photochemical reaction bonding cyanuric acid to hypochlorous acid

[Guest bearventures]

First off, I'm not a chemical engineer or even a student. That being said, I'm looking for some insight 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 indicates 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 becomes shaded (starting about 5 PM). There was no chlorine feed during this period, the pH remained constant at 7.5 and the temperature remained constant at 81 - 82 F. In fact, the hypochlorous acid was subject to biological load (bathers) and would have been expected to decrease without chlorine feed. 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 according to bather load.

 

This occurs every day at different 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. It is repeatable and verifiable.

 

It appears 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. It appears that the bond is at least in some part a photchemical reaction and not a strict chemical bond as much of the literature suggests.

 

Any help with understanding these occurrences?

[Guest mx_tang]

Hi bearventures,

 

I hope you got all of the replies that I wrote on this thread, since it was somewhat educational but has been deleted/erased. Hopefully, it will come back, but I'm not sure, since I just had to re-register (I couldn't login) because this forum had no record of me registering in the first place.

 

MX