HOT TUBS: Calcium (and Bromine)

[chem geek]

Thank you so much for the thorough information. You think like a scientist even if you don't have the full background! Being observant is an important attribute.

I initially looked at using Bromine for our pool instead of Chlorine, but found that it was about 3 times as expensive and that you had to use about twice as much of it so it netted out to being way more expensive overall. I can see that this doesn't matter much for the much smaller volume of water in a spa, but it just doesn't make sense to use bromine in a pool. As for loss from degradation including the higher temperatures, the chemistry doesn't back that up. Yes, chlorine degrades from sunlight, but with a small amount of CYA you can avoid that. And yes, some have said you shouldn't use CYA in hot tubs and that is probably because you want more disinfection in hot tubs due to the higher "bather load to pool water volume" relationship, but I'm talking about very small amounts of perhaps 10 ppm CYA that would go a long way in preserving chlorine while cutting down your 5 ppm chlorine to 0.5 ppm HOCl which is still quite high (close to Ben's "shock" level, in fact).

Well, I suppose given how hard it might be to try and maintain low CYA levels in a spa (mostly the difficulty of measuring such low levels since current test kits tend to start at 30 ppm) and given the presumed higher risks of "spa itch" and other problems requiring disinfection, I guess using bromine in the spa makes some sense. If anyone out there is using CYA and chlorine in their spa sort of "like a pool", it would be interesting to know if they run into any problems. I've seen other threads (search "bromine" and under Advanced search "hot tubs") report users using BBB (with chlorine, not bromine) in their hot tubs without a problem.

As for the temperature effect, we keep our pool at 88F and there are many others on the forum who have their pool at 90F or above (though they may not want to -- they live in very hot climates) and I don't believe anyone is reporting unusual chlorine loss beyond that expected from the intense sunlight. It is true that chlorine breaks down from higher temperature and a 10C (18F) rise roughly doubles this breakdown rate (until you get to much higher temperatures), but at 77F a 10% chlorine (that is, VERY concentrated compared to your spa) has a half-life of 220 days while the very high 140F temperature shows a half-life of chlorine of 3.5 days, but for the concentrations in your spa the breakdown of chlorine due to temperature is neglible. So I can understand the concerns from the breakdown from sunlight (when no CYA is used), but not from temperature.

As for mixing bromine and chlorine -- DON'T! If you were to mix the actual concentrated chemicals (as opposed to pool and spa water), you can produce an explosive or certainly very, very hot mixture that is very dangerous! Chlorine and Bromine are chemically related, but chlorine is a more powerful oxidant. Chlorine (hypochlorous acid HOCl) will oxidize bromide ion (Br-) to produce bromine (hypobromous acid HOBr) and in fact when you add a non-chlorine "shock" to your spa water (such as MPS) it will regenerate HOBr from Br- in the same way. By adding spa water to your pool water you have added some HOBr hypobromous acid and/or Br- bromide ion which you will never get rid of. Unfortunately, your chlorine will waste its time converting bromide ion to HOBr instead of disinfecting and oxidizing other chemicals. It is true that if some HOBr got converted to Br- then this meant it actually did something useful (probably killing some "weak" bugs or oxidizing "easy" substances), but the point is that you want your chlorine to be maximally available in your pool and not wasting time creating the weaker sanitizer HOBr. Ironically, some brominating tablets contain both chlorine and bromine (since chlorine is a more effective sanitizer) though most contain just bromine.

Richard