Bromine Tablets Bad For Spa Components

[chuckboone64]

Getting my new tub started up. I have owned a hot tub for about 20 years. Old one finally bit the dust. The new tub (from Watkins) calls for using granular Sodium Dichlor. It says NOT to use Sodium Dichlor tablets. Well, I had been using bromine tablets for years. When I asked the company about this, their answer was, "Granular form of bromine is fine but we do not recommend using tablets and floaters in any form. These tablets are very concentrated and will damage your jet pump, heater and jets causing them to fail much sooner than they should from over-sanitizing the water.

Is this true? Indicative of "sensitive components"? Or not trusting the owner to NOT over-sanitize? I guess I could learn "new tricks", but wondered whether the "old tricks" (bromine tablets with some shock) are out of date.

[chem geek]

Bromine tabs and especially Trichlor pucks are acidic so if they are in a floating feeder and park themselves in one place without circulation, then the pH in the area near the floater can get quite low and potentially be damaging. I believe this is what they are referring to. There shouldn't be much of an issue if there is always circulation or if the floater is kept closer to the middle of the spa when the cover is closed. Also read below, because there shouldn't be an issue with bromine tabs specifically.

When I first got my pool 10 years ago, I initially used Trichlor pucks in a floating dispenser and it parked itself near some stainless steel underwater bars and the mounts closest to the dispenser rusted as a result. So the problem is very real, but it's not so commonly heard of in a spa -- Trichlor isn't normally used in a spa and bromine tabs don't have the same amount of acidity as Trichlor, though they are acidic. A comparison chemically is below:

Cl3CY + 3H2O ---> 3HOCl + H3CY

Trichlor + Water ---> Hypochlorous Acid + Cyanuric Acid

H3CYA ---> H2CYA + H+ ..... about 70% of this happens at pH 7.5

Cyanuric Acid ---> Cyanurate Ion + Hydrogen Ion

HOCl ---> OCl- + H+ ..... about half of this happens at pH 7.5

Hypochlorous Acid ---> Hypochorite Ion + Hydrogen Ion

BCDMH + 2H2O ---> HOCl + HOBr + H2DMH

Bromine/Chlorine Tabs + Water ---> Hypochlorous Acid + Hypobromous Acid + DMH

HOCl + Br- ---> HOBr ..... assuming one has a bromide bank so the chlorine creates more bromine

Hypochlorous Acid + Bromide Ion ---> Hypobromous Acid

H2DMH has a pKa of 9.19 while HOBr has a pKa of 8.70 so both are only slightly acidic at spa pH (7-8 range). Note that their concern seems to not be about the acidity, but that with tabs you will over-brominate your spa. In other words, they don't think you will set your floating dispenser at the right level nor will you check your bromine levels to make sure they don't get too high.

By the way, there is no such thing as sodium dichlor tablets. Can you please re-read the documentation? It probably says not to use Trichlor tablets.