How Long To Form Bromide Bank?

[njmurvin]

Last weekend I decontaminated my Marquis spa. After the final fill, I balanced the pH and added the 2 oz packet of sodium bromide. I ran the jets for 30 min on high to distribute the chemicals. Then, I shocked the spa with 7 oz of 6% bleach (for 350 gal). I ran the jets for another 30 minutes and checked the bromine using my Taylor K2106 kit. I was surprised to find it only up to about 5 ppm. Usually, shocking this spa in this manner takes me well over 10. I checked the next day and it was down to about 3ppm. So, I shocked it again with another 7 oz. This time, it went up to the expected 10ppm. Did I not allow enough time for the bromide to establish a reserve before the initial shock? If so, how long should I wait before the initial shocking and what happened with the first 7 oz of bleach?

[chem geek]

The test kit cannot distinguish between chlorine and bromine so whether or not your bromide bank was sufficient is not relevant to what you were seeing. If you saw high chlorine or bromine demand, then that is what was happening, independent of bromide in the spa. There must have been something in your spa that needed to get oxidized. You probably lost some chlorine or bromine from outgassing when you ran the jets, but that doesn't explain the overnight drop. I assume you don't have an ozonator in this spa.

See what your loss rate is over 24 hours. If it's stabilized to around a 25% loss, then you're back to normal again and got rid of whatever was there before. Strange though -- especially after a decontamination. Maybe the decontamination dislodged something that was oxidizable by chlorine or bromine and didn't get removed when you changed the water.

[njmurvin]

I considered that there might have been something in there using up the bromine. But, I recall seeing something similar the last time I changed the water. It took two shocks to get it inline. Could there be something in my tap water that needs oxidizing? BTW, I installed new filters, so nothing was hiding in there. I do have an ozonator but in this spa it is next to worthless (only runs a couple of hours a day). I don't even know if it's working. After 24 hours, it was down to 7, so it seems to be stabilizing.

Oddly, the pH adjustment also seemed to have a delayed effect. I added quite a bit of dry acid right after the fill (~5 oz by volume in 1 oz increments) and the pH still seemed high (8.0) after running the jets for awhile. So, again, I decided to let it settle overnight. The next day, I checked pH again and it was down to 7.2. I do have a bromine tablet floater and I know the tabs lower the pH over time, but not that fast. I'll keep checking over the next few days to ensure it's stable. I need to check the TA again. I have a feeling all that acid must have lowered it as well (started at 100).

[chem geek]

Municipal treated tap water shouldn't have that much of a chlorine demand since they generally try and remove much of the organics to avoid creation of chlorinated disinfection by-products. It might be chloraminated, but usually that's only 1 ppm CC or so and the maximum the EPA allows is 4 ppm. That would take roughly an equal amount of FC to get rid of it (the 10x "rule" is wrong in this case since it applies to ammonia in ppm N units and not to monochloramine in ppm Cl₂ units). Well water, on the other hand, could very well have organics in it that take time to oxidize.