Does anyone know of a reliable first-order or pseudo-first-order rate constant (or equivalent half-life time, from which I can get the rate constant) for the kinetic decay of aqueous chlorine in water (without photolysis or other reactions)? I have done a lot of web searching, read lots of research papers (mostly foreign, as the full text is more often provided) and come up with different answers in nearly all. The majority of papers report values over quite a large range, and most of these were related to chlorine residuals over time in water delivery systems. The two values I've seen which may be most reliable were in two different papers I found in the National Institute of Health website. One reported is a half-life time of 114*Ln(-1), which is 358.14 hours (no temperature given, but I'm assuming 25 degrees C). Another from a paper's summary on the NIH which gave a rate of .15 h-, which could mean .15 of the molar concentration loss/hour decay, or it could also mean a rate constant of .15 - totally different. Help?
Kinetic Decay of Aqueous Chlorine
- Thread starter Tom ONeill
- Start date
- Mar 5, 2020
- 3,211
- Pool Size
- 66000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Astral Viron V35
I don't have real scientific data on this. One thing to consider is the amount of stuff in the water that chlorine can oxidise, which makes pool water different to drinking water supply. One of those things is for example CYA, oxidation of CYA by chlorine is responsible for CYA decaying in hot climates.
My own experience: In winter my pool is fully shaded, with low UV to start with. To maintain 10ppm FC, my SWG produces about 0.25ppm of FC per day, water temperature around 9°C. This is a chlorine loss of about 2.5% over 24h. Or a half life of a bit less than 4 weeks. This is very independent from cloud coverage.
I have also confirmed this half life when bringing FC up to 21 before a longer holiday in winter and then turning the SWG off. After 30 days FC was down to 9.5ppm.
Assuming a doubling of the responsible chemical reactions every 10°C (some assume every 10°F or 13°F, something in this magnitude), I estimate about 8% FC-loss over 24h at 25°C due to UV-independant effects, but in summer I can't really distinguish between UV-dependant and -independant losses anymore to confirm this. This gives me a half-life of about a week due to UV-independant effects.
Nothing hard and fast...
My own experience: In winter my pool is fully shaded, with low UV to start with. To maintain 10ppm FC, my SWG produces about 0.25ppm of FC per day, water temperature around 9°C. This is a chlorine loss of about 2.5% over 24h. Or a half life of a bit less than 4 weeks. This is very independent from cloud coverage.
I have also confirmed this half life when bringing FC up to 21 before a longer holiday in winter and then turning the SWG off. After 30 days FC was down to 9.5ppm.
Assuming a doubling of the responsible chemical reactions every 10°C (some assume every 10°F or 13°F, something in this magnitude), I estimate about 8% FC-loss over 24h at 25°C due to UV-independant effects, but in summer I can't really distinguish between UV-dependant and -independant losses anymore to confirm this. This gives me a half-life of about a week due to UV-independant effects.
Nothing hard and fast...