Measuring free chlorine and chlorine dioxide independently of each other is quite a challenge, given their chemical similarities.

Many sensors struggle to differentiate between the two measurands, but did you know that…

…many chlorine probes suffer from interference in the presence of chlorine dioxide?
…DPD1 will read both chlorine and chlorine dioxide?
…you can have accurate chlorine dioxide control in water where chlorine is present?

Free chlorine and chlorine dioxide are both oxidants used for disinfection in water. Each act differently as a disinfectant but are measured in almost the same way; with an electrochemical sensor or with an online DPD sensor. It is sometimes beneficial to have both disinfectants in the water at the same time, particularly when chlorine dioxide is being added to mains water.

In most non-membraned (and some membraned) amperometric sensors, oxidants are detected by a current produced at the working electrode, at a particular voltage. The same technology can effectively be ‘tuned’ to different oxidizers by varying the voltage. Lots of oxidants are measurable over a range of voltages, and sometimes those response curves overlap.

The graph shows that at almost any voltage where you can measure free chlorine, the chlorine dioxide curve overlaps with the chlorine one. This means it can be very difficult to find a probe that measures free chlorine, but doesn’t measure chlorine dioxide. Although these response curves can shift depending on probe design, electrode material and other factors, it is very difficult to engineer a response curve that gives a good signal for free chlorine but not for chlorine dioxide.

The DioSense membraned chlorine dioxide sensor from Pi is not susceptible to interference by free chlorine. This means that the DioSense sensor can be used in conjunction with Pi’s HaloSense free chlorine sensor, to measure chlorine dioxide and free chlorine independently of each other in the same application, on Pi’s CRONOS^® or CRIUS^® analyzer. The analyzer takes the signal from the free chlorine probe, which does have a known interference from chlorine dioxide (1ppm of chlorine dioxide will show up as 0.75ppm of chlorine). The normalized signal from the chlorine dioxide sensor can then be removed.