Hypochlorous Acid Vs. Hypobromous Acid

[PaP dude]

I have a question for anyone on this forum. When a pool has an outbreak of algae, is it more effective to just use sodium hypochlorite (yielding hypochlorous acid) or does using a bromide salt (sodium bromide) with an activating agent prove to be more effective? I have some what of a basic idea of how bromine chemistry works, but I really don't know how (chemically) the "bromide bank" gets converted to hypobromous acid, then to bromamines (which aren't bad from what i have read) then back to the bromide bank. I do know that bromine is more effective over a broader range of pH than chlorine (which is why I guess it is popular for spas with warm water and aeration), but is it worth the money for using it in a pool? Any answers, chemical formulas and equations are much appreciated.

[chem geek]

Basically, sodium bromide is sometimes used as an algaecide in a chlorine pool because when added it converts to bromine (hypobromous acid) using up some chlorine (hypochlorous acid) in the process. Because bromine is not lowered in effective concentration by Cyanuric Acid (CYA), it is stronger than chlorine in terms of the rate of fighting algae. This is particularly effective against yellow/mustard algae since it takes a higher disinfectant level to kill it. However, higher levels of chlorine are also effective. For regular algae, a Free Chlorine (FC) level that is 40% of the CYA level works well while for yellow/mustard algae an FC that is 60% of the CYA level works well. So the use of sodium bromide is another approach, but not a necessary one.

If you use sodium bromide, your pool will partially be a bromine pool for a while with any extra chlorine going to convert bromide to bromine. That means anything that bromine kills or oxidizes will have it go to bromide and then chlorine will convert the bromide to bromine. Also, bromine does not combine with CYA so it breaks down in sunlight much faster than chlorine (that is mostly bound to CYA) so you will seemingly have a higher chlorine usage for a while as it attempts to restore the bromine that is broken down (since it is broken down to bromide that chlorine then reactivates to bromine, using up chlorine). Eventually, the bromine will outgas out of the pool though that will take some time.

So in general, it's better to avoid the bromide/bromine route, especially in a pool exposed to sunlight.

NaBr --> Na+ + Br- ..... Sodium Bromide added to pool

HOCl + Br- --> HOBr + Cl- ..... Bromide converted to Bromine by Chlorine that ends up as Chloride

2HOBr --> O2 + H+ + Br- ..... Bromine getting broken down by sunlight

etc.

Technically, CYA will still absorb UV rays and protect lower depths of both chlorine and bromine from breaking down as quickly, but closer to the surface there isn't much of that protection. Chlorine is mostly bound to CYA and is protected that way, but bromine is not.

Richard

[PaP dude]

All other factors aside, does one kill better than the other?

[chem geek]

All other factors aside, does one kill better than the other?

Not really, especially if your CYA level isn't very high so that you can readily get an FC level that is 60% or more of the CYA level. I would say that the sodium bromide approach is most useful if you've got yellow/mustard algae and your CYA level is high. However, you then have a bromine pool to deal with that will have higher-than-normal chlorine demand for a while (not sure if its days or weeks, but the bromine won't go away quickly).

Technically, an FC level that is 60% of the CYA level is equivalent to an FC of 1.4 to 1.6 ppm with no CYA. Typically, the amount of sodium bromide that is used is around 2-4 ppm bromine which is around 1-2 ppm FC equivalent. The bromine does have the advantage that any bromamines that are formed are still effective as algaecide unlike chloramines but at the shock levels with chlorine there is usually relatively little chloramine that is formed or it gets fully oxidized and one just adds more chlorine to keep the FC high.

The normal shock level for chlorine with green algae is an FC that is 40% of the CYA level and is equivalent to and FC of 0.5 to 0.6 ppm with no CYA. Since mustard/yellow algae is more resistant to disinfectants, it takes a higher level than this to kill it off thoroughly as noted above. So what usually happens is that people don't shock at a high enough chlorine level to kill off yellow/mustard algae.

I really can't recommend one approach over the other as there are pros and cons for each. Each approach can be equally effective. It's more a matter of needing high FC levels when using chlorine vs. dealing with a bromine pool for a while when using sodium bromide.

Richard