By Michael Francis, global product manager, Emerson Automation Solutions

Let’s talk chlorine. Obviously, balancing chlorine in water treatment is a critical issue, so having a friendly relationship with your chlorine analyzers is important. Customers ask us a number of basic questions about chlorine and its analysis. Here are a few of these questions with the essential answers from our Rosemount Analytical experts.

Q. How do you measure chlorine in aqueous solutions?
A. 
Reagent-less chlorine measurement requires an amperometric sensor and an analyzer to convert the current signal to a ppm reading. Unless the pH never changes more than 0.2 pH units, a separate pH sensor is strongly recommended when performing chlorine measurement. The preferred installation is in a bypass line with the sensors installed in a low flow cell. Other alternatives are installation in a 1-1/2 inch tee or submersed in a tank.

Q. How do chlorine sensors work?
A. 
Amperometric sensors use electric currents or changes in electric current to detect ions in a solution. The amperometric sensor tip consists of a membrane stretched over a noble metal cathode. The chlorine in solution diffuses through the membrane to the surface of the cathode. A voltage applied to the cathode reduces the chlorine to chloride. This process consumes electrons, which come from a second electrode (the anode) inside the sensor. To measure the amount of chlorine in the solution, the analyzer measures the number of electrons consumed at the cathode (the current) which is directly proportional to the concentration of chlorine in the sample. Since the sensor is constantly consuming chlorine from the process, it is necessary to have a continuous flow in front of the sensor, or else all the chlorine in front of the cathode will be destroyed, and the sensor will read zero chlorine in the sample.

Q. What types of chlorine sensors are available for real-time measurement and process control?
A. 
Monochloramine, free chlorine, and total chlorine sensors and systems are available for process control. It’s important to match the application and kind of chlorine to the measurement system.

Q. How do I calibrate a chlorine sensor?
A. 
Because stable, diluted chlorine and monochloramine standards are generally not available, the sensor must be calibrated against the results of a laboratory test run on a grab sample of the process liquid. (Learn how to zero the Free Chlorine Measuring System with a Rosemount Analytical 499ACL sensor here.)

Q. What affects the accuracy of chlorine measurements?
A. 
Chlorine measurement accuracy can be affected by fluctuations in temperature as it changes membrane permeability rate, electrolyte conductivity, and the sample pH levels. The need for additional sensors is reduced with the automatic temperature compensation and low sample conductivity requirement on the Rosemount 499ACL (Option -01) free chlorine sensors. Click HERE to learn more about the Rosemount Analytical 499ACL Chlorine Sensor.

There are many more basic questions on chlorine measurement that can be critical to successful water treatment. If you have pressing questions, please comment below.

We’ll address some more chlorine questions in the future.

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