Bleach & pH

[wes8398]

I have a ~450 gallon hot tub which I keep sanitized via 3 step bromine method (and ozonator), employing 10% Purox bleach for bromine activation. On average the tub sees 2 adults 2-3 times a week. I find that whenever I add the Purox (to either shock or just bump up sanitizer), I'm then fighting high pH levels. I try to keep pH around 7.5 and I've tried TA levels anywhere from 30 to 120, but (a few hours) after adding my bleach, pH shoots up to 8.2+. I understand this is normal with bromine levels over 10ppm, but even when I'm just adding the bleach to bump up to less than that, my pH raises significantly.

I shocked (to ~15ppm 'FB') 2 days ago, then brought pH down to about 7.6 via 4 tablespoons of dry acid. 'FB' is currently at about 9ppm, pH 7.6ish, and TA is about 80-90. I also have some borates in there from the addition of some Borax months ago, but that's been diluted by water top ups and so fourth. Likely a nominal amount.

So, suggestions anyone? Or is it normal to have to bring pH down after the addition of bleach?

 

[JohnT]

Re: Bleach & pH

Shock level chlorine can interfere with pH test results, especially if your pH test doesn't have chlorine neutralizer in it.

 

[JamesW]

Re: Bleach & pH

Upon addition of bleach, the pH will rise a little, but it should mostly go back down as the bleach is used up. Some bleach has more sodium hydroxide than other bleaches do, which will cause some net pH rise.

Maintaining a lower TA (perhaps around 40 to 50 ppm) should help quite a bit as far as pH rise due to carbon dioxide offgassing is concerned, but it will provide less buffering. Therefore, it would help to maintain the borates at 50 ppm for better control of pH rise due to the addition of bleach. It will also help if you maintain the pH closer to 7.9.

Some people use a phosphate buffer in tubs that are not exposed to sunlight and that have low levels of calcium, but I don't think that phosphate buffers are a good choice for most people.

http://www.spadepot.com/shop/pH-Proper- ... 64C47.aspx

 

[chem geek]

Re: Bleach & pH

In a bromine tub, the addition of bleach has the pH rise as it would when adding a hypochlorite source of chlorine to a pool. The activation of bromide to bromine doesn't change that effect. However, with no CYA in the water (or assuming all the chlorine oxidized bromide to bromine), the pH would rise from 7.5 to 7.84 if the TA level were around 50 ppm. If you added 20 ppm, then that would raise the pH to 8.1. However, it's more likely that the higher sanitizer level is interfering with the pH test as John noted.

The quote from Taylor's Pool & Spa Water Chemistry booklet on the issue of a high FC causing a false high pH reading is as follows:

FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

If you have distilled water available, you can use that to dilute your water sample with negligible effect on pH, so that can cut down the bromine concentration making the pH test more reliable. Note that you must use unbuffered water (such as distilled water with 0 TA) for the dilution and not use tap water which is buffered (i.e. has some TA).

 

[wes8398]

As I mentioned, I'm aware that high FC/FB levels can affect the phenol red pH test. I've had this happen to me before. I can assure you guys that the high pH readings I'm referring to are NOT the result of the phenol red converting to chlorphenol red (purple) due to high sanitizer levels.
Maybe I need to add some more Borax to the water, but I don't have a borate test besides strips unfortunately. Or maybe I just need to keep the TA lower. Or maybe I need to be ok with a pH closer to 7.8 or 7.9. Guess this is where the guessing begins. lol

 

[guamguy]

Re: Bleach & pH

I would try to work with a pH of 7.8 or so. I tried to work with a 7.2 pH for the first 6 months I had the pool and it was kind of a pain. After balancing the CH and TA to work with a 7.8 pH, life was a LOT easier! The pool is open and in use all year 'round, and I don't remember the last time I needed to add acid! This may not work for everyone, but it's worth a try if you are struggling with constantly rising pH.

 

[duraleigh]

Re: Bleach & pH

Adding my thoughts to guamguy's, my pool likes pH 7.8 as well. Holding it down below 7.6 seems troublesome but at 7.8 I add acid about twice each season.

 

[wes8398]

My pool's never been a problem. I've never had to add acid to it. This thread is regarding my hot tub, which seems to be another story.