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[bearventures]

First off, I'm not a chemical engineer. That being said, I'm looking for some insite into observations I've recently made since starting to use a new type of ORP/pH controller. All the literature I've read on the subject states that while cyanuric acid reduces the consumption of hypochlorous acid by UV, the mechanism by which that occurs is not well understood.

In chlorinated water (2 ppm) with 40 ppm cyanuric acid I have observed the ORP value increase from 740 mV to 815 mV over the course of about 4 hours beginning when the outdoor swimming pool became shaded (starting about 5 PM). There was no chlorine feed, the pH remained constant at 7.5 and the temperature remained constant at 81 - 82 F. When the pool is exposed to the sun (about 6 AM) the ORP drops back down to 740 mV within about 3 hours and remains constant throughout the day with chlorine and pH adjustments made automatically.

This occurs every day at several locations. The response varies directly with the amount of cyanuric acid. There seems to be no response in indoor pools with or without cyanuric acid. I have not observed this response in the same pools under the same conditions using other ORP/pH monitoring devices.

I propose that the hypochlorous acid only bonds to the cyanuric acid in the presence of sunlight and that it releases in the absence of sunlight. In other words, in the presence of UV the hypochlorous acid and cyanuric acid form a temporary isocyanurate compound which then separates in the absence of UV.

Any help with understanding these occurances?