Bleach Without Cya Not Good For Tub

[Vince22]

I just read in another post that manufacturers don't recommend bleach as a sanitizer because it can damage the tub. I believe it was Chem Geek (might be wrong) who said that with CYA, the bleach effect on the tub is reduced.

But what about people using bromine with bleach as the activator? I wouldn't have CYA in my tub, so does that mean that I'm damaging my tub?

Thanks for the help.

[chem geek]

When you have bromine in the tub, then if you add chlorine it will oxidize bromide ions to bromine. Now this doesn't happen instantly so during the time the chlorine is hanging around it is stronger without the CYA, but it won't be around for very long -- probably not more than an hour. So it shouldn't be a problem. If you were concerned about it, then you could use Dichlor instead of bleach to shock with your bromine system, at least for the first few times until you've built up some CYA. After that, you can shock with bleach, possibly switching to Dichlor once a month. It's not a big deal in your case with a bromine system since the chlorine won't be around for very long. Note that the CYA won't have any affect on the bromine; it will only reduce the active chlorine concentration during the time the chlorine hangs around. Also, with CYA in the water, the chlorine will take longer to oxidize the bromide to bromine -- I don't know exactly how much longer, but I doubt it's more than a day (I can't find the rate constant for bromide to bromine from chlorine, but have heard that it's not immediate).

[chem geek]

I found a rate constant for the oxidation of bromide ion with chlorine (hypochlorous acid). At a pH of 7.5, the rate constant is from around 1550 to 6840 depending on the source. The question is what the bromide concentration will be in a typical spa. When using brominating tablets, the bromide level will climb over time. If I assume around 10 ppm bromine forming a very small initial bromide bank when it gets used up, then this is 6.3x10^-5 moles/liter concentration. If you have 30 ppm CYA in the water and shock with 4 ppm FC, then this is 3.1x10^-6 moles/liter concentration of hypochlorous acid (at 100F temp). So the reaction rate is (taking the low-end rate constant) 1550 * 6.3x10^-5 * 3.1x10^-6 = 3.0x10^-7 moles/liter/second or around 0.5% of the bromide level converted per second. That's around 30% of the bromide level converted per minute so you can see that even in these worst-case conditions with a small bromide bank and having CYA in the water that the conversion of bromide to bromine happens pretty quickly in minutes (and that's with CYA in the water; with no CYA, the reaction is mostly complete in less than a minute). Also, the rate constant was for 25C (77F) so would be faster at spa temps.

So the bottom line is that the conversion of bromide to bromine by oxidation from chlorine should happen relatively quickly, meaning that the chlorine won't stick around for very long. Therefore, using chlorine without CYA in the water should be fine for the purpose of reactivating bromide to bromine without harm to the tub.

[quantumchromodynamics]

I think that people using a bromine maintenance program should establish a 30-ppm bromide bank by initially adding sodium bromide. This will ensure rapid and complete conversion of the chlorine to chloride and bromide to hypobromous acid.

Gallons....................Sodium bromide salt needed to add 30-ppm bromide ion concentration.

100.........................15 grams (0.53 ounces)

200.........................30 grams (1.1 ounces)

300.........................45 grams (1.6 ounces)

400.........................60 grams (2.1 ounces)

500.........................75 grams (2.6 ounces)

[Vince22]

Thank you. I sincerely appreciate you taking the time to provide your calculations. I do use a bromide starter packet when I fill the tub.