Free Chlorine and Total Chlorine analyzers and controllers measure the free residual chlorine (OCl– and HOCl), and the total residual chlorine (free chlorine + combined chloramines). The Pi residual chlorine analyzers are amperometric membraned sensors that bring a number of advantages over traditional colorimetric analyzers, in that they are cheaper to own with no reagants and very little maintenance (once per year).
They are extremely stable and work almost anywhere you have a need to measure residual chlorine from drinking water, to pools and spas, produce washing, cooling tower disinfection and disinfection in paper and pulp industries.
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The HaloSense chlorine monitor range is particularly suited to working in sites where reliability and ease of use are most important.
Many companies want to measure free chlorine residuals without the need for chemical buffers and reagants traditionally associated with measuring chlorine in water. 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.
That leaves colorimetric analyzers that mix chemicals with the water which changes color and the resultant color is related to the residual chlorine. Whilst still popular issues with reagant cost, optical fouling and the length of time between reporting results means that these analyzers are increasingly out of favor.
Most amperometric cells and 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.
The HaloSense Free Chlorine Sensor from Pi measures all the HOCl and the majority of the OCl– present. This results in a vastly reduced pH effect and means that most chlorine monitoring applications require no buffer and no pH compensation at all.
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 analysers 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 OCl– plus 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 of 1 year. HaloSense is fast becoming the instrument of choice for the engineer who wants the best instrument at the best price.
After chlorine has been added and disinfected the process water, any surplus chlorine that remains is referred to as ‘Residual Chlorine’.
When added to pure water, chlorine is present as hypochlorite (OCl–) and hypochlorous acid (HOCl). Together these are referred to as ‘free chlorine’.
‘Combined’ chlorine collectively refers to three chloramine compounds: monochloramine, dichloramine and nitrogen trichloride. Monochloramine is formed from the reaction of hypochlorite and ammonia:
NH3 + OCl– → NH2Cl + OH–
If the conditions are acidic, further reactions occur that form dichloramine and nitrogen trichloride:
2NH2Cl + H+ → NHCl2 + NH4+
3NHCl2 + H+ → 2NCl3 + NH4+
The familiar ‘chlorine smell’ is often due to these chloramines rather than ‘actual’ chlorine.
Free chlorine and combined chlorine in water can be collectively referred to as ‘total chlorine’. It can be thought of as ‘free chlorine + combined chlorine = total chlorine’.
Our free and total chlorine sensors can be ordered in 0.005-2ppm, 0.05-5ppm and 0.05-20ppm ranges. The total chlorine sensor can also be provided with a range of 0.005-0.5ppm, and our free chlorine sensor is also available in a 0.5-200ppm version.
Our chlorine sensors have a very low rate of drift, so the frequency of calibration depends upon the needs of your application. Different customers are known to calibrate weekly, monthly or even biannually.
Changing the electrolyte annually is recommended for our Total and Free Chlorine sensors, and every 3-6 months for our Zero Chlorine sensor.
Once a year.
Our sensors are resistant to changes in pH and will only vary by a very small amount. This is acceptable to most users.
Both the membrane and the electrolyte can be stored for two years in a cool, dry place.
All three sensors are constructed of stainless steel, PVC-U, silicon and hydrophilic membrane. The Total and Zero sensors also contain PEEK.
0 – 40°C for the Zero sensor, and 0 – 45°C for the Free and Total sensors.
A zero adjustment is not necessary because the sensor always operates at a positive voltage. Because of this any drift on the zero is negligible compared to the operating voltage.
Our sensors have an internal thermistor that allows for automatic temperature compensation, so there is no need to worry about temperature variation.
Many of our customers use handheld meters to determine the chlorine concentration for calibration, generally using colorimetric DPD analysis. These are available from a wide variety of suppliers.
Three things; timing, location and execution. Take your sample when the concentration is varying as little as possible, take it from the same location as the sensor and follow the instructions for your handheld device carefully.
The analyzer monitors the stability of the probe signal during the calibration process. If this varies by more than 10% during the calibration countdown, then the analyzer will not accept the result. This is to prevent the calibration routine from introducing inaccuracies into the sensor reading.
Pi passes another milestone In April, Process Instruments shipped their 4000th Version 3 controller and their 8000th controller since the company launched. In January 2017,
Process Instruments (Pi) have signed a new distributor agreement with EquipSolutions, ensuring a continued relationship for years to come. Welcoming EquipSolutions To The Pi Family
In 2009 Northern Ireland Water bought 334 residual chlorine analyzers from Pi after a years trial of a range of chlorine monitors. Pi Analyzers Performing
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