Sodium Hypochlorite in Bottle vs in Pool

[spinPHD]

It's pretty common knowledge on here that bottles of sodium hypochlorite (bleach, liquid chlorine) will lose potency over time. It will lose strength faster the higher the concentration. It is often mentioned that heat increases this process so it's recommended to store your bottles in a dark/cool location.

In contrast, once chlorine is added to the pool it is explained that the chlorine loss in your pool is only impacted by two things: UV rays (sun) and organics (algae). The heat of the water is not a factor, which is a reason why the OCLT works no matter your pool water temperature.

In any event, the mystery to me is why does temperature matter when chlorine is in the bottle, but doesn't matter when it's in the pool?

 

[JoyfulNoise]

Concentrated bleach in a bottle is different than chlorine in your pool. Concentrated bleach has excess lye to keep the bleach bottle pH around 12 or so. At that pH, the chlorine is entirely in the form of hypochlorite anion (OCl-). The degradation pathway for hypochlorite is mainly dominated by trace metal contamination (iron mostly) and the reaction products are oxygen gas (O2) and chloride ion (Cl-) (chlorate can also be formed but let's just ignore that for now). This reaction is temperature dependent and doubles in rate every 13F or so.

Bleach, when poured into a pool, converts into three forms - 95% of it is chlorinated cyanurates and the rest splits up into hypochlorous acid (HOCl) and hypochlorite (OCl-). The split between HOCl/OCl- is 50/50 at a pH of 7.5. Chlorine bound to CYA doesn't do much, stays in it's +1 oxidation state and is released into the water as HOCl/OCl- gets used up. The autolysis of chlorinated cyanurates is very, very slow. HOCl/OCl- reacts with sunlight at a much faster rate than it's own automatic degradation but all of the chlorine reactions double in rate with a 13F rise in temperature. So, in that sense, a hotter pool will go through chlorine faster than a cooler pool. However, the rates aren't really that appreciable until one gets above 90F or so, thus one can ignore them in favor of chlorine oxidation and UV reduction reactions.

 

[spinPHD]

Thank you very much for that explanation. I knew there had to be science behind it rather than just observations.

 

[JoyfulNoise]

In case you really like chemistry, there's more details here - Pool Water Chemistry

 

[dfahrion]

With our autocover closed that takes the UV out of the equation so without even plotting the data it is clear water temperature is directly correlated to chlorine loss in the pool. At somewhere around 75 degrees our pool needs 1ppm FC per day (if it isn't used/open), when it gets warmer or colder that doubling at 13 degrees is very noticeable. A couple of warm weeks without rain and our pool can climb above 90 which sends it over 2fc/day and if I'm not paying attention to the fact it warmed up and keeping up with testing that can be bad news fast. I can't just add 3FC and ignore the pool for a couple of days anymore, now if I can't test tomorrow I better add closer to 5FC and make sure I don't skip more than one day. On the other hand spring/fall waiting for the pool to warm up or cool off enough to close I will get under 0.5fc/day at which point daily testing would be silly as the test results may not even change from day to day.

The problem with trying to add temperature to the OCLT is you can't really test for less than 1FC loss due to testing accuracy, so how it is defined is probably the best that can be done, and just be aware a really hot pool (I'd guess near 100) may fail even when clear and a cold pool could pass when not quite clear.

 

[pooldv]

The higher FC loss in warmer water is due to higher biological activity more than higher FC decay rates. Everything grows faster in warmer water, bacteria, viruses, algae, etc. That is one reason why hot tubs are more challenging.

 

[spinPHD]

So I have an additional question that came up. If I mix 1/2 gallon of water with 1/2 gallon of 12% bleach, will that then allow me to store the gallon of now 6% bleach for longer and without the same concern of temperature?

 

[JoyfulNoise]

So I have an additional question that came up. If I mix 1/2 gallon of water with 1/2 gallon of 12% bleach, will that then allow me to store the gallon of now 6% bleach for longer and without the same concern of temperature?

Not really. Diluted bleach would have lower pH and therefore be LESS stable. Hypochlorite at pH values less than 11 will breakdown rapidly into oxygen gas and chloride ion. This is why all bleach has excess lye in it, to keep the pH up above 12 in order to stabilize the hypochlorite anion.