does an automatic compensation.

What should I use to calibrate the sensor?
Use a handheld meter. These are available from a variety of suppliers and nearly all of them utilize colorimetric DPD to determine the ozone concentration in the sample.

What do I have to think about when I am taking a sample to do a DPD test?
Firstly take the sample from right at the instrument. Secondly don’t take the sample when the concentration is varying quickly, and thirdly use a good quality handheld and follow the instructions carefully.

I have tried to calibrate and the analyzer says that the sample wasn’t stable?
During calibration the analyzer looks at the stability (rate of change) of the signal from the probe and if it varies by more than 10% over the countdown then the analyzer prevents calibration to avoid the calibration routine introducing errors.

Focus Ons are a series of emails providing technical information regarding instrumentation, process measurement in potable, waste, process and pool waters.

You probably know that some instruments use ORP to control chlorine dosing and others use ppm chlorine sensors but…… did you know that ORP over about 3ppm won’t work?… did you know that swimming pools in the USA use ORP and in Europe use ppm chlorine sensors?… that the ORP of towns water can vary a great deal?

In the USA nearly all pools and spas use ORP sensors to control their chlorine dose, yet conversely in the UK and Western Europe most ORP systems have been replaced with systems that measure the concentration of free chlorine in water. Pi provides systems that utilizes either or both technologies.


Oxidation reduction potential (ORP or REDOX) sensors, measure the tendency of water to gain or lose electrons from anything in the water. The more positive a reading from an ORP the greater the tendency the water has to oxidize (gain electrons from) organisms or other material in the water, thereby killing or destroying them.

Why do so many pools in the USA use ORP?

When chlorine is dosed into a pool it forms OCl and HOCl. Disinfection is largely done by the HOCl and ORP responds to the concentration of HOCl in the water, which makes it a good measure of the tendency of the chlorine in the water to kill bugs. Despite this, ORP is a secondary measure of HOCl and is affected by a multitude of other factors, some of which will be touched on below. The main attractions of ORP are; low purchase cost, no calibration and little or no maintenance. Swimming pool

What are the problems with ORP sensors?

Unfortunately, what ORP sensors measure is tendency and not capacity, i.e. ORP measures the likelihood or the ability of the water to kill bugs, but not how many bugs that water can kill, a subtle but very important difference. A sample with high ORP may be able to kill a small number of bugs very quickly but then not be able to kill future pollution. What’s more, although chlorine affects ORP very strongly it is not the only variable involved. The pH of water affects ORP directly and also affects the concentration ratio of OCl/HOCl, the two main disinfectant components. A lower pH (higher acidity) will cause an increase in the relative concentrations of HOCl causing an increase in ORP.

Perhaps the biggest issue with ORP is that the ORP readings on water with no chlorine in it will be different depending on the source of that water. This means that an ORP of 750mV in one part of the country is not the same chlorine concentration as 750mV in another part of the country. Also the ORP response to HOCl is not linear and increasing residual chlorine above 3ppm has little effect on ORP readings making control above 3ppm extremely difficult.These issues typically lead to overdosing the water with chlorine, in order to compensate for these effects. This can be seen very clearly in US pools which often have more than 2ppm of chlorine compared to European pools which typically operate around 0.8 – 1.5ppm (The World Health Organisation recommends 1ppm residual). ORP Sensor

ppm Chlorine

These sensors use electrochemistry to measure the free chlorine concentration directly. They tend to be slightly more expensive than an ORP sensor, but are more reproducible and precise, and therefore tend to give better control (and therefore reduced chemical cost). They are specific to free chlorine (the disinfectant) and can be easily calibrated using a DPD test for free chlorine. Whilst the capital cost for a ppm chlorine sensor is higher, total cost of ownership tends to be lower as ORP sensors are typically replaced every year and ppm sensors last for ten years or more.

Problems with ppm Chlorine sensors

A ppm sensor measures the capacity of water to kill organisms, the only problem is that is doesn’t measure how fast the bugs are killed, a variable largely down to pH. There are two different types of ppm sensors. The first measure only HOCl, and have very similar problems to ORP sensors. The other type of sensor, in pHs below 8.0, measure both HOCl and OCl. Pi only recommends the use of sensors that (for use in pools) are independent of pH, and the use of pH control that is independent of chlorine dosage. This leads to tighter control of both pH and free chlorine meaning chlorine residuals can be more tightly controlled and reduced, which in turn leads to lower costs and a more pleasant bathing experience.




ORP Sensors

Simple (no calibration)


ORP Sensors

Doesn’t measure disinfection capacity

Affected more by pH than by free chlorine

Non- Linear

Not reproducible (not the same from site to site)

Affected by changing water chemistry

Affected by all oxidants

Using ORP control normally leads to higher residuals and less stable control

ppm Sensors

Measure free chlorine directly

Results comparable across different sites

Linear response

Only affected by free chlorine

Using a ppm sensors leads to lower residuals, more stable control and better swimmer experiences

ppm Sensors

Requires calibration

More expensive – but not much

More maintenance – but not much

1. Did you know that when you dose chlorine into seawater it is bromine that does the disinfection?

2. Did you know that DPD1 measures free chlorine or total bromine and not free bromine?

Chlorination Chemistry of Seawater

The chemistry of the chlorination of seawater is more complex than many people realise and although the measurement of chlorine residuals is possible in seawater (and therefore automatic control of chlorine dosing), better results will be obtained if this is fully understood.

Is it Chlorine or is it Bromine?

Seawater contains about 70ppm dissolved bromides most of which are sodium bromide. When you put chlorine in water it displaces (because it’s more reactive) the bromine from the bromide and becomes a chloride. So for up to about 70ppm of total chlorine dosed what you actually have in the water is free bromine and combined bromine (NOT free and combined chlorine) so it is the total bromine that actually does the disinfection [1]. So why does everyone call it chlorination when technically it is bromination? Mainly because most people don’t know this interesting bit of chemistry. So what? Normally it makes no difference at all as bromine is an effective disinfectant, however there can be a lot of confusion when it comes to monitoring residuals and controlling dosing. Choosing the correct sensor to control the dosing is crucial as is choosing the correct DPD test.

Pi offer a specialist range of seawater chlorination controllers, but to choose the right controller we need to understand the chemistry going on. A technical note on the same subject is available here.

Free Chlorine and Total Bromine

Due to the confusion on what is being measured it is easy for an engineer to specify the wrong equipment and calibrate it incorrectly. For example, it is common for a free chlorine sensor to be specified for seawater chlorination control. Most electrochemical free chlorine sensors will react to free bromine (not all so be careful!) but this isn’t necessarily what you need for bromination control. Most authors agree that whilst the disinfection capability between free chlorine and combined chlorine differs, when it comes to free bromine and combined bromine, both forms of the chemical are equally good at disinfection so a better measurement would be total bromine, which requires a total bromine sensor.

DPD and Seawater Chlorination

To add to this already confusing environment we need to look at calibrating online sensors or using handheld photometers to track the residual. DPD is used extensively to measure chlorine residuals and it also reacts to bromine so can be used for both, however, DPD1 measures FREE chlorine or TOTAL bromine. The situation can therefore arise where you have an online instrument such as a CRONOS, CRIUS or CRATOS specified as a free chlorine, actually measuring free bromine but calibrated as a total bromine (against DPD1)! Typically the best results are obtained by specifying a total bromine (total chlorine) sensor and calibrating it using DPD1. That, however, isn’t the end of the story! When specifying an analyzer it is crucial that we suppliers know that it is for use with seawater because the physical and chemical makeup of seawater is very different to potable or process water and this can affect what we would supply to customers.

The effect of Salinity on Membrane Sensors

It is crucial for us to know if you are going to use a Pi sensor in seawater so we can provide you with a saltier electrolyte. Osmosis means that water moves from a low solute concentration to a higher solute concentration across a semi-permeable membrane. The electrolyte in our sensors is saltier than potable or process water so osmosis forces water into the end of the sensor, which the sensor is designed to cope with, however, with seawater the process is reversed and the water in the electrolyte can be forced out of the sensor into the sample. To solve the problem we supply electrolyte especially designed for seawater, with a higher salinity.

Estuarine Waters

Many seawater chlorination applications are estuarine in nature (partly seawater and partly freshwater) and it is the degree of dilution which determines which sensor and which electrolyte you should use. Seawater has approximately 70ppm bromides and so up to 70ppm chlorine the replacement will be 100%. If the seawater is 50% fresh water then up to 35ppm chlorine will give 100% displacement. For example, if we looked at a 2ppm residual then the water could be only 3% seawater and 97% fresh water and you would still be measuring bromine, so a total bromine sensor calibrated with DPD1 would be appropriate. For any water that is contaminated with seawater the seawater electrolyte is likely to be the most appropriate.

The Solution!

If all of this is too much to take in and remember, then don’t worry! Just remember to talk to Pi for any online chlorination application and we will do the rest… guaranteed!

Total Bromine Sensor


[1]. White’s Handbook of Chlorination and Alternative Disinfectants, 5th Edition ­ Wiley – page 874, pages 122-129