Mr. Ed said:
chemgeek definitely knows his stuff....that's obvious....but testing of the water 24 hours later shows that the bromide isn't present
When I wrote that bromine only works well for high CYA situations, I didn't mean that it doesn't work unless the CYA is high. As you wrote, it works regardless of CYA level. I meant that it is only really needed when the CYA is high because when the CYA is lower you can shock with reasonable amounts of chlorine. By "works well" I meant in an overall sense balancing against other options such as using chlorine.
How are you able to test for bromide and distinguish it from chloride? And how are you able to test for bromine and distinguish it from chlorine? The standard test kits that test for chloride also react with bromide and quite frankly there is always so much chloride in the water that it will swamp any bromide result anyway. As for bromine, the tests are for halogen where bromine and chlorine will both react whether it is an OTO, DPD or FAS-DPD test. The only difference is that technically bromine is 2.25 times heavier than chlorine which is why the test kits have dual scales with bromine twice as high as chlorine. If you measure a zero bromine reading, that just means there is no bromine or chlorine, but it does not mean there is no bromide. Any chlorine that is then added to the water would then reactivate the bromide to bromine and would show up in your test kit as if you had chlorine, but it's really bromine because the test kit does not know the difference.
If you only add a fairly small amount of sodium bromide to the pool so that you are only generating a few ppm, say less than 5 ppm, of bromine (equivalent to around 2.2 ppm of chlorine, but unbound to CYA), then that might outgas from the pool in a week or a month, but it's not going to be hours or days. The reason is that unlike chlorine which breaks down to become chloride and then gets stuck there (unless you have an SWG system), bromine that gets broken down by sunlight or that oxidizes an organic becomes bromide which then gets reactivated by chlorine in the water to become bromine again. So the pool becomes a bromine pool and bromine or bromide only leave the system through 1) slow outgassing (accelerated by more aeration if you want to make it happen faster), 2) dilution of the pool water and maybe some physical removal in backwashing if you are lucky enough to have bromine attach to some organic particles large enough to get filtered out.
Bromide + Chlorine ---> Bromine + Chloride
Bromine broken down by sunlight or oxidizing ammonia or an organic ---> Bromide
So you can see from the above cycle, that you don't get rid of bromide/bromine except by its physical removal from the water. This is very different than chlorine which goes to chloride (salt) and stays there.
To a large extent this is all somewhat of a moot point since what does it mean to have a bromine pool? It's not a disaster, but it usually means a somewhat higher sanitizer demand because bromine does not bind to CYA so gets broken down by sunlight, though not as quickly as unbound chlorine. Actual reports from users are mixed, however, in terms of sanitizer demand when using bromine -- some see a higher demand, some see about the same (no one sees a lower demand). So other than carefully measuring the sanitizer demand and noticing a difference in consistent conditions, you can't really know when you are back to chlorine instead of bromine. Technically, you might smell a difference if you are very familiar of bromine odor vs. chlorine.
If I were going to use a product that bypassed CYA, then ammonium sulfate to form monochloramine would be an alternative and at least in this situation one knows that one can add chlorine to get rid of the monochloramine and can measure to know that the Combined Chlorine (CC) has truly dropped.
Richard
